My, how time flies. This is a post I have been meaning to write for nearly 3 years now. Back in late 2018, after a search for a small portable SW receiver, I purchased a C Crane Skywave SSB. It had a lot of the things I wanted in a portable radio and, at the time, I felt that it offered a lot for a receiver of it’s diminutive size.

Then I became aware of an even smaller receiver called the Belka-DX. Designed and manufactured by Alex EU1ME in Belarus, this positively tiny radio used SDR technology and, judging by the reviews I was reading and videos I was watching on YouTube, there was no other receiver it’s size that felt and performed like a much larger communications receiver in the way that this one apparently did. It all seemed very encouraging, so I went ahead and ordered one directly from Alex in Belarus. The first time I tried to order direct from his site, my bank denied the payment. I got on the phone to advise them that it was a legitimate charge, and they gave me a one hour window in which to put the transaction through again. I returned to the site, ordered the Belka-DX, and the order was accepted.

There are 3 ways I know of to purchase Belka receivers –

1. Directly from Alex EU1ME, in Belarus. Alex supplies 2 versions, with and without a built-in speaker. In order to accommodate the built-in speaker, that particular version has a slightly smaller battery. The version with the built-in speaker currently costs 475BYN, which at the exchange rate at the time of writing, is about US$145. As of Aug 9th 2024, Alex’s site carries the message that international shipping is currently unavailable. I notice that in the Q&A, Alex noted in July that he hopes to resume shipping to the US in about a month. This was the message he posted, on July 18th 2024 – “We do ship to the USA but at the moment we need to to undergo technical expert appraisal so that our Belarusian customs could allow export abroad. We hope to restart shipping in a month. Shipping cost to the USA is 13 USD.”
   
   
2. From Mobimax in Bulgaria. This is the same receiver, but with a larger speaker back that also has two small fold-out legs. This increases the depth of the Belka, but allows you to have a speaker as well as the larger 2500mAH battery. The markings for the input and output connections on the sides are etched into the metal. In addition, the LCD display is fitted with a screen protector into which are etched the words “HAM tactical RCVR”. Mobimax sell two different packages containing this receiver. The only difference is that the very slightly more expensive package includes a 3.5mm stereo to 3.5mm stereo cable, for plugging the IQ output of the Belka to your computer for use with SDR software. The version of the Belka that Mobimax supply is currently 227 Euros, which is about US$247. The package with the IQ cable is just a few Euros more.
   
   
3. There is an eBay seller in Bulgaria who sells the same version that Mobimax does, but for the (in my opinion) rather high price of US$350. The main reason I can see that buyers might go for this seller is the convenience and comfort factor of being able to pay with Paypal on a site they are familiar with.

A few weeks after ordering, a small box arrived from Belarus. In it was the Belka-DX wrapped in bubble wrap, and a small telescopic whip antenna of about 28.5″ in length. No documentation was included in the box, though it is available online. I didn’t take any pictures of it then, so here are some of it now, 3 years later.

In the above image, the backlight is on. In the next one, it is off. The backlight can be customized to be on all the time, off all the time, or to stay on for 12 seconds after any button is pushed or the tuning knob is turned. In the next photo, to the left of the BNC is a micro-USB connector that is used for charging the receiver. Underneath it is a red LED that lights when charging. To the right of the BNC antenna connector is the 3.5mm earphone jack. It is important that a stereo TRS jack is used here. A mono jack will short out one of the channels and can damage the audio IC –

At 82mm x 50mm x 20mm, this receiver is small!

I’m not sure if this is still true of the version that comes with the internal speaker direct from Alex, but the speaker holes in mine were slightly imperfect. It was evident that they had been drilled by hand. Not a big deal, but I thought it worth noting –

On the right side is the tuning encoder. It rotates smoothly with no click stops, for that “big receiver” smooth tuning feel. Also on the right side is a 3.5mm jack for the IQ output –

This was the second iteration of this receiver. The first one, named the Belka-DSP, covered 3.5-31 MHz. The Belka-DX covers 1.5-31 MHz. There is now a newer version, known simply as “Belka”, that has impressively continuous coverage from 0.1 MHz – 31MHz. Yes – 100KHz to 31MHz!

There are, by now, a number of quite detailed reviews of this series of receivers online. One such review, which gives a good overview of the capabilities of this pint-sized communications receiver, was written by Dave N9EWO. Since purchasing my Belka-DX almost 3 years ago, I have used it regularly. There are several features that I find very compelling, which distinguish it from many other shortwave portables –

1. There is no chuffing or soft-muting in between frequency steps. As a result, when on the smallest frequency step of 10Hz, the effect is of smooth, continuous tuning.
   
   
2. Unlike the CC Skywave SSB and, I believe, many other portables of it’s type, it is absolutely solid on SSB and CW receive. The carrier injection on my C Crane receiver was unsteady on strong signals, leading to chirping on CW, and similar frequency instability on SSB. For a short while, I owned a CountyComm GP-5 SSB, which was even more unsteady. The Belka-DX handles like a proper communications receiver in this regard, being rock steady on strong and weak signals alike.
   
   
3. The frequency display is accurate. As far as I can tell, it is accurate to better than 10-20Hz across the frequency range. Because of this, I can easily tune it to a frequency, and know that it is there. The Belka-DX employs a 0.5ppm TXCO and as a result, has a high level of frequency stability. My Skywave SSB only tunes in 1 KHz steps. To interpolate between those steps, you have to push a button to engage the fine tuning, but are not able to read the frequency accurately in between those 1 KHz points. For many users, this might not be an issue, but for those who listen out for weak beacons and other signals that are not on 1 KHz “channels”, the continuous tuning and accurate frequency readout on the Belka makes such monitoring much easier. A couple of years ago, I went on a 4 /12 month long campervan trip around 6 Western states. At the time, one of my interests was listening out for low-powered unlicensed HF beacons. Standing in the vast expanses of the desert with the little Belka-DX in my hand, hearing a weak low-powered CW beacon from hundreds of miles away was magical. These beacons are home-made affairs, and usually running somewhere between 30mW and a watt. Being able to dial in the precise frequency on a handheld receiver that is even sensitive with the set-top whip is a boon with such pursuits.
   
   
4. You can tailor the passband for each mode. With my C Crane Skywave, and I believe many other similar receivers, the adjustable filtering (if available) is audio filtering, and doesn’t occur in the RF stages. The Belka is an SDR, and the custom adjustable filtering is the equivalent of filtering in the RF or IF stages of a conventional superhet.

Two things that I wasn’t too keen on, and which have been amended in the 0.1-31MHz version –

1. In CW mode, the frequency display doesn’t indicate the operating frequency. For example, with a 700Hz sidetone pitch selected, if you want to receive a CW signal on 7030 KHz, you have to tune the receiver 700 Hz below that frequency i.e. to 7029.3 KHz. You do get used to it, but it would be nice to have it display the actual operating frequency in CW mode. In the newest version, the 0.1-31MHz version, I have read that the receiver displays the actual operating frequency in CW. If you’re concerned with being able to read out the exact frequency (if searching for weak beacons on non-standard frequencies, for example) you’ll still need to ensure that you tune to the correct sidetone frequency in order for the frequency readout to be accurate. Personally, I’d love a sidetone feature for this, but I doubt that too many others would consider it to be an essential feature in a receiver (as opposed to a transceiver).
   
   
2. When stepping through the memory channels, you cannot hear those channels as you cycle through them. In order to hear the selected channel, you have to press the appropriate button to select and load it. Thus, you cannot easily scan through a number of preset memory channels to listen for activity. As with 1 above, I this has been remedied in the newest version of the Belka.

The audio quality from the Belka-DX is excellent when used with earbuds. The internal speaker doesn’t do it justice, though it is very useful when taking the receiver on outings. Power from the audio amplifier is adequate for most applications, though when plugging an external speaker in, it helps to use one that is sensitive. Some people use powered speakers. I have two external speakers on the bench, both unpowered, that I use with it. The main one is an MFJ-281 ClearTone speaker. It is sensitive, and produces good volume. The audio response from the mylar cone is restricted, and what I would characterize as communications quality. Audio is clear and intelligible. The speaker appears to have a natural resonance at around 650-700Hz, which is useful for CW. For those times when I need a little more fidelity, such as when listening to strong SWBC stations, or hams on AM, I use an old and compact hi-fi speaker manufactured by Cambridge Soundworks. It was discarded by one of my neighbors, and appears to be a mid-range unit. It is not as sensitive as the MFJ ClearTone speaker, but the Belka will still provide enough drive in a small, quiet room, which perfectly describes the conditions in my shack.

The small size and slim dimensions of the Belka-DX make it ideal for traveling. When using it at home with an external antenna connected, the ergonomics and ease of use are much improved when mounted on some kind of stand. There are a number of stands available, as well as files for those who wish to 3D-print their own. I remembered a clamp I once bought, that was designed to hold a cellphone for attaching it to a tripod, for making videos. The Belka is not quite as wide as a cellphone, so I used a couple of pieces of dense foam to pad it out, and screwed it to a small tabletop tripod. It works quite well, and improves the ease of use drastically when listening at home. When mounted like this, it feels like a serious and very usable SWL set-up. In the various SWL groups on FB that I frequent, I often see questions from folk asking about receivers that are good for SWL’ing. Portables such as the Tecsun PL-880, along with other similar receivers are often recommended. I think that this Belka makes an excellent receiver for all-round shortwave listening. It is not available in as many outlets as the more traditional shortwave portables, which is why I think that it isn’t as popular in the SWL community as it should be. If you are listening mainly to AM broadcasts on shortwave, then many of the portables will most likely work well. If you do a lot of SSB and CW listening though, the Belka is a solid and, to my mind, preferable option.

In the following picture, my Belka is mounted on a mini tripod (an Ultrapod) and connected to the MFJ ClearTone speaker. Behind and underneath the Belka, you can just see an Altoids tin which contains a high pass filter with a cut-off at about 2700 KHz. It was designed to prevent overload from strong local AM broadcast stations. More on that later in this post.

The Belka-DX is surprisingly sensitive when listening outdoors with the included telescopic whip. It does need a counterpoise, or the received signal strength suffers greatly. If you are holding the receiver, then your body acts as the counterpoise. If you are listening on earbuds or headphones, then the headphone cord acts as a counterpoise. If the receiver is sitting on a surface and using the internal speaker, then you’ll need to connect a counterpoise wire somehow. At home, it works really well when connected to my outdoor antena, which is a doublet at 47 feet, cut roughly for 40M, fed with 300 ohm twinlead, and matched to coax at the entrance to the shack, with a balun and Elecraft T1 tuner. I tune the T1 by squirting RF into it on the nearest amateur band. If you are using your outdoor antenna for listening only, then a simpler arrangement would suffice. This just happens to be the one antenna I also use for my ham exploits.

I live in a densely populated urban area, within a few miles of several medium power (5KW) AM broadcast stations. They often break through when I am using the external antenna with receivers that don’t have narrow filtering on the antenna input. My Belka-DX experiences strong AM breakthrough when used on the external antenna at all frequencies up to 4530 KHz. The moment I tune above 4530 – even by a single 10Hz step, the breakthrough stops instantly, suggesting that a different bandpass filter is switched in at the point. According to the manufacturer-supplied block diagram of the first version of Belka, known as the Belka-DSP, the input bandpass filters are from 3.5-7.5MHz, 7.5-15MHz, and 15-30MHz. The block diagram can be seen on this page by Fernando Duarte. I assume that for the Belka-DX, with it’s extended coverage down to 1.5 MHz, one of the bandpass filter crossover points is at 4530 KHz, the point above which all AM BC band breakthrough stops.

To solve this problem of breakthrough, I resorted to a little high pass filter that I have used successfully with other receiver projects. It’s a high pass filter that was designed by David WA7JHZ, details of which were given in K4SWL’s very wonderful and informative SWLing Post blog. You can see it here. David built his with molded chokes. I built versions with both molded chokes and toroids, and compared the response curves.

Trusty Altoids tins to the rescue. Here’s the version built with molded chokes, purchased from Tayda Electronics. The chokes are mounted vertically, and are a little hard to see in this image –

The small holes in the base of the tin were left over from a previous project that didn’t work out.

Then I built another high pass filter with toroids instead of molded chokes. I figured the toroids should have slightly higher Q and would present a better response curve. All 4 inductors were wound on T37-6 toroid cores with 26 AWG wire. The 2.7µH inductors had 30 turns and the 1µH ones 18 turns. The wires supported them about 4 or 5mm above the ground plane of the Altoid tin –

Dang, after all these years, Altoids tins still make very serviceable and cheap enclosures for small projects!

These filters were designed for input and output impedances of 50Ω. My one external HF antenna is a 40M doublet fed with 300Ω twinlead, and matched to 50Ω coax with a 1:1 balun and Elecraft T1 tuner. As mentioned previously, I briefly transmit a small amount of power on the nearest ham band to where I want to listen, to tune the T1. A manual tuner could be used here instead, and tuned for maximum noise. For listening, this is not too critical a procedure, and a single tune will cover the receiver for listening on a wide range of frequencies. The antenna input of the Belka is matched for the high impedance of the supplied short telescopic whip, and not for a 50Ω antenna. Nevertheless, I went ahead and plugged both versions of this HPF in between the antenna and the receiver, and they both served to completely eradicate every single trace of AM BC band breakthrough.

Using a NanoVNA, I measured the response curve of both filters from 1.5 MHz to 30MHz. Here’s the curve for the filter built with molded chokes –

FREQUENCY (MHz)	INSERTION LOSS (dB)
30	0
20	0.1
15	0.25
10	0.5
3.7	1
3	3
1.7 (1700 KHz)	41
1.5 (1500 KHz)	49

The 3dB cutoff point of this filter was actually 3 MHz, and the insertion loss small, with a virtually flat response from the 80M band up to the top of the 10M band. Here’s a close-up of the response between 1.5 MHz and 4 MHz –

The insertion loss of the toroid filter in the passband was a little lower, For all practical purposes though, there would be no discernible difference between the two filters. If you hate winding toroids, then by all means, build this filter with molded chokes, and it will kill your AM BC band breakthrough just as effectively as if you’d built it with toroids. Here’s the response curve of the toroid version from 1500 KHz to 30 MHz –

And from 1500 KHz to 4 MHz, giving a closer look at the area around the the 3dB cutoff point –

FREQUENCY (MHz)	INSERTION LOSS (dB)
30	0
20	0.04
15	0.12
10	0.25
3.7	0.82
2.8	3
1.7 (1700 KHz)	40
1.5 (1500 KHz)	48

There are quite a few internally generated birdies throughout the whole coverage range. However, the majority of them are only audible with no antenna connected, and are masked by band noise. The others, although audible over the band noise, are not troublesome. For a receiver this compact, and with this overall level of performance, it’s a small price to pay. I rarely noticed them during normal use. It would be nice for the end-user to have a way to update the firmware, though the extended coverage down to 100KHz that the new (V3) Belka enjoys required a hardware upgrade in the form of an extra bandpass filter.

For a more complete description of the improvements made with the newest Belka version, see 13dka’s guest post on Thomas K4SWl’s excellent SWLing Post blog. In short, the Belka is a fantastic general coverage shortwave receiver. It performs and handles like much larger tabletop communications receivers. It is so small that it can be carried anywhere with great ease, making the decision to do a little SWL’ing while on a hike, a walk, or any trip, a no-brainer. You can do a lot of serious shortwave listening with this receiver. Ordering direct from Alex in Belarus offers by far the lowest price and is, in my opinion, the way to go. When I think of my first communications receiver, an old, huge and very heavy British military R107 boat anchor, it is amazing to think that this light and svelte pocketable Belka-DX handily runs circles around it. An SWL can positively rule the shortwaves with this tiny and light miracle of wireless!

There are many other, far more comprehensive reviews on this receiver, but I have been wanting to sing the praises of the Belka (which is Russian for squirrel) for a long time now. I needed to get this out.

Before building Jim W4LF’s Hobbydyne crystal set, I put together an impedance matchbox, for matching the detector diode to a variety of different headsets and earphones, so that I could determine the best ones to use. The world of serious crystal set listening was new to me, so I did some reading up. To give you an idea of how serious this gets, many committed crystal set listeners have heard over 100 different stations on their sets, on the AM broadcast band (thanks to nighttime skywave propagation)!

It appeared that there are a few different kinds of headset that crop up often as being the favored types among crystal set enthusiasts. Of these, perhaps the most storied is the Baldwin Type C radio headset, or “Baldies” as their owners affectionately call them –

There are a couple of reasons why crystal set aficionados often have a set of Baldies in their collection. One of these is for their historical significance. Baldwins are considered to be the first mass-produced headset that significantly resembles modern headphones. Developed by Nathaniel Baldwin in Utah in the early part of the 20th century, they were first patented in 1910. He got his big break after sending 4 pairs to the US Navy. They were very impressed with their sensitivity and performance, and put in an initial order for a hundred pairs. Baldwins were a high quality headset. In the early 1920’s a set cost $14-$16 – about $250 in today’s money.

The other main reason for their continued popularity with crystal set folk, is that even by modern standards, they are quite sensitive. The design is different from most other headsets of the era, being that they employ a balanced armature and a mica diaphragm. A small proportion of Baldwin headsets had a phenolic diaphragm, and were slightly less sensitive as a result. It’s easy to tell if your Baldwins have mica diaphragms. If you unscrew the ear caps and see clear diaphragms like this, yours are mica –

Here’s a view of the bottom of the element. Looking at it from this angle requires that the diaphragm side is downwards. Do not set it in this orientation without placing it on top of the ear cap, as you could damage the diaphragm or the drive rod (see how, in the above picture, the very end of the drive rod protrudes very slightly above the level of the diaphragm?) In this following view, if you look carefully, you can how the plate (circled) that supports the drive rod, is centered between the two poles of the magnet. This is how it should be – slap bang in the center, and not touching one or other of the poles –

Looking straight down at the back of the element, you can see the 2 screws that are used to connect the ends of the headphone cords. I wish that modern headsets were as simple to disassemble and reassemble as these vintage models are –

Scott (from oldheadphones.com) is a big collector of vintage headsets. He told me that of the Baldwin headsets he has seen that have weak output, it has rarely been because of weak magnets, especially with the units that have purple painted magnets, as seen above. I believe that the purple painted magnet variety are far more common than the other type, which have black painted magnets. If your Baldwins have weak output, things to look for, Scott advised me, are armature plates that are not centered between the magnet poles, or are actually touching one of the poles (keep an eye out for debris between the plate and the poles). Another thing to look for, is warped or broken diaphragms.

Nathaniel Baldwin’s story is an interesting and dramatic one. Read it here – it’s well worth it. My Baldwins, like most of the vintage parts and gear I acquire, came from eBay and, contrary to what seems to be popular opinion, overpaying is not necessarily the norm. As the buyer, the power is in your hands. I paid US$15 + $12 shipping for these, which I think was a very fair price. I did clean them up, but only a little. This is the condition they arrived in, which wasn’t bad at all –

They were in pretty good shape, the main areas in which some shabbiness could be seen being in the headband, which showed very little fraying, though there was some dirt and stains present, and just a small amount of corrosion on the metal parts –

Scott from oldheadphones.com has a useful page on the restoration of old headphones. On this page, he is specifically referencing Western Electric 509-W’s, but the same advice can be applied to other types. Following this advice, I gave the metal parts of the Baldies a very light polish with Brasso (only very cursory, as I really wasn’t that bothered with making them gleam), and cleaned the bakelite earcups up a little with Bon Ami, which is a very minimally abrasive household cleaner. The construction of these headsets makes it very easy to take them apart for cleaning. The headphone cord unscrews from the elements – no soldering needed, and the elements themselves can be taken out from the bakelite earcups with no unscrewing at all – they simply drop out. It is a pleasingly very modular type of construction.

He also gave me some useful advice on cleaning the cords and headbands via email. Following this advice, I sprayed some prewash onto the headband, left it to soak for a short while, then immersed it for a while in a basin of cold soapy water, made with clothes detergent, while giving it a light scrubbing with an old toothbrush. Be careful when doing this, as the old fabric can fray quite easily. Then I rinsed it with cold water and left it out to dry overnight. I didn’t do anything with the cord, as it was in good shape. Incidentally, if you can buy something from him, it helps to fund his hobby of collecting vintage headsets.

Here’s the result. You can see the most improvement in the condition of the headband. Not gleaming, but with the aura of respectability that comes with the evidence of having lived an honest working life –

A very usable pair of Baldwins for everyday use, I think!

While on the subject of Nathaniel Baldwin Type C headsets, I want to share with you a very exciting find that I made while looking for the everyday use pair above. This was also on eBay and, unlike with the above pair, I paid quite a lot for these. They are a set of completely unused Baldwins, in the original box, with the instruction leaflet. The box was a little banged and frayed, but the headset was mint, and looked as if it had never even been assembled, let alone used! I am not an experienced collector of these but, on seeing them, something told me that it was not very common to see a pair in this condition. I ummed and aahed over them, told myself I wasn’t going to pay that much for them, and moved on. Shortly after, I came back, and opened up a correspondence with the seller. He had bought them at an estate sale 30 years ago, he told me, and they had been in storage ever since. He wasn’t sure if they worked, and didn’t know how to test them, but he did own a multimeter. I told him how to do a continuity check on the coils, and they tested out fine. This pushed me over the edge and, after a bit of back and forth, we agreed on a price. They were mine!

I hadn’t seen a pair of Baldies with a green headband before. I think they look very handsome!

This model was first patented in 1910, and the entire Baldwin operation closed down between 1930 and 1932, so I think it’s fair to say that they are about 100 years old. It’s hard to put into words the appeal of seeing a product that has been sitting in it’s original box for 100 years, in the same unused state the original buyer would have seen it. I can’t help but wonder how this came about. Perhaps this headset sat on the shelves of a radio store somewhere that went out of business and then, along with other remaining stock, was sold, and sat in storage for years? Perhaps it was purchased by a well-heeled customer who bought it and forgot about it?

Just look at that shiny bakelite ear cap, that completely clear mica diaphragm, and that shiny metal, without nary a smidgin of corrosion in sight. Magnificent!

I mean, if you could get in a time machine, walk down Radio Row in NYC in the 1920’s, dive into a store and buy a pair of Baldwins, this is exactly what they’d look like –

Oh lawdy. The shiny bakelite. That is original factory shine. The beautiful green headband. It’s almost too much for me to bear!

That leaflet, that came in the box with the headset? Here it is –

That’s enough of Baldies for now. The other headset of this vintage that I was keen to land a working pair of, was the Western Electric 509-W. Patented in 1918, this headset was also very popular with telegraph and radio operators in the late teens and 1920’s, mainly due to it’s ruggedness and sensitivity. Many believe 509-W’s to be equal to Baldwins due to their sensitivity, robust nature, and quality of manufacture. In the opinion of Scott, from oldheadphones.com, they are ideal for crystal set use, and their performance rival that of Baldwins, only being surpassed by Navy sound-powered headphones.

Continuing my quest to land a bargain, I scored a set of 509-W’s for just $10.50 + $14.85 shipping. They arrived in good, though obviously well used, condition –

A bit shabby, to be sure. Allow me to show you a few more pictures, before I reveal how they cleaned up –

The metal cans were dull, but with no major scratching or other damage. The slots on the two screws that hold the sound element to the nickel-plated brass cans are often knurled or otherwise damaged. Not so in this case, which I took as a good sign –

These are not balanced armature units, like the Baldwins. They are the more traditional style, which makes the poles of the magnets very easy to access for the purpose of re-magnetizing, if that becomes necessary. Like the Baldwins, the headset cord is easily attached and attached with 2 screws. The general rule of thumb for determining if the unit needs re-magnetizing, is if the magnet holds the metal diaphragm on, it is strong enough. These headsets are the more traditional type, with both poles exposed, making remagnetizing easier, if necessary. For these traditional types of headsets, of remagnetizing, Scott says, “I just use a strong rare earth magnet and a couple taps on one pole of the weak magnet takes care of it.”

I polished the metal parts with Brasso, washed the bakelite ear caps with Bon Ami, and soaked the headband in cold, soapy water, then lightly scrubbed them and left them to dry overnight. The headphone cord smelled of tobacco and was a bit greasy, so I also soaked it in cold, soapy water (yes, really) and scrubbed it lightly with an old toothbrush. I then patted it down on a towel, gave it an initial drying with a hair dryer, and hung it up to dry thoroughly, on a hot day.

During this initial cleaning, I did lightly polish the metal cans, but didn’t pay too much attention to the other metal parts. For minimal effort, the 509-W’s came out quite well, I think –

One feature of this set was a little curious. It came with this washer hanging from a piece of string on one side of the headset. To me, it looked as if it belonged there and was an integral part of the unit. I wondered what it was for. I wasn’t able to locate any photos of other instances of this headset with such a washer, and no mentions of a washer anywhere else. After asking a few other 509-W owners, it became apparent that whatever the washer’s purpose was, it was not a stock item, and was not a known 509-W accessory. Eventually, I removed it, but not before taking these pictures. I think the first picture (the very next one) was taken partway through the cleaning process, before all the metal had been cleaned up –

The headband came out a lot cleaner. The cord looked a little better, though the main improvement was that it felt a lot less greasy –

Shiny!

Western Electric 509-W headsets are not hard to find in fairly good, working condition. There are still a lot of them around. They were well made, and cost $12 in the early 1920’s, the equivalent of a week’s wages for the average worker. Other brands of headphones, that were less well-made and not as sensitive, cost less. Brandes were $8, and other brands even less. I wonder how many sets of headphones being used today will still be around 100 years from now and working well?

Cloth headset cords of this era came with strips of material on the ends that attached to tabs on the cans for strain relief –

The headband is looking pretty spiffy and clean from this side!

Next on the agenda was to put together a sound-powered headset. Sound-powered units are used by the military; primarily, I believe, by the Navy. The sound elements are so sensitive, you can connect two together and they will operate without the need for outside power i.e. the tiny current produced by talking into the mouthpiece will produce a sound in the earpiece. This very informative page on Darryl Boyd’s website has information on many of the available sound-powered headsets you’re likely to find. Jim Frederick W4LF’s favorite is the WW2 sound-powered headset known as the RCA “Big Cans”, which I believe are also referred to as the US Navy “Deck Talkers” (see linked page in previous sentence). He says that they are hard to find and expensive, but so far ahead of the others that they are worth it.

Some of the sound-powered units, such as the US Navy Deck Talkers, have a headset, so little physical modification will be needed in order to use them. With others, the sound elements are in a telephone-style handset. In this case, you’ll probably want to remove the elements from the handset and install them in a headset of your choosing. This page on Darryl’s site lists the specs of sound elements in the various units, and also notes the handsets that have identical elements used for both microphone and earpiece. This can be useful, as you will only need one handset to make a complete headset. I set about looking for one of these handsets, and managed to come up with this Canadian RCA MI-2215-E model, which has 2 identical sound elements –

Touching the two leads coming from the handset across the terminals of a AA battery resulted in absolutely no clicking from the headset whatsoever, which was not a very encouraging sign.

Back view of one of the elements, after the leads had been desoldered, with a strip of tape over the holes to prevent ingress of debris –

Front view of the same element –

Removing the metal back cover from the first element, the armature plate was clearly visible. If you look closely (easier if you’re viewing this on a computer, as opposed to a phone), the armature plate, which was connected to the threaded drive rod, appeared to be jammed up against one of the poles of the magnet –

Looking at the threaded drive rod, you may be able to see that there are two small nuts, one above and one below the plate. Adjusting those nuts determines the default positioning of the plate. I adjusted them with a pair of needle-nosed pliers and successfully repositioned the plate halfway between the top and bottom magnet, so it was no longer in contact with the top magnet. At this point, I didn’t have a crystal set to test it out with (I hadn’t yet built the beta kit in my last post). Instead, I connected it to the output of my impedance matchbox, with the output impedance set to somewhere in the region of 300 ohms (the impedance of this element), connected a longwire antenna and ground to the two input terminals of the matchbox, and connected a 1N34A diode across the input terminals of the matchbox. Success, as the sound of a cacophony of MW AM stations sprang forth from the sound element. It was working!

Buoyed by this success, I set about tightening the nuts, and disaster struck. I overtightened one nut, and snapped the very delicate threaded drive rod. I had lost sight of the fact that it is a thin rod. Unfortunately, the break occurred right next to the nuts. If it had been further away from the nuts, I might have been able to solder or epoxy the rod pieces back together. As the break was up against one of the nuts, repairing it would have reduced my ability to adjust the positioning of the nuts on the rod.

Back to the drawing board and, before long, I acquired this H-203/U handset, manufactured by the Dynalec Corporation. This handset has a push-to-talk button –

When this handset is connected to another handset, pushing the PTT button connects the handset to the other handset so that, when the PTT is not pressed, the other station cannot hear what you are saying. If you connect the two handset leads together and talk into the microphone, you should be able to hear your own voice in the earpiece. If you’re unsure whether you’re hearing your own voice via the earpiece or via bone conduction, alternately pressing and releasing the PTT while talking will clarify.

Unlike the previous handset, the identical microphone and earpiece are not soldered in, so it is easy to remove them. They just pull out, revealing the neatly-wired supporting circuitry –

Even the capacitor leads across the earpiece have been neatly preformed. I very much approve!

These are the identical sound elements from this handset –

The terminals are easily soldered to, though you have to be careful to apply the iron for the absolute minimum of time, so as not to melt the plastic. I used a little flux from a flux pen, to help things along –

Next on the agenda was to find a suitable headset or old pair of headphones to install these elements in. I found this new unused pair of earmuffs on Facebook Marketplace for $5 –

This set of earmuffs turned out to be well suited for these sound elements. The aperture in one end of each side of the earmuffs is very slightly wider, and there is a good thickness of foam lining the back of the earmuff –

It was possible to push each sound element in at an angle, such that it ended up being held firmly in place by being pressed up against the ledge/flange around the opening, by the foam in the back –

The headset lead enters one side, at the bottom of the can. Heat-shrink tubing over the lead and tie-wraps on the inside and outside of the can hold the lead in place –

A lead runs in between the cans to connect the elements in series (in phase). It runs between the tops of the cans, underneath the cushioned piece that surrounds the headband –

These ProCase brand earmuffs are available on Amazon at the time of writing for $16.99, although the FB Marketplace price of $5 from a local private seller was a no-brainer, of course. The only possible drawback to them is that the fit is very tight, though as the purpose of earmuffs is to keep ambient noise out, this is an intentional part of the design. One could argue that a sound-powered headset will only be worn for the very weak DX stations, and the absence of outside ambient noise is helpful when trying to copy them. I used an old headphone lead from a pair of AKG 240’s with the molded plug at each end chopped off, but you can use anything that works for you.

Also included in the assortment of headsets/headphones/earbuds/earpieces that I tested with my newly built crystal set and impedance matchbox, was a classic piezo earpiece, often known as a crystal earphone. The styling hasn’t changed over the years, and many of the cheaper crystal radios marketed to youngsters in the past, such as the rocket ship crystal radio, came with one of these. The metal diaphragm is connected to one of the leads, and used to be soldered to it. In recent years, many of the units sold had a foil diaphragm with the lead held to it partially by glue and partially by the pressure of the plastic case against the lead. As a result, there was a high rate of failure. Both the piezo earpieces of this type that I bought failed soon after I acquired them. However, the old-style with a soldered connection are still available. Mike’s Electronics sells them. There is also a seller on eBay called protechtrader who sells them (they recently increased the price significantly, from $10.70 to $14.99, both prices including shipping). The earpieces from the eBay seller have a characteristic black lead and plug. You can see the brass color of the diaphragm too, which I assume is also the case with the one from Mike’s –

These piezo earpieces have a 3.5mm mono plug. The 3.5mm jack on my impedance matchbox is wired to a stereo plug, so a 3.5mm stereo male to mono female adapter was pressed into service.

Also tested were an MDR-W14 yellow headset from an old Sports Sony Walkman cassette player/ AM/FM radio combination, and a pair of C Crane earbuds, both low impedance –

I forgot. I also tested my beloved AKG 240’s, of which I own 2 pairs. I used these for years, when working as a DJ/announcer, and voiceover guy in Los Angeles. I also use them with my Elecraft K2 on CW, as they are very comfortable to wear for long periods. I wasn’t expecting a lot from them for crystal set use though, as they are known for not being as sensitive as the more consumer type low impedance models. They are intended mainly to be driven by the headphone amplifiers present on mixing boards and similar professional equipment, which is capable of providing a greater drive level than the amplifiers in small consumer products such as Walkmans/radios/MP3 players etc –

OK, the big test. How do all these different headsets/headphones/earpieces/earbuds stack up against each other? Firstly, allow me to say that I haven’t yet performed extensive testing with many very weak stations, so take these preliminary results with a pinch of salt. Also, at this point, I still have some significant improvements to make to my antenna, by lengthening the outside portion of it from 45 feet to about 75 feet. This will very possibly yield a far more noticeable improvement than which headset I choose to use will.

That said, here are my initial impressions.

As expected, the AKG 240’s are the least sensitive, though not by as large a margin as I expected. They was, at a rough guess, about a 6 dB difference between them and most of the other headsets which, surprisingly, all seemed to be roughly the same in sensitivity. However, the AKG’s have a wonderfully flat audio response, and AM radio sounds great on them. They’re really good for listening to stations that are moderately strong, or greater.

The piezo earpiece was sensitive, though the sound was very restricted and tinny and, because it’s just one earpiece as opposed to two, doesn’t sound as loud as the other units as it’s only in one ear. Wiring two in series or parallel might help with volume, though the frequency response will still be very tinny. Plus, it kept falling out of my ear, which was annoying.

The Baldwin Type C, Western Electric 509-W, homemade sound-powered headset, Sony Walkman MDR-W14 headset, and C Crane earbuds all seemed to be quite sensitive, about as sensitive as each other, and about the same volume on weak to moderate strength signals. On strong signals, the Baldwins and sound-powered headset weren’t quite as loud as the others due, presumably, to the physical limitations imposed by the balanced armatures.

My initial impression was that, if anything, the Sony Walkman MDR-W14 headset was very slightly more sensitive than any of the others tested here. However, I’m not sure if that’s really the case, or has more to do with the fact that sound is transferred more efficiently to the ear because of the way that the headset earpieces sit in the ear canal. Being earbuds, the C Cranes also sit in the ear canal, but the Sony headset has a slight edge over them. However, I believe this is mainly because the Sony set has more response over the whole audible frequency range, while the C Crane earbuds purposely have their frequency response shaped to favor voice, with a sharp drop-off above about 7-8KHz.

The Baldwins and Western Electric 509-W’s both have somewhat restricted frequency response; the WE’s have slightly more bottom end.

Both the C Crane earbuds and Sony MDR-W14 headset sound a lot louder on strong signals, yet are also sensitive on weak signals. Of the two, the Sony has the widest frequency response. For most types of listening, my favorite headset, of all of them, is the Sony. My sense is that it would also be good for listening for weak DX stations. It’s possible that a headset with less bottom end might increase intelligibility and copy on the very weakest of stations. It’s for this reason that I’m thinking it’s worth keeping several headsets to hand, just as many serious listeners keep a selection of diode detectors on hand, for the most challenging of DX catches. Interestingly, my initial take on these headsets aligns with what Al Klase has said, namely that in his experience, modern earbuds, even the cheap ones, are about as sensitive as sound-powered headsets.

I find all of this a bit frustrating, because I went to quite a lot of expense, time, and trouble, only to discover that my favorite headset to listen to my new crystal set on, was a cheap Sony Walkman model that I already owned! I am not too surprised by this finding, as I had already read Al’s remarks, but needed to find out for myself. I could have saved myself quite a lot of money. On the other hand, I do enjoy owning a few pairs of vintage and antique headphones. The Baldwins and WE 509-W’s both occupy significant places in radio headset history, and my mint Baldies are museum grade. There’s a definite pride of ownership at play here.

Bear in mind that with vintage headsets, there can be variation in their performance, especially if they’ve been treated poorly throughout their long life. Sound-powered headsets often received rough treatment while in service. If you have a set that are in poor physical shape, they may have received a lot of knocks during their life that degraded their performance. In other words, my very brief test drive of these different sound-producing devices was preliminary at best. Nevertheless, things are looking good for the combination of matching transformer and a modern headset/earbuds using neodymium disc magnets and lightweight components.

It turns out, after all those vintage headsets, and a homebuilt one, that my favorite way to listen to this crackin’ little crystal set is on a pair of Sony Walkman MDR-W14 headphones. They have great fidelity, are the loudest on strong stations, and appear to be sensitive as well. I do wonder if a pair of US Navy Decktalkers (the famed RCA “Big Cans”) would beat them on very weak signals, but Al Klase appears to have a pair of those, and still said that modern earbuds are about as sensitive as anything else he has used. Who’d have thought! Crystal set enthusiasts – what are your experiences?

My, how time flies. This is a post I have been meaning to write for nearly 3 years now. Back in late 2018, after a search for a small portable SW receiver, I purchased a C Crane Skywave SSB. It had a lot of the things I wanted in a portable radio and, at the time, I felt that it offered a lot for a receiver of it’s diminutive size.

Then I became aware of an even smaller receiver called the Belka-DX. Designed and manufactured by Alex EU1ME in Belarus, this positively tiny radio used SDR technology and, judging by the reviews I was reading and videos I was watching on YouTube, there was no other receiver it’s size that felt and performed like a much larger communications receiver in the way that this one apparently did. It all seemed very encouraging, so I went ahead and ordered one directly from Alex in Belarus. The first time I tried to order direct from his site, my bank denied the payment. I got on the phone to advise them that it was a legitimate charge, and they gave me a one hour window in which to put the transaction through again. I returned to the site, ordered the Belka-DX, and the order was accepted.

There are 3 ways I know of to purchase Belka receivers –

1. Directly from Alex EU1ME, in Belarus. Alex supplies 2 versions, with and without a built-in speaker. In order to accommodate the built-in speaker, that particular version has a slightly smaller battery. The version with the built-in speaker currently costs 475BYN, which at the exchange rate at the time of writing, is about US$145. As of Aug 9th 2024, Alex’s site carries the message that international shipping is currently unavailable. I notice that in the Q&A, Alex noted in July that he hopes to resume shipping to the US in about a month. This was the message he posted, on July 18th 2024 – “We do ship to the USA but at the moment we need to to undergo technical expert appraisal so that our Belarusian customs could allow export abroad. We hope to restart shipping in a month. Shipping cost to the USA is 13 USD.”
   
   
2. From Mobimax in Bulgaria. This is the same receiver, but with a larger speaker back that also has two small fold-out legs. This increases the depth of the Belka, but allows you to have a speaker as well as the larger 2500mAH battery. The markings for the input and output connections on the sides are etched into the metal. In addition, the LCD display is fitted with a screen protector into which are etched the words “HAM tactical RCVR”. Mobimax sell two different packages containing this receiver. The only difference is that the very slightly more expensive package includes a 3.5mm stereo to 3.5mm stereo cable, for plugging the IQ output of the Belka to your computer for use with SDR software. The version of the Belka that Mobimax supply is currently 227 Euros, which is about US$247. The package with the IQ cable is just a few Euros more.
   
   
3. There is an eBay seller in Bulgaria who sells the same version that Mobimax does, but for the (in my opinion) rather high price of US$350. The main reason I can see that buyers might go for this seller is the convenience and comfort factor of being able to pay with Paypal on a site they are familiar with.

A few weeks after ordering, a small box arrived from Belarus. In it was the Belka-DX wrapped in bubble wrap, and a small telescopic whip antenna of about 28.5″ in length. No documentation was included in the box, though it is available online. I didn’t take any pictures of it then, so here are some of it now, 3 years later.

In the above image, the backlight is on. In the next one, it is off. The backlight can be customized to be on all the time, off all the time, or to stay on for 12 seconds after any button is pushed or the tuning knob is turned. In the next photo, to the left of the BNC is a micro-USB connector that is used for charging the receiver. Underneath it is a red LED that lights when charging. To the right of the BNC antenna connector is the 3.5mm earphone jack. It is important that a stereo TRS jack is used here. A mono jack will short out one of the channels and can damage the audio IC –

At 82mm x 50mm x 20mm, this receiver is small!

I’m not sure if this is still true of the version that comes with the internal speaker direct from Alex, but the speaker holes in mine were slightly imperfect. It was evident that they had been drilled by hand. Not a big deal, but I thought it worth noting –

On the right side is the tuning encoder. It rotates smoothly with no click stops, for that “big receiver” smooth tuning feel. Also on the right side is a 3.5mm jack for the IQ output –

This was the second iteration of this receiver. The first one, named the Belka-DSP, covered 3.5-31 MHz. The Belka-DX covers 1.5-31 MHz. There is now a newer version, known simply as “Belka”, that has impressively continuous coverage from 0.1 MHz – 31MHz. Yes – 100KHz to 31MHz!

There are, by now, a number of quite detailed reviews of this series of receivers online. One such review, which gives a good overview of the capabilities of this pint-sized communications receiver, was written by Dave N9EWO. Since purchasing my Belka-DX almost 3 years ago, I have used it regularly. There are several features that I find very compelling, which distinguish it from many other shortwave portables –

1. There is no chuffing or soft-muting in between frequency steps. As a result, when on the smallest frequency step of 10Hz, the effect is of smooth, continuous tuning.
   
   
2. Unlike the CC Skywave SSB and, I believe, many other portables of it’s type, it is absolutely solid on SSB and CW receive. The carrier injection on my C Crane receiver was unsteady on strong signals, leading to chirping on CW, and similar frequency instability on SSB. For a short while, I owned a CountyComm GP-5 SSB, which was even more unsteady. The Belka-DX handles like a proper communications receiver in this regard, being rock steady on strong and weak signals alike.
   
   
3. The frequency display is accurate. As far as I can tell, it is accurate to better than 10-20Hz across the frequency range. Because of this, I can easily tune it to a frequency, and know that it is there. The Belka-DX employs a 0.5ppm TXCO and as a result, has a high level of frequency stability. My Skywave SSB only tunes in 1 KHz steps. To interpolate between those steps, you have to push a button to engage the fine tuning, but are not able to read the frequency accurately in between those 1 KHz points. For many users, this might not be an issue, but for those who listen out for weak beacons and other signals that are not on 1 KHz “channels”, the continuous tuning and accurate frequency readout on the Belka makes such monitoring much easier. A couple of years ago, I went on a 4 /12 month long campervan trip around 6 Western states. At the time, one of my interests was listening out for low-powered unlicensed HF beacons. Standing in the vast expanses of the desert with the little Belka-DX in my hand, hearing a weak low-powered CW beacon from hundreds of miles away was magical. These beacons are home-made affairs, and usually running somewhere between 30mW and a watt. Being able to dial in the precise frequency on a handheld receiver that is even sensitive with the set-top whip is a boon with such pursuits.
   
   
4. You can tailor the passband for each mode. With my C Crane Skywave, and I believe many other similar receivers, the adjustable filtering (if available) is audio filtering, and doesn’t occur in the RF stages. The Belka is an SDR, and the custom adjustable filtering is the equivalent of filtering in the RF or IF stages of a conventional superhet.

Two things that I wasn’t too keen on, and which have been amended in the 0.1-31MHz version –

1. In CW mode, the frequency display doesn’t indicate the operating frequency. For example, with a 700Hz sidetone pitch selected, if you want to receive a CW signal on 7030 KHz, you have to tune the receiver 700 Hz below that frequency i.e. to 7029.3 KHz. You do get used to it, but it would be nice to have it display the actual operating frequency in CW mode. In the newest version, the 0.1-31MHz version, I have read that the receiver displays the actual operating frequency in CW. If you’re concerned with being able to read out the exact frequency (if searching for weak beacons on non-standard frequencies, for example) you’ll still need to ensure that you tune to the correct sidetone frequency in order for the frequency readout to be accurate. Personally, I’d love a sidetone feature for this, but I doubt that too many others would consider it to be an essential feature in a receiver (as opposed to a transceiver).
   
   
2. When stepping through the memory channels, you cannot hear those channels as you cycle through them. In order to hear the selected channel, you have to press the appropriate button to select and load it. Thus, you cannot easily scan through a number of preset memory channels to listen for activity. As with 1 above, I this has been remedied in the newest version of the Belka.

The audio quality from the Belka-DX is excellent when used with earbuds. The internal speaker doesn’t do it justice, though it is very useful when taking the receiver on outings. Power from the audio amplifier is adequate for most applications, though when plugging an external speaker in, it helps to use one that is sensitive. Some people use powered speakers. I have two external speakers on the bench, both unpowered, that I use with it. The main one is an MFJ-281 ClearTone speaker. It is sensitive, and produces good volume. The audio response from the mylar cone is restricted, and what I would characterize as communications quality. Audio is clear and intelligible. The speaker appears to have a natural resonance at around 650-700Hz, which is useful for CW. For those times when I need a little more fidelity, such as when listening to strong SWBC stations, or hams on AM, I use an old and compact hi-fi speaker manufactured by Cambridge Soundworks. It was discarded by one of my neighbors, and appears to be a mid-range unit. It is not as sensitive as the MFJ ClearTone speaker, but the Belka will still provide enough drive in a small, quiet room, which perfectly describes the conditions in my shack.

The small size and slim dimensions of the Belka-DX make it ideal for traveling. When using it at home with an external antenna connected, the ergonomics and ease of use are much improved when mounted on some kind of stand. There are a number of stands available, as well as files for those who wish to 3D-print their own. I remembered a clamp I once bought, that was designed to hold a cellphone for attaching it to a tripod, for making videos. The Belka is not quite as wide as a cellphone, so I used a couple of pieces of dense foam to pad it out, and screwed it to a small tabletop tripod. It works quite well, and improves the ease of use drastically when listening at home. When mounted like this, it feels like a serious and very usable SWL set-up. In the various SWL groups on FB that I frequent, I often see questions from folk asking about receivers that are good for SWL’ing. Portables such as the Tecsun PL-880, along with other similar receivers are often recommended. I think that this Belka makes an excellent receiver for all-round shortwave listening. It is not available in as many outlets as the more traditional shortwave portables, which is why I think that it isn’t as popular in the SWL community as it should be. If you are listening mainly to AM broadcasts on shortwave, then many of the portables will most likely work well. If you do a lot of SSB and CW listening though, the Belka is a solid and, to my mind, preferable option.

In the following picture, my Belka is mounted on a mini tripod (an Ultrapod) and connected to the MFJ ClearTone speaker. Behind and underneath the Belka, you can just see an Altoids tin which contains a high pass filter with a cut-off at about 2700 KHz. It was designed to prevent overload from strong local AM broadcast stations. More on that later in this post.

The Belka-DX is surprisingly sensitive when listening outdoors with the included telescopic whip. It does need a counterpoise, or the received signal strength suffers greatly. If you are holding the receiver, then your body acts as the counterpoise. If you are listening on earbuds or headphones, then the headphone cord acts as a counterpoise. If the receiver is sitting on a surface and using the internal speaker, then you’ll need to connect a counterpoise wire somehow. At home, it works really well when connected to my outdoor antena, which is a doublet at 47 feet, cut roughly for 40M, fed with 300 ohm twinlead, and matched to coax at the entrance to the shack, with a balun and Elecraft T1 tuner. I tune the T1 by squirting RF into it on the nearest amateur band. If you are using your outdoor antenna for listening only, then a simpler arrangement would suffice. This just happens to be the one antenna I also use for my ham exploits.

I live in a densely populated urban area, within a few miles of several medium power (5KW) AM broadcast stations. They often break through when I am using the external antenna with receivers that don’t have narrow filtering on the antenna input. My Belka-DX experiences strong AM breakthrough when used on the external antenna at all frequencies up to 4530 KHz. The moment I tune above 4530 – even by a single 10Hz step, the breakthrough stops instantly, suggesting that a different bandpass filter is switched in at the point. According to the manufacturer-supplied block diagram of the first version of Belka, known as the Belka-DSP, the input bandpass filters are from 3.5-7.5MHz, 7.5-15MHz, and 15-30MHz. The block diagram can be seen on this page by Fernando Duarte. I assume that for the Belka-DX, with it’s extended coverage down to 1.5 MHz, one of the bandpass filter crossover points is at 4530 KHz, the point above which all AM BC band breakthrough stops.

To solve this problem of breakthrough, I resorted to a little high pass filter that I have used successfully with other receiver projects. It’s a high pass filter that was designed by David WA7JHZ, details of which were given in K4SWL’s very wonderful and informative SWLing Post blog. You can see it here. David built his with molded chokes. I built versions with both molded chokes and toroids, and compared the response curves.

Trusty Altoids tins to the rescue. Here’s the version built with molded chokes, purchased from Tayda Electronics. The chokes are mounted vertically, and are a little hard to see in this image –

The small holes in the base of the tin were left over from a previous project that didn’t work out.

Then I built another high pass filter with toroids instead of molded chokes. I figured the toroids should have slightly higher Q and would present a better response curve. All 4 inductors were wound on T37-6 toroid cores with 26 AWG wire. The 2.7µH inductors had 30 turns and the 1µH ones 18 turns. The wires supported them about 4 or 5mm above the ground plane of the Altoid tin –

Dang, after all these years, Altoids tins still make very serviceable and cheap enclosures for small projects!

These filters were designed for input and output impedances of 50Ω. My one external HF antenna is a 40M doublet fed with 300Ω twinlead, and matched to 50Ω coax with a 1:1 balun and Elecraft T1 tuner. As mentioned previously, I briefly transmit a small amount of power on the nearest ham band to where I want to listen, to tune the T1. A manual tuner could be used here instead, and tuned for maximum noise. For listening, this is not too critical a procedure, and a single tune will cover the receiver for listening on a wide range of frequencies. The antenna input of the Belka is matched for the high impedance of the supplied short telescopic whip, and not for a 50Ω antenna. Nevertheless, I went ahead and plugged both versions of this HPF in between the antenna and the receiver, and they both served to completely eradicate every single trace of AM BC band breakthrough.

Using a NanoVNA, I measured the response curve of both filters from 1.5 MHz to 30MHz. Here’s the curve for the filter built with molded chokes –

FREQUENCY (MHz)	INSERTION LOSS (dB)
30	0
20	0.1
15	0.25
10	0.5
3.7	1
3	3
1.7 (1700 KHz)	41
1.5 (1500 KHz)	49

The 3dB cutoff point of this filter was actually 3 MHz, and the insertion loss small, with a virtually flat response from the 80M band up to the top of the 10M band. Here’s a close-up of the response between 1.5 MHz and 4 MHz –

The insertion loss of the toroid filter in the passband was a little lower, For all practical purposes though, there would be no discernible difference between the two filters. If you hate winding toroids, then by all means, build this filter with molded chokes, and it will kill your AM BC band breakthrough just as effectively as if you’d built it with toroids. Here’s the response curve of the toroid version from 1500 KHz to 30 MHz –

And from 1500 KHz to 4 MHz, giving a closer look at the area around the the 3dB cutoff point –

FREQUENCY (MHz)	INSERTION LOSS (dB)
30	0
20	0.04
15	0.12
10	0.25
3.7	0.82
2.8	3
1.7 (1700 KHz)	40
1.5 (1500 KHz)	48

There are quite a few internally generated birdies throughout the whole coverage range. However, the majority of them are only audible with no antenna connected, and are masked by band noise. The others, although audible over the band noise, are not troublesome. For a receiver this compact, and with this overall level of performance, it’s a small price to pay. I rarely noticed them during normal use. It would be nice for the end-user to have a way to update the firmware, though the extended coverage down to 100KHz that the new (V3) Belka enjoys required a hardware upgrade in the form of an extra bandpass filter.

For a more complete description of the improvements made with the newest Belka version, see 13dka’s guest post on Thomas K4SWl’s excellent SWLing Post blog. In short, the Belka is a fantastic general coverage shortwave receiver. It performs and handles like much larger tabletop communications receivers. It is so small that it can be carried anywhere with great ease, making the decision to do a little SWL’ing while on a hike, a walk, or any trip, a no-brainer. You can do a lot of serious shortwave listening with this receiver. Ordering direct from Alex in Belarus offers by far the lowest price and is, in my opinion, the way to go. When I think of my first communications receiver, an old, huge and very heavy British military R107 boat anchor, it is amazing to think that this light and svelte pocketable Belka-DX handily runs circles around it. An SWL can positively rule the shortwaves with this tiny and light miracle of wireless!

There are many other, far more comprehensive reviews on this receiver, but I have been wanting to sing the praises of the Belka (which is Russian for squirrel) for a long time now. I needed to get this out.

As a follow-up to the previous post, in which I discovered that the Sony SRF-59, though cheap to purchase, offered surprisingly good performance due to a rather creative and interesting receiver architecture. I did some reading up on external antennas to help pull in weak stations.  Among the Ultralight DX’ing crowd (those who DX the AMBC band with small, cheap receivers) FSL antennas are a source of great interest – they offer good gain and directivity in a small and portable package.  However, I had almost all the materials on hand to build a simple tuned loop and as, typically, I don’t pursue these things in too much depth, figured this would be the way to go.

First off, let’s get to grips with the rather complex schematic of this thing. The SRF-59 doesn’t have an antenna jack, so the external antenna will need to be coupled to the receiver inductively, which just makes the circuit diagram even simpler (at this point, it couldn’t really be any simpler) –

There are many different ways to construct a loop of this type. Big ones give more gain with deeper nulls, but space is at a premium for me and as this was an initial experiment, I decided to go for something modest in size.  You can use a cardboard box, plastic crate, or any number of things on which to wind the turns, but I opted to construct a frame specifically for the purpose.  Hardwood is nice, but I don’t have any woodworking tools. A trip to Michael’s craft store yielded a display of balsa and basswood in pre-cut and finished sizes. Balsa is very easy to cut, but is also very soft, and wouldn’t be very hard wearing in duty as a portable loop antenna.  Basswood is a little harder, but can still be cut with a sharp craft knife, so I decided to try a frame made form basswood. I bought 2 pieces of basswood pre-cut to 3/16″ x 3″ x 24″ and a length of 1/2″ square rod to strengthen the frame. At this stage, I have cut 2 slots in each of the 2 main pieces –

I slotted the 2 pieces together and glued 2 pieces of the square section to them with epoxy, to act as strengthening pieces. The square section was held in place with small clamps while the glue was setting. Here’s the finished result –

I wanted to have a rough idea how many turns would be needed, so found an online calculator for exactly this purpose.  I had a nice air-spaced variable capacitor that had been donated by a friend (thanks Jason!) With both gangs in parallel, it has a capacitance swing of 16 – 705pF.  This frame has sides equal to about 16.5″ in length and using the calculator, I figured that 10 turns, with 0.25″ spacing, should tune the AM BC band. Before winding the loop, I mounted the variable capacitor –

I split a length of narrow-gauge zip cord in two for the loop. Halfway through winding it, Sprat The QRP Cat bit clean through the wire while my back was turned, so I had to solder a new length on in order to continue winding. She also chewed a small part of the frame while I wasn’t looking. It’s a good thing I love that little kitty!

Here’s the finished loop –

The space between the windings is 1/4″, with a wider 1/2″ gap in the middle. This is in case I later decide to use a rod or piece of square section wood as a supporting mast – it can fit through that larger gap –

Another view of the completed loop –

Of course I was keen to try it out, so I switched the SRF-59 on, placed it close to the loop, tuned to a weak station, then tried tuning the loop and moving the receiver around for optimum coupling. Nothing I tried seemed to work and although I could tune the loop to resonate at the frequency I was listening on, it wasn’t enhancing the received signal at all. In fact, reception was better without it. This was all rather dispiriting and I was about ready to throw the towel in and think about adding a few parts to convert the loop to a novel crystal set receiver when, after taking some shots of it outside on my balcony (the 2 pictures above with the concrete on the floor, and the one below), I decided to set up the radio and try it there. It worked! (All the previous tests had been made in my apartment indoors).

For good inductive coupling between the loop and receiver, you want to orient the loop so that both it’s turns, and the turns on the ferrite rod of the receiver, are in the same plane.  The rod in the SRF-59 runs across the top of the case, so this is how it is oriented (you can also place it inside the loop) –

In the above picture, the loop will receive maximum signal from stations to the left and right of the picture (broadside to the winding) – and it does!  My test was only brief, conducted in the daytime, with signals that were of moderate strength. They were of such a strength that there was some noise and static when receiving them with just the radio. On placing the radio next to the loop and tuning it to resonance, all static and noise disappeared, yielding a more pleasant signal to listen to.  To make operation easier,  when orienting the loop for maximum signal, I rested the receiver on one of the diagonal arms in the frame. If the loop were on a stand, one of the arms would be horizontal.

My loop seems to tune well above the top end of the BC band, but doesn’t cover the bit from 530 to about 600KHz.  A fixed capacitor across the variable should bring the tuning range down a bit.  I’ll fiddle around with it in the next few days. I may also make a recording if the spirit moves me EDIT – I did. See below.

I already had the wire and variable capacitor, so this loop cost me $8.58 in wood from the craft store. The SRF-59 receiver cost me $6.50 inc shipping from eBay, so my complete AM BC band DXing set up set me back a whopping $15.08. I like the kind of fun that can be had for such a small outlay

This afternoon, I went out onto my balcony and made a short recording of KZSF in San Jose.

The recording starts with the SRF-59 receiver without the loop, then I place the receiver inside the loop, which has been pre-tuned to resonance and oriented in the direction for maximum signal. I remove the receiver, and then place it back in the loop for comparison. KZSF is not a DX station from my location in Oakland. It is a 5KW station in San Jose – just 40 miles away. It is entirely possible that I could have found a nearby position from which to get a better signal on the receiver without the loop, but this recording was made to show how a loop such as this can provide a meaningful and useful boost to a marginal signal.

8/03/2024 Update – I recently received an email from James W8FDV. He writes, “Made one as close to yours as possible. Works well”. That looks like the Sony SRF-39FP Federal Prison clear version that you are using it with James. Your FB loop antenna project reminds me that it’s been a while since I’ve used mine. I should take it down from the shelf and give it a spin. Hope yours continues to work out well for you OM!

Next month (August) will mark 10 years since I built my little Sproutie HF Regen Receiver. I recently received an email from Bob W3BBO. A few years ago, he built a Sproutie, and he has just built another one – more on that later. Bob’s first Sproutie is shown in this post, along with a few of his other projects. Take a look – there’s some good ones there. His email got me thinking about my Sproutie. I looked up my post on it, and realized that it has now been in the world for 10 years. This seemed like a good time to haul it down from the shelf and see how it has fared. From time to time, I see comments on Manhattan construction, questioning what the copper-clad boards look like after a few years. What a good time to find out! To be fair, I drag The Sproutie out every few months and give it a whirl, so I knew what to expect. It looks essentially the same as it did when built, with the exception of some Dymo labeling that I recently added, and some light dust. I live in an old house, built in 1908, that is very dusty. Comparing the National N dial to how it looked 10 years ago, there does appear to be some light corrosion breaking through the plating, but nothing serious –

All is looking good on the outside, but what about those boards? Well, it turns out they’re looking pretty good too –

These boards were sprayed with clear lacquer before use, and it has protected them from oxidation quite well. In a few of my projects, I didn’t lacquer the boards at all, and they went dark with oxidation after a while. They are still perfectly functional; just not quite as attractive. Over the course of a few years and a few different projects, I learned that how the lacquer is applied makes a difference as well. Very light coats result in a somewhat stippled appearance and, if you don’t apply enough coats, can result in light oxidation with the passage of time. Applying the lacquer more heavily creates a smoother finish, but care is required here – too heavily, and the lacquer pools. If I remember correctly, it will also wrinkle as it sets if multiple thick layers are applied. Both the distance between the spray can and the board makes a difference, as does the length of time you spend spraying. Experimentation is key.

My WBR receiver was built in a case made out of double-sided copper-clad. After 13 years, the circuit board inside still looks good. The outside of the case hasn’t fared as well though. The lacquer was applied quite lightly. In the areas where the receiver was handled a lot, the lacquer must have worn thin, as the board has oxidized in those areas. From the outside, it doesn’t look as bright and shiny as it did when it was built in 2011. This is one of the reasons why, with subsequent projects that I built a case for out of copper-clad, I used single-sided board, and kept the copper side on the inside.

Anyway, The Sproutie still looks presentable, and it sounds just as good as it did when built. Occasionally, the AF and RF gain, and LPF bandwidth pots are a little scratchy, but rotating them a few times cures that. It is still a fun receiver to travel around the HF bands on, though I wish there were more SW AM BC stations to listen to. This receiver would have been a real hoot in the 70’s and 80’s. If I could have built and used this when I was 16, I would have felt as if I’d died and gone to heaven!

The coil box was made from an old cigar case, and some basswood for the compartment divisions. I made this coil box 10 years ago and these, like the Sproutie pictures in this post, are current photos –

Building things is fun, and it’s even better when the project you’ve built remains very usable year after year. If I was building any iteration of The Sproutie again, I’d leave out the fine tune control, as I found it unnecessary. Not sure how I’d make the front panel look balanced without that knob on the right side though. That would have to be given some thought.

As I mentioned at the beginning of this post, Bob W3BBO just finished building his second Sproutie. He didn’t have an AD820AN on hand for the variable bandwidth audio LPF, so he used half of an NE5532 op-amp as a preamp with 6dB of gain and a cut-off at 20KHz, which is essentially just an audio amp with a little gain. See the post on my Sproutie MK II for details on those active filters. With the coil he had, shortly after switching it on, he was able to hear the CHU time signals at 7850 KHz –

These are the two boards that Bob mounted underneath the chassis, and which comprise most of the circuitry for his Sproutie regen –

Congratulations Bob, on bringing another HF regen receiver into the world, and thank you for sharing details of your FB project!

Next month (August) will mark 10 years since I built my little Sproutie HF Regen Receiver. I recently received an email from Bob W3BBO. A few years ago, he built a Sproutie, and he has just built another one – more on that later. Bob’s first Sproutie is shown in this post, along with a few of his other projects. Take a look – there’s some good ones there. His email got me thinking about my Sproutie. I looked up my post on it, and realized that it has now been in the world for 10 years. This seemed like a good time to haul it down from the shelf and see how it has fared. From time to time, I see comments on Manhattan construction, questioning what the copper-clad boards look like after a few years. What a good time to find out! To be fair, I drag The Sproutie out every few months and give it a whirl, so I knew what to expect. It looks essentially the same as it did when built, with the exception of some Dymo labeling that I recently added, and some light dust. I live in an old house, built in 1908, that is very dusty. Comparing the National N dial to how it looked 10 years ago, there does appear to be some light corrosion breaking through the plating, but nothing serious –

All is looking good on the outside, but what about those boards? Well, it turns out they’re looking pretty good too –

These boards were sprayed with clear lacquer before use, and it has protected them from oxidation quite well. In a few of my projects, I didn’t lacquer the boards at all, and they went dark with oxidation after a while. They are still perfectly functional; just not quite as attractive. Over the course of a few years and a few different projects, I learned that how the lacquer is applied makes a difference as well. Very light coats result in a somewhat stippled appearance and, if you don’t apply enough coats, can result in light oxidation with the passage of time. Applying the lacquer more heavily creates a smoother finish, but care is required here – too heavily, and the lacquer pools. If I remember correctly, it will also wrinkle as it sets if multiple thick layers are applied. Both the distance between the spray can and the board makes a difference, as does the length of time you spend spraying. Experimentation is key.

My WBR receiver was built in a case made out of double-sided copper-clad. After 13 years, the circuit board inside still looks good. The outside of the case hasn’t fared as well though. The lacquer was applied quite lightly. In the areas where the receiver was handled a lot, the lacquer must have worn thin, as the board has oxidized in those areas. From the outside, it doesn’t look as bright and shiny as it did when it was built in 2011. This is one of the reasons why, with subsequent projects that I built a case for out of copper-clad, I used single-sided board, and kept the copper side on the inside.

Anyway, The Sproutie still looks presentable, and it sounds just as good as it did when built. Occasionally, the AF and RF gain, and LPF bandwidth pots are a little scratchy, but rotating them a few times cures that. It is still a fun receiver to travel around the HF bands on, though I wish there were more SW AM BC stations to listen to. This receiver would have been a real hoot in the 70’s and 80’s. If I could have built and used this when I was 16, I would have felt as if I’d died and gone to heaven!

The coil box was made from an old cigar case, and some basswood for the compartment divisions. I made this coil box 10 years ago and these, like the Sproutie pictures in this post, are current photos –

Building things is fun, and it’s even better when the project you’ve built remains very usable year after year. If I was building any iteration of The Sproutie again, I’d leave out the fine tune control, as I found it unnecessary. Not sure how I’d make the front panel look balanced without that knob on the right side though. That would have to be given some thought.

As I mentioned at the beginning of this post, Bob W3BBO just finished building his second Sproutie. He didn’t have an AD820AN on hand for the variable bandwidth audio LPF, so he used half of an NE5532 op-amp as a preamp with 6dB of gain and a cut-off at 20KHz, which is essentially just an audio amp with a little gain. See the post on my Sproutie MK II for details on those active filters. With the coil he had, shortly after switching it on, he was able to hear the CHU time signals at 7850 KHz –

These are the two boards that Bob mounted underneath the chassis, and which comprise most of the circuitry for his Sproutie regen –

Congratulations Bob, on bringing another HF regen receiver into the world, and thank you for sharing details of your FB project!

I don’t need to explain the attraction of low power operation; if you’re reading this, the chances are that you are already a convert. I’ve been operating with low power ever since first being licensed in the UK in the late 70’s as G8RYQ, and then G4IFA. One of my first rigs was a homebrew VFO-controlled FM rig for 2M. I don’t remember how much power it put out, but it was at most only a few watts. Then there was an 80M DSB rig, built from a kit, that put out a watt or two. I had a series of 2M FM rigs, including an Icom IC-22A and a Trio TR2200 (1 watt out). I had a hand-rotated 5 element 2 meter beam. To change the beam heading, I leaned out of my bedroom window and twisted the aluminum pole that was supporting it. One of the PA transistors in my Icom IC-22A was blown, so the rig only put out 4 or 5W. I remember using that rig and the beam to talk regularly on simplex with a fellow young ham who was in Wales, 90-100 miles away. I think I used the TR2200 to talk with him as well, which was even more impressive, as the Trio only put out 1 watt of RF power.

My checkered amateur radio life did include one 100W rig. It was a TS520 that I owned for a couple of years in the early 1990’s. Other than that though, every rig I have ever built or owned has been 5 watts or less. After a while doing QRP, 5 watts becomes the norm. 5W is known as “the full QRP gallon”, and it does feel like it! I still run 5W as my default most of the time, on both CW and SSB. Recently though, I’ve been turning the power down, to see how lower power levels get out. A fun moment recently, was when I clearly heard the backwave from my little two-transistor transmitter on the KPH SDR, which is 41 miles away as the crow flies. That backwave was about 1mW, and being able to hear it on a remote receiver was something of a revelation. It was that moment that kickstarted my interest in even lower power levels than the mighty force of the full QRP 5 watts.

Once a day, I check into The Noontime Net on 7284KHz. It is virtually the only time I use SSB. They welcome check-ins from QRP stations. Once a year, they have a QRP day, when operators are encouraged (though not required) to check-in using QRP power levels. There is an honorable mention on their website for the station who checks in using the least power and this year, I took the prize for checking in with 10mW of SSB. The check-in was with Don KY7X in Wellington, NV. The distance between us is 168.5 miles as the crow flies. At an equivalent distance of 16,850 miles per watt, I think that would easily qualify for the QRPARCI 1,000 miles per watt award. Given that most members who apply for that award have achieved it with CW, I think that a QSO of 168.5 miles with 10mW of SSB is even more inspiring.

The lowest power I can turn my Elecraft K2 down to on SSB is 1 watt. I achieved the power of 10mW out by connecting an inline attenuator with a fixed attenuation level of 20dB in the antenna lead. It is one of those “barrel” attenuators, with a BNC connector on each end. With the success of a check-in with 10mW under my belt, I resolved to try for even lower power in next year’s QRP Day. This led me to a nifty little kit offered by QRP Guys. It is an inline attenuator, with switchable levels of attenuation of 10, 20, and 30dB. Also included is a bypass switch that allows the operator to easily switch the attenuator out of circuit when on receive. For $25 + shipping, it was the obvious solution to my QRPp needs. I was very close to pulling the trigger, when my “QRPp extreme sports” gene kicked in, and I thought it could be useful to be able to attenuate a signal by an extra 10dB, for a total of 40dB attenuation. This would reduce the 1 watt of SSB from my K2 down to the truly flea power level of 100µW, and the 100mW CW output to the mind-bogglingly low level of just 10µW! Granted, this extra 10dB of attenuation may never be needed, but if and when I succeed in making a QSO with 30dB of attenuation in the antenna line, I may always wonder if it could also have been made with an extra 10dB. It’s the desire to constantly push our achievements just that little bit further.

With that in mind, I decided to build my own attenuator box. There is no circuit design involved really, as it is simply a series of 50 ohm pi-attenuator pads and a few DPDT switches. I took the circuit from the QRP Guys’ attenuator and added an extra pi-section and switch –

All resistors are metal film 3W types. The 100 ohms ones are 1%, while the other values were 5%. If you can get 1% tolerance for all values, then all the better. If you want to calculate resistor values for any other degree of attenuation, you can use this online calculator.

There’s not much to the build. The diecast enclosure, switches, and BNC connectors all came from Tayda. I am not thrilled with the quality of the connectors and switches from Tayda. The terminals were hard to solder to, presumably due to the use of a cheaper alloy than the quality brands such as Kobiconn and Switchcraft use. Nevetheless, I persevered, and managed to obtain a reasonably satisfactory result.

The bypass/attenuate switch is useful when going from transmit back to receive, for ensuring that your reception is not also attenuated.

Daytime band conditions weren’t too good on first finishing this attenuator, though I did manage to just be heard by Don KY7X, 168.5 miles away, with an output power of 10mW SSB, using 20dB of attenuation from an original 1W signal. I’m not sure if it would have been enough for a positive ID of my signal if he didn’t already know who I was. Nevertheless, band conditions were poor that day, so this was a good sign. I decided to hook it up to my VK3HN WSPR beacon (thanks Paul), which puts out 200mW. I applied 30dB of attenuation, for a 200µW WSPR signal – that’s just 0.2mW! Incidentally, to figure out the various attenuation levels, and what output power they give you, there are several online calculators. I found this one to be useful.

Unfortunately, the lowest power level that can be encoded into a WSPR signal is 0dBM, equivalent to 1mW. I’m not keen on misrepresenting the power level, but as I was so eager to see what a mighty 200µW of WSPR could snag me, and as I considered a power level of 20mW (10dB of attenuation of the 200mW signal) to be too high, I decided to WSPR for the night on 40M, and encode the signal at 0dBm. Check out the following results from a night of WSPR’ing. There were 486 spots in total. This is not a lot by normal standards but a good result, I think, for such a low power signal. In this screen grab, they are sorted in order of distance, so these are the most remote spots. AI6VN/KH6 in Maui tops the list, for a distance of 3778km = 2347 miles. That’s 11.735 million miles per watt! In a normal night of WSPRing on 40M with the relatively high power of 200mW, I would expect multiple spots from Hawaii, VK and ZL land, as well as a spot or two from DP0GVN in Antarctica. However, 0.2 mW is a whole new ballgame, and I was very happy to get just one spot from HI –

Here’s the attenuator sitting on my desk, on top of the VK3HN WSPR beacon. It would be nice to have a tidy desk and a nice, clean operating position but every time I tidy it, it slowly gets like this again (the 3rd law of thermodynamics in action!) At the bottom of the stack is the Sproutie SPT Part 15 Beacon, which is currently not QRV. DK, if you’re reading this, you may notice something familiar at the very bottom of this picture –

I want to be able to WSPR on a more regular basis, and have the encoded power information on my transmissions actually be fairly accurate, so the next step was to build an attenuator with a fixed attenuation level of 23dB, to reduce the output of the WSPR transmitter to 1mW. This way, when a spot from me shows up with a power level of 0 dBm, it actually is 0 dBm. This online pi-attenuator calculator was pressed into service, and yielded the following values. If you don’t have a 12 ohm resistor, then a single 330 ohm part will be close enough –

As this attenuator will only be used to attenuate the 200mW output of the WSPR beacon, I used 1/4 W resistors. The two 56 ohm resistors were left over from a cheap resistor kit that I bought years ago. They had short, thin leads, and were ostensibly 1/4W parts. The left-hand one (the one closest to the transmitter) dissipates the most amount of power, and was getting very warm during 2 minute transmissions from the 200mW transmitter. I do think it was sustainable, but would have preferred it to run cooler. I didn’t have any 1/2W or bigger resistors in appropriate values, so decided to parallel 2 x 1/4W parts. There are plenty of fresh sheets of white paper here, but drawing schematics on envelopes is more fun. A 100 ohm and 150 ohm resistor in parallel makes 60 ohms. Using 352 ohms as the “top” resistor (achieved with a 330 and a 22 ohm resistor in series) makes for an attenuation level of 22.7dB, which is pretty dang close –

The two resistors on the left-hand side run much cooler than the single 56 ohm did. The 56 ohm one was a cheapie resistor, and I suspect it’s stated power dissipation of 1/4W was being a bit optimistic. All the resistors in the final attenuator are 1/4W metal film types. The project box came in a pack of 5 from Amazon, for $7.50. I have used the same ones recently to build QRP baluns and ununs. There are all sorts of fun and novelty projects they would be useful for. The lid snaps on. I can supply the link to anyone who is interested –

The increase in power from 200µW was noticeable after the first night of WSPR’ing on 40M with 1mW. Here is a screen grab of the most distant spots received. In just under 10 hours, I had 1114 spots. It’s a lot fewer spots than what I would receive with 200mW, but that many spots with just 1mW sounds very encouraging. I love this little WSPR beacon (thanks to Paul VK3HN once again). Out of 1114 spots, all of the drift figures were a big honking zero, with the exception of a single 1 and a single -1. Instead of the single spot from AI6VN’s remote listening station in Maui, I now had 7. Sure, propagation on different nights could be some of it, but I’m pretty sure the 7dB power increase from 200µW to a gigantic 1 milliwatt was a significant factor.

This QRPp experiment has been a huge success so far, and I haven’t even begun to work on the goal that I had in mind when beginning this. That was to use the switched step attenuator to see how far I can go with very low power on CW, my mode of choice. WSPR is very instructive and interesting, but an actual QSO, even a brief one, carries the extra appeal of contact with a distant person, with all the unpredictable intangibles that come along with that. I do wish there was a way of encoding lower powers than 0 dBm (1mW) into a WSPR transmission, as I would then be WSPR’ing with successively lower powers. I’ve already received a significant number of spots with 200µW of transmitted power. It would be great to see what could be done with, say, just 10µW (0.01mW), if anything. In the meantime though, there will be a lot of 1mW WSPR’ing emanating from the AA7EE radio ranch, as well as some extreme QRPp CW too, with the help of the switchable step attenuator.

As well as the small stash of finished projects that grace my living space, I also have two small boxes containing various boards. Some of them are boards from part-finished projects that didn’t work. For whatever reason, I ran out of steam and, instead of troubleshooting them, put them carefully into a small box along with their cellmates, and conveniently put them out of my mind. A few of the boards actually worked, but I decided not to case them up. One of these is the two-transistor transmitter I built a few years ago, that was basically the TX side of the Pixie 2 design. I like to pull boards like this out of the box from time to time and power them up. Then they go back in the box, waiting for the next time I feel partial to some extra-curricular fun.

Extricated unceremoniously from it’s cardboard box on the shelf, this little transmitter (pictured above) was putting out about 200mW when connected to a 12V supply. I got a real kick from hearing the signals from this simple circuit on the KPH online SDR at the Point Reyes coastal station, 41 miles away as the crow flies. This design has the oscillator running continuously. In the Pixie, from which it was taken, the oscillator is needed on receive as well. As the circuit consists of just an oscillator and a PA stage, there is a small amount of oscillator leakage into the PA even when the PA stage is not being keyed. Imagine my surprise on discovering that I could hear this backwave on the KPH online SDR! Putting the transmitter on my OHR QRP Wattmeter revealed that although accurate measurement was not possible at such low power levels, it looked as if something of the order of a milliwatt of RF energy was making it’s way to the antenna when the transmitter was not being keyed. Now, the fact that 1mW could be heard 41 miles away, though impressive, is by no means unprecedented. Nevertheless, it fired up my imagination, and made me want to build another little transmitter.

The train of logic that I followed in order to finally settle on building GM3OXX’s little OXO Transmitter was, as it turns out, not very logical and quite convoluted. I will not attempt to explain it here, as it will only serve to confuse! However, I did want a design in which the oscillator wasn’t running on key-up, so that I could monitor the signal in the receiver to serve as sidetone.

The OXO transmitter was first featured in the Autumn 1981 issue of SPRAT, the journal of the G-QRP club. The original circuit didn’t include an LPF; the builder was expected to provide their own. Here is the original circuit, with the addition of an LPF –

The BCY39 PNP transistor keys the +12V supply to the 2N3866 PA. It is not necessary to key the oscillator, as the oscillator won’t run unless the PA transistor is switched on. In the LPF, the 1.38µH inductors can be made with 21 turns on a T37-6 toroid, and the 1.7µH with 24 turns on a T37-6 toroid. Values were taken from the assembly instructions for the QRP Labs low pass filter kits. Numbers of turns for different toroids can be figured out from the very useful calculators on the Kits and Parts website.

I mocked it up on a breadboard and it worked well. I noticed that the VXO, although not oscillating on key-up, was emitting some very low level spurii. In retrospect, this could well have been due to stray capacitances in the breadboard, and the fact that the circuit wasn’t built over a ground plane. Nevertheless, I decided to have the keying transistor key both the oscillator and PA. This is what I came up with, drawn on the back of an envelope –

George GM3OXX added a 0.1µF cap across the key contacts to help with shaping. It wasn’t on the original schematic as published in SPRAT, but I added it here. The RFC in the collector of the PA transistor can be a molded choke. I wound 17 turns on an FT37-43 toroid to serve the same purpose. I also added a spotting switch, with the 1N5817 diode to prevent the PA from being switched on when only the oscillator signal is wanted, for “netting” the transmitter frequency on a receiver. Unfortunately, the spotting switch didn’t work out quite as planned. More on that later.

I had an old LMB Heeger 143 enclosure lying around from my build of N6KR’s SST. I had drilled the front panel holes in the wrong places, so ditched it and used a fresh enclosure. I had considered this enclosure to be unusable for another project, until realizing that I could simply place a piece of PCB material over the front panel to cover up the unused holes, and keep it attached with the nuts and screws that were holding the controls in place. It worked well and looks pretty good. The slide switch is for transmit/receive switching, the red button is for spotting, and the big knob is the VXO tuning –

The board was scrubbed with a steel wool soapy pot scrubber and given a couple of thin coats of clear spray-on lacquer. A fresh board holds so much potential, and never looks as good as before construction begins. It’s almost a shame to glue any pads onto it!

The slide switch was part of an order that I received from Dan’s Small Parts & Kits back in 2011. The polyvaricon was from the VRX-1 direct conversion receiver, designed by NT7S and kitted by 4SQRP. It was one of my first ever Manhattan construction attempts and, although the receiver worked well, I wasn’t happy with my layout and the way my build looked.

All the back panel connectors (key jack, DC power connectors, and BNC’s) were from Tayda. I’m not blown away by the quality of the connectors from Tayda but considering the very reasonable prices, I am making an exception.

I built the VXO first, and tested the frequency coverage. Most any small signal NPN transistor will work in this position. I used a 2N3904. A 2N3866 was used in the original circuit for the PA. I didn’t have one of those, but I did have some 2N3866 equivalents in the form of the Motorola 4-247 CG9949. A while back, the G-QRP club were giving away small quantities of these transistors for free to their members. I took advantage of the offer, knowing that I could use a 2N3866 equivalent or three. This is the exact same transistor that Kanga UK are using for their kit version of the OXO transmitter.

At this point, after having built the VXO and PA circuits, the OXO will function as a transmitter, by keying the +12V line. If you only ever intend to use a manual key with this transmitter, it is not necessary to build the keying switch, and you’ve got yourself a nice and simple two-transistor transmitter. However, I like using a paddle, and the keying circuit is very simple. Here’s the board with the basic transmitter built. You can see the crystal, in a holder made from an SIP strip, along with the VXO transistor, right next to the polyvaricon. The PA transistor is the one with the big honking heatsink on it, just behind the toroid. The keying transistor, a 2N3906, is to the right of the toroid –

For transmit/receive switching, a primitive solution was found, in the form of a DPDT slide switch that came from Dan’s Small Parts and Kits years ago, and has been sitting in one of my parts drawers, just waiting for an opportunity to be used. It was wired up like this (another diagram drawn on the back of an envelope!) –

On receive, the antenna is connected to the antenna input of the receiver. The output of the transmitter is connected to a 50 ohm dummy load. In case the transmitter is accidentally keyed while in receive mode, it’s output will be protected by the 50 ohm load. On transmit, the output of the transmitter is routed to the antenna, while the receiver antenna input is connected to the 50 ohm load. The receiver is being used for sidetone, so the idea behind this is to do whatever can be done to prevent receiver overloading while in transmit mode.

The OXO transmitter, all wired up and ready to go. I used an LPF from QRP-Labs. Band changing can be accomplished by plugging in a different crystal and plugging in a different LPF –

Regarding the PA emitter resistor that is marked as 39Ω. You can fine tune the value of that resistor, depending on the output power you want. Be careful not to go too low, or the transistor could overheat and be destroyed. In the Kanga UK kit version that uses the exact same PA transistor, the emitter resistor is 2 x 33Ω resistors in parallel, for a total effective resistance of 16.5Ω. In the build instructions, Paul reports that with a 13.8V power supply, he gets 1.5W out on 80M and 1W on 40M. I was cautious, and began with 2 x 100Ω resistors (=50Ω). I added resistors in parallel, until I got to 4 x 100Ω (=25Ω). My OHR QRP Wattmeter indicated an RF power out of 400mW. My NM0S QRPometer indicated a power out of 580mW. I wasn’t sure which one was more accurate, so split the difference and called it 500mW. I could get more power by adding another couple of 100Ω resistors, but I rather like the idea of having a 500mW transmitter. By comparison, 1W seems so pedestrian! With an emitter resistance of 25Ω, the voltage across it was 3.2V. According to ohm’s law, the current through it was 128mA, for a dissipated power of 0.41W. The resistors are 1/4W parts, so their total power dissipation capability is 1W. Sounds well within the margin of their capability.

In the following picture, you can see the two 100 ohm 1 watt resistors that form the 50 ohm dummy load in the background –

The heatsink on the PA transistor is probably overkill for a power output of just 500mW, but it gives a good safety margin. I held the key down for 2 minutes (into a dummy load, of course), and it only became mildly warm to the touch. I think it would safely survive even if my cat fell asleep on the key

The two 12V DC connectors on the back panel are wired in parallel, to allow one 12V lead to power both the transmitter and a companion receiver. It is not shown in the schematic, and cannot be easily seen in any of these pictures, but a 1N5817 diode is wired between the +ve side of the 12V DC connectors and the board, for reverse polarity protection.

I paired it up with my Rugster direct conversion receiver and QSO’ed with K6KWV in Diamond Bar, CA on 40M – a distance of 363 miles as the crow flies. Not mega-DX, but it was a very enjoyable contact. He said that I was the first contact he’d had with a station using a homebrew rig, and I was also running the lowest power of any station he’d QSO’ed with. That was nice to hear! He has only been a ham for 4 months, and has a good fist. His code is pleasant to listen to, and easy copy.

When using the Rugster, as well as throwing the TX-RX switch when going from receive to transmit, I also have to turn the RF gain control down to zero to prevent the receiver from overloading. If on headphones, I also have to turn the AF gain down somewhat. Although it is an easy process to get used to, I find my Belka-DX even easier. Due to the AGC in the Belka, I don’t have to change a thing when going from receive to transmit, and vice-versa, other than flipping the TX-RX slide switch on the OXO transmitter.

The CW note sounds good on 40, and chirp-free. There is a little chirp with my 14060 crystal, and a lot with the 21060 and 28060 crystals. I credit this to the PA loading down the oscillator, and figure that a buffer stage between the oscillator and buffer would cure it. Thanks to John KC9ON, at 3rd Planet Solar, I have a pack of 40M crystals that are in HC49/S cases – the short cases. My 14060, 21060, and 28060 crystals are all in the tall HC49/U cases. Perhaps the higher band crystals are not fundamentals?

On a related note, comparing my HC49/U 7030 crystal with the HC49/S one, the tall crystal pulls over a wider frequency range. I didn’t think to measure the capacitance of the polyvaricon before installing it, but I think it is 270pF per gang. Putting both gangs in parallel made little difference to the pulling range. With the tall HC49/U 7030 crystal, the range was 7028.77 – 7032.83KHz, representing a swing of 3.61KHz. By contrast, the short HC49/S crystal pulled from 7029.57 to 7031.1KHz – a swing of just 1.53KHz. Pulling range became increasingly greater with the higher frequency crystals. The 28060KHz crystal pulled over a range of 13.42Khz, but chirped so much it was comic.

Oh, about that spotting button. I thought I’d be able to use it to net the transmitter precisely on a received station’s frequency. Unfortunately, the frequency of the VXO is significantly lower in spot mode than on full key-down. On 40M with a short crystal, the difference is 250Hz. I assume this issue wouldn’t exist with a buffer stage between the VXO and the PA.

For the time being, I am going to give this project a rest and concentrate on other things. We all need a break sometimes. However, if and when I revisit this little transmitter, I’d like to rewire it so that just the PA is keyed, as was intended with the original circuit. Given that the oscillator doesn’t run unless the PA transistor is switched on, I see no reason not to do this. It is an easy change to make. I’d also like to acquire and try different crystals for the higher bands, in the hope that will eliminate the chirpiness.

Another future possibility would be to add an SMA connector on the back panel for an Si5351 VFO. At that point though, the project is becoming more complex, perhaps negating the point of such a simple transmitter to begin with.

Here’s a little project I put together on a whim about 18 months ago. It was my tribute to the unlicensed (i.e. pirate) beacon cluster around 4096 KHz. There were several of them operating in full force, with powers ranging from around 100mW to a watt or so a few years ago. Their heyday was about 20 years ago. They were located somewhere in the southwestern deserts of the US, and were powered by batteries and solar panels. The feller who placed and maintained them has stopped their upkeep and, as a result, most, if not all of them, are no longer operational. (There was a small group who also placed these beacons, and I believe they too stopped doing so years ago).

This little beacon, using a commonly available crystal, put out around 30 or 40mW, if I remember correctly, into a 51 ohm resistor as a dummy load. If it were to be connected to an antenna, which will not happen at this frequency, it would need a low pass filter. I haven’t posted the schematic here, for two reasons –

1. I don’t want to encourage unlicensed operation and
2. This particular circuit is a bit of a kludgy design, and not one I’d use anyway.

I’m posting the photos as a reminder that if you don’t have Manhatttan pads, or don’t want to make your own, then so-called “ugly construction” is a very viable way of putting circuits together. If you do it well, your circuits can be quite robust and long-lasting. One of Rex’s MePADS was used for the IC, but everything else was built ugly-style.

In the next overhead picture, you can see the ATTiny85 that keys the transmitter. Above and to the right of it, is a 78L05 that supplies 5V to the chip. Directly below the crystal is the oscillator transistor, which is keyed by the transistor to the left of it. On the right is the PA transistor. All 3 transistors are 2N3904’s.

Before ending this post, there is a key part of ugly construction that is worth mentioning. High value resistors (1MΩ and above) can be used as standoffs. To circuit components, they look like insulators. In this circuit, there are two 1MΩ resistors used for this purpose. I later bought 200 x 10MΩ resistors from Mouser, and will be using them as standoffs in the future. They won’t increase your current draw by any significant amount either. A 1MΩ resistor to ground from a point at 12V potential will only cause an increase in current consumption of 0.012mA. A 10MΩ resistor will have even less of an effect, incurring an increase of just 0.0012mA. That’s 120µA. I can live with that!

Anyway, that’s it. I hope this inspires you to get the soldering iron out and build a little circuit. A simple receiver or transmitter, a novelty circuit, or anything really. Building these little things and getting them to work is fun. Incidentally, if you’re interested in listening out for these unlicensed HF beacons, the best source of information is the forums at HF Underground. The forum you’ll want is the one called “HF Beacons” though there are many other great sub-forums there as well.

Back in 2009/2010 the Arizona QRP Scorpions released a little kit designed by Dan N7VE, called the Fort Tuthill 80. It was a QRP CW transceiver, with a direct conversion receiver and a transmitter capable of putting out about 3W. It caught my imagination, and I just had to build it. I have never had a great antenna for 80, and don’t operate on that band a lot, but one of my successes with the Tut 80, was a QSO with the T32C DXpedition team in Kiritimati. I was running 3W from the Tut80 into a 40M coax-fed dipole! This made the QSO feel like a great victory.

The first run of Tut 80 kits was over fairly swiftly. This run was, I think, either 100 or, at the most, 200 kits. There was talk of another run, but it never materialized. Doug Hendricks began selling kits of the design for 160M and 15M through his QRP Kits website. Ownership of QRP Kits has now changed, and both Fort Tuthill kits have been retired. In other words, like a lot of QRP kit rigs of that era, if you have one, you are sitting on a limited edition classic!

I have never used my Tut 80 a lot, though every now and again it comes down off the shelf for a jaunt around the lower end of 80. There used to be a Yahoo Group for Tut 80 builders. Sadly, that wonderful repository of information on so many subjects was all lost several years ago when the plug was pulled on Yahoo Groups. One of those valuable bits of info was a post by noted QRPer Cam Hartford N6GA (now sadly, SK). He had modified his Tut 80 for varactor tuning, which struck me as a very desirable mod. The stock polyvaricon, though serviceable, didn’t make for the best user experience, in my opinion. The polyvaricon is just a little “spongy” in feel, making precise spotting a bit tricky at times. Also, I had added the suggested toggle switch to separate the tuning into two bands. It switched in an extra NPO capacitor for the lower band. Although it worked, I didn’t love the idea of using a toggle switch in a frequency determining circuit. Cam’s varactor mod had been on my mental list of things to do for years.

Several times over the years, I have come close to performing this mod, but never quite found the necessary mojo. Then, very recently, inspiration struck, the rig came down from the shelf, the Bournes 10-turn 10K pot that had been stashed in my parts drawer for a rainy day was retrieved from the parts drawer, and I got to work.

A side-benefit of making the decision to do this was that, in the process, I could tidy up the wiring a bit. At the time I assembled the Tut 80, my soldering was already good, but I had not yet become quite as focused on making the wiring to the boards in my projects quite as tidy. This photo of the Tut 80 before the modifications makes my point. Not a huge mess, but it could use some improvement. This picture was taken in 2010, when I had just finished building my Tut 80 –

The next few pictures were taken recently, just before the mods were performed. This wiring needs to be tidied up! –

The “Hi Lo” toggle switch in the above picture will be removed, as there will be no need for it. No idea what I will do with the hole.

The rig on my desk, with the polyvaricon and 10-turn pot getting ready to swap places. The 10-turn pot is made by Bourns. They are not cheap, but they are quality pieces. The part # is 3590S-2-103L –

Here is the schematic of Cam’s mod, as drawn by him, and posted to the Tut 80 Yahoo Group –

And here is what he wrote about it –

Sorry about the pencil/paper rendering. All of the parts came from my inventory. The MVAM108 was left over from another project, probably a 2NXX. The ten-turn pot came from a swap meet. It’s a Bourns 100K unit, but the value is not critical. A 50K or 10K would probably do as well. For starters, I did this more than ten minutes ago so short-term memory vacancy has taken it’s toll. I reverse-engineered the schematic but can’t really remember the why of some of the decisions. Lots of trial and error, no real science involved. It works, however. The radio covers the bottom 100 KHz of 80 meters almost exactly. As far as linearity is concerned, the first turn at the bottom of the band yields 6 KHz, this increases to about 13 KHz in the middle of the band, then drops back down to 6 KHz at the top end. So it tunes faster mid-band but is still vastly better than 100 KHz in one turn. I added a digital dial so the linearity really isn’t an issue. And it retains the stability of the original design, which is to say, like a rock. I’m thinking that the on-board regulated 5 volts didn’t give me enough band spread so I went to a little 3-terminal 78L09 9 volt regulator. I mounted this Manhattan-style on a little scrap piece of pc board and mounted it in an available corner of the box. The rest of the pieces soldered directly to the terminals of the 10-turn pot. The lead from the varactor soldered to the hole in the board that was vacated by the polyvaricon. In looking at the board now I see a couple other caps that I added to fine tune the frequency range. I tack-soldered a 33 pF NPO cap across C73, as I’ve noted on the hand-drawn schematic. This was probably the cap that was supplied with the kit for location C83. The notation on the original schematic says “C83 optional, use if needed.” In that location I used a 47pF NPO from my collection. The changes I made were necessary to accommodate the parts I had on hand, so the usual disclaimers apply. Caveat emptor, YMMV etc. It’s a fun bit to play with, IMHO.

72, Cam
N6GA

I had an MVAM108, though it was an unmarked part, and I didn’t completely trust the source. I could have tested it, but I had some MVAM109’s from Kits and Parts, so decided to use one of those instead. Here is my version of Cam’s mod. My values are a little different, and I have added one part –

The 78L09 regulator was mounted at the back of the board, next to the existing 5V regulator IC. The input and ground legs of the part were poked through the holes in the board for 12V power and ground respectively. The 9v regulated output leg was bent up away from the board. The next two pictures may help to make it a bit clearer. The existing 5V regulator, the 78L05, is the one furthest away from the back of the enclosure (i.e. closest to the CA3086 IC). Also, in this picture, you will see that I moved the 12V DC input power connector higher up the rear of the enclosure. This was to avoid fouling the board, which had been moved. A small piece of PCB material blocks the old hole, and is held in place by the new connector –

In the next picture, you can see how the cable (lavalier mic cable, incidentally) that carries the regulated 9V supply to the rest of the tuning circuit is soldered to the bent-up output leg of the 78L09. You can also see the little 0.1µF bypass cap that connects from the output of the 78L05 to ground –

The MVAM109 varactor was mounted on the board, into the two holes marked “Tune” that had previously been used to connect to the main tuning polyvaricon. It is circled in red in this picture –

With the MVAM109, I found that a 68pF NPO cap across C73 gave me coverage from 3487 to 3615 KHz. In retrospect, I am not entirely sure why I used a 68pF cap at that point. The shorter and much less convoluted version of the story is that, by the time I had soldered that cap to the bottom of the board, I was past the point of wanting to fine tune the coverage. I figured that having that extra cap in parallel with C73 could only help the long-term stability, by spreading any heating effect over two capacitors instead of one, so I left it there. The reason I didn’t adjust trimcap C82 (which is on the main Tut 80 board and not shown in any schematic here) to bring the coverage higher, is that it was already at minimum capacitance. Perhaps one day I will further fiddle with the capacitance values to bring the coverage more in line with my ideal of ~3495-3585 KHz but tuning with the 10-turn pot is already a massive improvement over the half-turn with the polyvaricon, so I’m not bothered. The 10-turn pot has a much lower friction and smoother feel, in addition to the fact that it covers the tuning range in 10 whole turns.

The 47µF capacitor between the slider of the 10K tuning pot and ground was added to remove the slight “whizzing” sound that occurs in the headphones when a wirewound pot is used for varactor tuning. If your pot has a carbon track, you don’t need this part. In fact, the whizzing sound is so faint that 80M band noise usually covers it, but I like to add that capacitor anyway.

Another mod that piqued my curiosity was one posted by Dan N7VE. He had modeled some changes to the PA to make it more efficient and put out more power. Although at the time, he hadn’t tried it, he felt sure it would work. I found mention of it online by someone who did the mod and found that, as a result, his Tut 80 put out 4.5W. Here is the description of N7VE’s PA mod, in his own words, as posted to the now-defunct Yahoo Group –

Tut 80 PA Mod for Class D

The more efficient output network that I have simulated involves:

1) Changing L4 from 24 turns to 19 turns
2) Shorting out C58 so that C57 and C59 together provide 2000 pF of C at
that point rather than the current 1500 pf.
3) Replacing L6 (a FT37-43 core) with a T37-2 core with 22 turns. There
is no tap on this new coil. The coil end points go to pad 1 and pad 2.
Pad 3, where the tap used to go, is shorted to pad 2.

This simulates well to give a non-critical, class E type output, even
though it is being driven with a less than optimum sine wave input. The
output would be somewhere in the 4.5w range.

I want to test this when I get back, but someone could try this if they
wanted to see how it does. I think this would take unwanted heat and
change it into extra output power, allowing the finals to run cooler
than they currently do.

– Dan, N7VE

I made the above mods to the PA, and my Tut 80 now puts out 4W instead of 3W. Score!

The final task to accomplish was to tidy up the wiring a bit. This was accomplished by connecting all the wiring to the underside of the board, and running it underneath the board. Shielded cable was used to connect the AF gain pot to the board. Shielded cable wasn’t really necessary, but the stuff that is intended for lavalier mics is thin and flexible, and works well for this purpose. I use Mogami W2697. The twisted wires for the key and headphone sockets were scavenged from a multi-conductor cable. The feed from the board to the KD1JV Digital Dial is RG174/U. You can’t see it in the pictures, as it is hidden quite well. I think you’ll agree that it looks tidier inside the enclosure now –

My little Tut 80 now had nicer tuning and more power out, but what about the stability of the VFO? I ran a series of tests, from a few hours long, to one that was 36 hours in total. All the tests gave very similar results. At 3560KHz, from a cold start, it drifted steadily downwards about 260Hz in the first 30 mins, and a total of about 300Hz in the first hour. Thereafter, it didn’t drift more than ±30Hz for the next 10 hours. Most of that time, it didn’t drift more than 10Hz in an hour. In the one very long test, that lasted for 36 hours, after 10 hours of holding relatively steady, it began a new steady downward drift, reaching a maximum excursion of an extra 270Hz from the frequency it was at 1 hour after the cold start. Nearly all of this extra drift was steady though, and for most of this time, drift was no more than 10 or 20 Hz in an hour. This level of drift is good enough for the type of operating I do with my Tut 80.

Well, I rather wish I’d had a brand new exciting build to share with you, but that’s not how things have been going here at the AA7EE radio ranch recently. I figured that there might be a few Tut 80 owners who would be interested in these mods and, as the Tut 80 Yahoo Group no longer exists, decided to share them here. It it helps one person, my mission has been accomplished. FYI, the manuals, including schematics, for the 160 and 15M versions of this rig are hosted in the “Retired Kits” section of this site. For anyone who needs a copy of the assembly manual for the original Tut 80, drop me a line. My email address is good on QRZ.

Cam N6GA and I exchanged friendly emails some years ago. He had family in Oakland, and talked of visiting when he was next up here. I do wish he were still around, so I could say – hey Cam. I tried your Tut 80 varactor mod, and it works like a champ!

Some years ago, I purchased and assembled an Oak Hills Research WM-2 QRP Wattmeter from Milestone Technologies. As far as QRP wattmeter kits go, it was something of a classic at the time, and as such, I wanted one. I’m glad I made this purchase, as they are no longer available – at least, in this form. Another company is offering a very similar kit, but without the decals on the case. I was told that they acquired the rights to it from Milestone Technologies, so this would be appear to be a direct clone of the WM-2. The WM-2 is a great little wattmeter, with 3 ranges representing 100mW, 1W, and 10W full-scale – an ideal selection of ranges for the QRPer. You can read both forward and reflected power. While direct readout of SWR is not offered, it can be calculated from the forward and reverse power readings. The WM-2 has an analog meter and can be left inline while operating. Apart from the satisfaction of being able to see the needle bob up and down when transmitting, this type of indicator is very useful when peaking circuits for maximum output. It can certainly be done with a digital readout, but an extra stage of “translation” needs to happen in the brain, converting the number on the readout to a “level”. This process when looking at an analog meter is more immediately intuitive.

That was so many photos of my WM-2, that you might be thinking, “Hang on – isn’t this a post about the QRPoMeter? Well, it is – and we’ll get to that very soon. I don’t think I ever blogged about the WM-2 when I built mine years ago, so felt it was time to give it some air time on my blog.

For my purposes, the WM-2 meets my needs. However, I don’t have any other instruments with which to check the accuracy of it’s readings. A Bird wattmeter would be nice, but the expenditure is hard to justify. Another option is to use an oscilloscope to measure the peak to peak voltage a transmitter develops across a 50 ohm dummy load, and use that to calculate power. This is a definite possibility in the future, as I do intend to add a digital storage oscilloscope to the shack at some point. In the meantime, it would be good just to have another wattmeter of similar accuracy, simply to increase my confidence in the readings I am getting from either one. For the kind of operating us QRPers do, absolute accuracy is not essential. 5% of full scale is good enough which, on a 10W scale, means ±0.5W. If I claim to be transmitting with 5W, then the difference between 4.5 and 5.5 W is unlikely to even be noticed at the receiving end.

The QRPoMeter, designed by Dave Cripe NM0S, has been on my radar for a very long time. Originally offered by the 4SQRP group, it is a very affordable instrument for measuring power and SWR. It has a built-in dummy load, to make measuring the power into a 50 ohm load an easy task. Also, when measuring SWR, it uses a resistive bridge, so that the maximum SWR your transmitter will see is 2:1. I’ve long wanted to assemble this kit. A few times, I’ve waited too long to purchase a kit, only to find that it was no longer available (the SST, once offered by Wilderness Radio, was one example). With that in mind, and also because the QRPoMeter is so reasonably priced, I went ahead and placed an order with NM0S Electronics.

A few days later it arrived, in a small flat rate Priority Mail box. I love getting radio parts and kits in the mail. It’s exciting! The PCB pieces that, as well as forming the circuit board, also comprise the case, were slipped in between pages of the assembly manual to protect them. There was also a little bag of goodies. I love little bags of radio parts!

The bag of parts, emptied out into a styrofoam tray –

Also included was a piece of thin 2″ x 3.5″ PCB material, etched and silkscreened on both sides. On one side was the business card of NM0S Electronics. On the other side were these handy little band plans –

The pieces that form the sides of the case have to be broken off from the larger pieces of PCB material, and given a very light filing to remove the rough edges. It was immediately obvious how smart the final product was going to look. It was raining very, very lightly when I took this photo. If you look carefully, you might see some very small raindrops on the panels –

For anyone who has assembled a few kits, construction is uncomplicated. Dave’s instructions are clear and straightforward, consisting mainly of a checklist for populating the board, and instructions for constructing the included PCB case. The switches and input/output BNC connectors are all mounted directly on the board. The only wiring required is a 4-conductor ribbon cable that is used for the connections between the board and the LCD panel meter. Other than wiring up this meter, and soldering the case pieces together, construction of the QRPoMeter consists of populating the single PCB with the parts. This picture is of the finished board before the two switches were installed –

I only discovered two very slight issues during assembly. Neither will present a problem for anyone with a little experience, but they might slightly confuse a beginner. These were due to a change in sourcing parts, and Dave said he will take care of them in future versions of the construction manual. These were –

1. U4, the TLC2272, had no dot or u-shaped indentation to denote the correct orientation. I used the printing of the device number on the top of the IC as a guide instead, and this turned out to be right. See picture below, with U4 circled in red –

2. When using the 4-conductor ribbon cable to connect the LCD panel meter to the board, because the instructions refer to an earlier version of the meter, a beginner might experience some uncertainty as to which holes on the meter board to connect to which holes on the main board. On the main board, there are two pads next to each other marked +Vin and -Vin. These are connected to the pads on the meter board that are marked “IN” and “COM” respectively. The other two connections are more obvious. The pads on the main board, next to the schematic symbol for a 9V battery, marked + and -, are connected to the pads on the back of the meter board, next to the “DC 9V” text, that are marked + and respectively.

A portion of the board in greater detail, showing the 8 large surface mount resistors that form the dummy load/resistive bridge (or, to be more accurate, 7 of them, and a small part of the remaining one) –

Calibration is straightforward, and requires a fairly accurate DMM. I used my Brymen BM235 (the EEV Blog version). The only other piece of equipment needed is any HF QRP transmitter with an output of between 2 and 10 watts. The output power doesn’t need to be known, as long as it falls within that range. When the unit is calibrated, you have a very handsome and useful piece of QRP kit!

The QRPoMeter seems to be accurate enough for my purposes. Power measurements are in line with the ones reported by my WM-2, taking into account the accuracies of both instruments. SWR measurements are similar at the lower readings. They differ by fairly large amounts at higher SWR’s. This doesn’t concern me though, as once the SWR goes much above 2 or 3:1, it’s exact value is of little interest to me. I just know that I want to get it back down below 2:1! A useful feature of the resistive bridge in the QRPoMeter that is used to measure the values of forward and reflected power, is that when SWR is being measured, the transmitter never sees an SWR higher than 2:1. This was verified with the SWR indicator in my Elecraft K2.

Thanks Dave. A good-looking and worthy little piece of QRP test gear! The QRPoMeter is available from NM0S Electronics.

Before building Jim W4LF’s Hobbydyne crystal set, I put together an impedance matchbox, for matching the detector diode to a variety of different headsets and earphones, so that I could determine the best ones to use. The world of serious crystal set listening was new to me, so I did some reading up. To give you an idea of how serious this gets, many committed crystal set listeners have heard over 100 different stations on their sets, on the AM broadcast band (thanks to nighttime skywave propagation)!

It appeared that there are a few different kinds of headset that crop up often as being the favored types among crystal set enthusiasts. Of these, perhaps the most storied is the Baldwin Type C radio headset, or “Baldies” as their owners affectionately call them –

There are a couple of reasons why crystal set aficionados often have a set of Baldies in their collection. One of these is for their historical significance. Baldwins are considered to be the first mass-produced headset that significantly resembles modern headphones. Developed by Nathaniel Baldwin in Utah in the early part of the 20th century, they were first patented in 1910. He got his big break after sending 4 pairs to the US Navy. They were very impressed with their sensitivity and performance, and put in an initial order for a hundred pairs. Baldwins were a high quality headset. In the early 1920’s a set cost $14-$16 – about $250 in today’s money.

The other main reason for their continued popularity with crystal set folk, is that even by modern standards, they are quite sensitive. The design is different from most other headsets of the era, being that they employ a balanced armature and a mica diaphragm. A small proportion of Baldwin headsets had a phenolic diaphragm, and were slightly less sensitive as a result. It’s easy to tell if your Baldwins have mica diaphragms. If you unscrew the ear caps and see clear diaphragms like this, yours are mica –

Here’s a view of the bottom of the element. Looking at it from this angle requires that the diaphragm side is downwards. Do not set it in this orientation without placing it on top of the ear cap, as you could damage the diaphragm or the drive rod (see how, in the above picture, the very end of the drive rod protrudes very slightly above the level of the diaphragm?) In this following view, if you look carefully, you can how the plate (circled) that supports the drive rod, is centered between the two poles of the magnet. This is how it should be – slap bang in the center, and not touching one or other of the poles –

Looking straight down at the back of the element, you can see the 2 screws that are used to connect the ends of the headphone cords. I wish that modern headsets were as simple to disassemble and reassemble as these vintage models are –

Scott (from oldheadphones.com) is a big collector of vintage headsets. He told me that of the Baldwin headsets he has seen that have weak output, it has rarely been because of weak magnets, especially with the units that have purple painted magnets, as seen above. I believe that the purple painted magnet variety are far more common than the other type, which have black painted magnets. If your Baldwins have weak output, things to look for, Scott advised me, are armature plates that are not centered between the magnet poles, or are actually touching one of the poles (keep an eye out for debris between the plate and the poles). Another thing to look for, is warped or broken diaphragms.

Nathaniel Baldwin’s story is an interesting and dramatic one. Read it here – it’s well worth it. My Baldwins, like most of the vintage parts and gear I acquire, came from eBay and, contrary to what seems to be popular opinion, overpaying is not necessarily the norm. As the buyer, the power is in your hands. I paid US$15 + $12 shipping for these, which I think was a very fair price. I did clean them up, but only a little. This is the condition they arrived in, which wasn’t bad at all –

They were in pretty good shape, the main areas in which some shabbiness could be seen being in the headband, which showed very little fraying, though there was some dirt and stains present, and just a small amount of corrosion on the metal parts –

Scott from oldheadphones.com has a useful page on the restoration of old headphones. On this page, he is specifically referencing Western Electric 509-W’s, but the same advice can be applied to other types. Following this advice, I gave the metal parts of the Baldies a very light polish with Brasso (only very cursory, as I really wasn’t that bothered with making them gleam), and cleaned the bakelite earcups up a little with Bon Ami, which is a very minimally abrasive household cleaner. The construction of these headsets makes it very easy to take them apart for cleaning. The headphone cord unscrews from the elements – no soldering needed, and the elements themselves can be taken out from the bakelite earcups with no unscrewing at all – they simply drop out. It is a pleasingly very modular type of construction.

He also gave me some useful advice on cleaning the cords and headbands via email. Following this advice, I sprayed some prewash onto the headband, left it to soak for a short while, then immersed it for a while in a basin of cold soapy water, made with clothes detergent, while giving it a light scrubbing with an old toothbrush. Be careful when doing this, as the old fabric can fray quite easily. Then I rinsed it with cold water and left it out to dry overnight. I didn’t do anything with the cord, as it was in good shape. Incidentally, if you can buy something from him, it helps to fund his hobby of collecting vintage headsets.

Here’s the result. You can see the most improvement in the condition of the headband. Not gleaming, but with the aura of respectability that comes with the evidence of having lived an honest working life –

A very usable pair of Baldwins for everyday use, I think!

While on the subject of Nathaniel Baldwin Type C headsets, I want to share with you a very exciting find that I made while looking for the everyday use pair above. This was also on eBay and, unlike with the above pair, I paid quite a lot for these. They are a set of completely unused Baldwins, in the original box, with the instruction leaflet. The box was a little banged and frayed, but the headset was mint, and looked as if it had never even been assembled, let alone used! I am not an experienced collector of these but, on seeing them, something told me that it was not very common to see a pair in this condition. I ummed and aahed over them, told myself I wasn’t going to pay that much for them, and moved on. Shortly after, I came back, and opened up a correspondence with the seller. He had bought them at an estate sale 30 years ago, he told me, and they had been in storage ever since. He wasn’t sure if they worked, and didn’t know how to test them, but he did own a multimeter. I told him how to do a continuity check on the coils, and they tested out fine. This pushed me over the edge and, after a bit of back and forth, we agreed on a price. They were mine!

I hadn’t seen a pair of Baldies with a green headband before. I think they look very handsome!

This model was first patented in 1910, and the entire Baldwin operation closed down between 1930 and 1932, so I think it’s fair to say that they are about 100 years old. It’s hard to put into words the appeal of seeing a product that has been sitting in it’s original box for 100 years, in the same unused state the original buyer would have seen it. I can’t help but wonder how this came about. Perhaps this headset sat on the shelves of a radio store somewhere that went out of business and then, along with other remaining stock, was sold, and sat in storage for years? Perhaps it was purchased by a well-heeled customer who bought it and forgot about it?

Just look at that shiny bakelite ear cap, that completely clear mica diaphragm, and that shiny metal, without nary a smidgin of corrosion in sight. Magnificent!

I mean, if you could get in a time machine, walk down Radio Row in NYC in the 1920’s, dive into a store and buy a pair of Baldwins, this is exactly what they’d look like –

Oh lawdy. The shiny bakelite. That is original factory shine. The beautiful green headband. It’s almost too much for me to bear!

That leaflet, that came in the box with the headset? Here it is –

That’s enough of Baldies for now. The other headset of this vintage that I was keen to land a working pair of, was the Western Electric 509-W. Patented in 1918, this headset was also very popular with telegraph and radio operators in the late teens and 1920’s, mainly due to it’s ruggedness and sensitivity. Many believe 509-W’s to be equal to Baldwins due to their sensitivity, robust nature, and quality of manufacture. In the opinion of Scott, from oldheadphones.com, they are ideal for crystal set use, and their performance rival that of Baldwins, only being surpassed by Navy sound-powered headphones.

Continuing my quest to land a bargain, I scored a set of 509-W’s for just $10.50 + $14.85 shipping. They arrived in good, though obviously well used, condition –

A bit shabby, to be sure. Allow me to show you a few more pictures, before I reveal how they cleaned up –

The metal cans were dull, but with no major scratching or other damage. The slots on the two screws that hold the sound element to the nickel-plated brass cans are often knurled or otherwise damaged. Not so in this case, which I took as a good sign –

These are not balanced armature units, like the Baldwins. They are the more traditional style, which makes the poles of the magnets very easy to access for the purpose of re-magnetizing, if that becomes necessary. Like the Baldwins, the headset cord is easily attached and attached with 2 screws. The general rule of thumb for determining if the unit needs re-magnetizing, is if the magnet holds the metal diaphragm on, it is strong enough. These headsets are the more traditional type, with both poles exposed, making remagnetizing easier, if necessary. For these traditional types of headsets, of remagnetizing, Scott says, “I just use a strong rare earth magnet and a couple taps on one pole of the weak magnet takes care of it.”

I polished the metal parts with Brasso, washed the bakelite ear caps with Bon Ami, and soaked the headband in cold, soapy water, then lightly scrubbed them and left them to dry overnight. The headphone cord smelled of tobacco and was a bit greasy, so I also soaked it in cold, soapy water (yes, really) and scrubbed it lightly with an old toothbrush. I then patted it down on a towel, gave it an initial drying with a hair dryer, and hung it up to dry thoroughly, on a hot day.

During this initial cleaning, I did lightly polish the metal cans, but didn’t pay too much attention to the other metal parts. For minimal effort, the 509-W’s came out quite well, I think –

One feature of this set was a little curious. It came with this washer hanging from a piece of string on one side of the headset. To me, it looked as if it belonged there and was an integral part of the unit. I wondered what it was for. I wasn’t able to locate any photos of other instances of this headset with such a washer, and no mentions of a washer anywhere else. After asking a few other 509-W owners, it became apparent that whatever the washer’s purpose was, it was not a stock item, and was not a known 509-W accessory. Eventually, I removed it, but not before taking these pictures. I think the first picture (the very next one) was taken partway through the cleaning process, before all the metal had been cleaned up –

The headband came out a lot cleaner. The cord looked a little better, though the main improvement was that it felt a lot less greasy –

Shiny!

Western Electric 509-W headsets are not hard to find in fairly good, working condition. There are still a lot of them around. They were well made, and cost $12 in the early 1920’s, the equivalent of a week’s wages for the average worker. Other brands of headphones, that were less well-made and not as sensitive, cost less. Brandes were $8, and other brands even less. I wonder how many sets of headphones being used today will still be around 100 years from now and working well?

Cloth headset cords of this era came with strips of material on the ends that attached to tabs on the cans for strain relief –

The headband is looking pretty spiffy and clean from this side!

Next on the agenda was to put together a sound-powered headset. Sound-powered units are used by the military; primarily, I believe, by the Navy. The sound elements are so sensitive, you can connect two together and they will operate without the need for outside power i.e. the tiny current produced by talking into the mouthpiece will produce a sound in the earpiece. This very informative page on Darryl Boyd’s website has information on many of the available sound-powered headsets you’re likely to find. Jim Frederick W4LF’s favorite is the WW2 sound-powered headset known as the RCA “Big Cans”, which I believe are also referred to as the US Navy “Deck Talkers” (see linked page in previous sentence). He says that they are hard to find and expensive, but so far ahead of the others that they are worth it.

Some of the sound-powered units, such as the US Navy Deck Talkers, have a headset, so little physical modification will be needed in order to use them. With others, the sound elements are in a telephone-style handset. In this case, you’ll probably want to remove the elements from the handset and install them in a headset of your choosing. This page on Darryl’s site lists the specs of sound elements in the various units, and also notes the handsets that have identical elements used for both microphone and earpiece. This can be useful, as you will only need one handset to make a complete headset. I set about looking for one of these handsets, and managed to come up with this Canadian RCA MI-2215-E model, which has 2 identical sound elements –

Touching the two leads coming from the handset across the terminals of a AA battery resulted in absolutely no clicking from the headset whatsoever, which was not a very encouraging sign.

Back view of one of the elements, after the leads had been desoldered, with a strip of tape over the holes to prevent ingress of debris –

Front view of the same element –

Removing the metal back cover from the first element, the armature plate was clearly visible. If you look closely (easier if you’re viewing this on a computer, as opposed to a phone), the armature plate, which was connected to the threaded drive rod, appeared to be jammed up against one of the poles of the magnet –

Looking at the threaded drive rod, you may be able to see that there are two small nuts, one above and one below the plate. Adjusting those nuts determines the default positioning of the plate. I adjusted them with a pair of needle-nosed pliers and successfully repositioned the plate halfway between the top and bottom magnet, so it was no longer in contact with the top magnet. At this point, I didn’t have a crystal set to test it out with (I hadn’t yet built the beta kit in my last post). Instead, I connected it to the output of my impedance matchbox, with the output impedance set to somewhere in the region of 300 ohms (the impedance of this element), connected a longwire antenna and ground to the two input terminals of the matchbox, and connected a 1N34A diode across the input terminals of the matchbox. Success, as the sound of a cacophony of MW AM stations sprang forth from the sound element. It was working!

Buoyed by this success, I set about tightening the nuts, and disaster struck. I overtightened one nut, and snapped the very delicate threaded drive rod. I had lost sight of the fact that it is a thin rod. Unfortunately, the break occurred right next to the nuts. If it had been further away from the nuts, I might have been able to solder or epoxy the rod pieces back together. As the break was up against one of the nuts, repairing it would have reduced my ability to adjust the positioning of the nuts on the rod.

Back to the drawing board and, before long, I acquired this H-203/U handset, manufactured by the Dynalec Corporation. This handset has a push-to-talk button –

When this handset is connected to another handset, pushing the PTT button connects the handset to the other handset so that, when the PTT is not pressed, the other station cannot hear what you are saying. If you connect the two handset leads together and talk into the microphone, you should be able to hear your own voice in the earpiece. If you’re unsure whether you’re hearing your own voice via the earpiece or via bone conduction, alternately pressing and releasing the PTT while talking will clarify.

Unlike the previous handset, the identical microphone and earpiece are not soldered in, so it is easy to remove them. They just pull out, revealing the neatly-wired supporting circuitry –

Even the capacitor leads across the earpiece have been neatly preformed. I very much approve!

These are the identical sound elements from this handset –

The terminals are easily soldered to, though you have to be careful to apply the iron for the absolute minimum of time, so as not to melt the plastic. I used a little flux from a flux pen, to help things along –

Next on the agenda was to find a suitable headset or old pair of headphones to install these elements in. I found this new unused pair of earmuffs on Facebook Marketplace for $5 –

This set of earmuffs turned out to be well suited for these sound elements. The aperture in one end of each side of the earmuffs is very slightly wider, and there is a good thickness of foam lining the back of the earmuff –

It was possible to push each sound element in at an angle, such that it ended up being held firmly in place by being pressed up against the ledge/flange around the opening, by the foam in the back –

The headset lead enters one side, at the bottom of the can. Heat-shrink tubing over the lead and tie-wraps on the inside and outside of the can hold the lead in place –

A lead runs in between the cans to connect the elements in series (in phase). It runs between the tops of the cans, underneath the cushioned piece that surrounds the headband –

These ProCase brand earmuffs are available on Amazon at the time of writing for $16.99, although the FB Marketplace price of $5 from a local private seller was a no-brainer, of course. The only possible drawback to them is that the fit is very tight, though as the purpose of earmuffs is to keep ambient noise out, this is an intentional part of the design. One could argue that a sound-powered headset will only be worn for the very weak DX stations, and the absence of outside ambient noise is helpful when trying to copy them. I used an old headphone lead from a pair of AKG 240’s with the molded plug at each end chopped off, but you can use anything that works for you.

Also included in the assortment of headsets/headphones/earbuds/earpieces that I tested with my newly built crystal set and impedance matchbox, was a classic piezo earpiece, often known as a crystal earphone. The styling hasn’t changed over the years, and many of the cheaper crystal radios marketed to youngsters in the past, such as the rocket ship crystal radio, came with one of these. The metal diaphragm is connected to one of the leads, and used to be soldered to it. In recent years, many of the units sold had a foil diaphragm with the lead held to it partially by glue and partially by the pressure of the plastic case against the lead. As a result, there was a high rate of failure. Both the piezo earpieces of this type that I bought failed soon after I acquired them. However, the old-style with a soldered connection are still available. Mike’s Electronics sells them. There is also a seller on eBay called protechtrader who sells them (they recently increased the price significantly, from $10.70 to $14.99, both prices including shipping). The earpieces from the eBay seller have a characteristic black lead and plug. You can see the brass color of the diaphragm too, which I assume is also the case with the one from Mike’s –

These piezo earpieces have a 3.5mm mono plug. The 3.5mm jack on my impedance matchbox is wired to a stereo plug, so a 3.5mm stereo male to mono female adapter was pressed into service.

Also tested were an MDR-W14 yellow headset from an old Sports Sony Walkman cassette player/ AM/FM radio combination, and a pair of C Crane earbuds, both low impedance –

I forgot. I also tested my beloved AKG 240’s, of which I own 2 pairs. I used these for years, when working as a DJ/announcer, and voiceover guy in Los Angeles. I also use them with my Elecraft K2 on CW, as they are very comfortable to wear for long periods. I wasn’t expecting a lot from them for crystal set use though, as they are known for not being as sensitive as the more consumer type low impedance models. They are intended mainly to be driven by the headphone amplifiers present on mixing boards and similar professional equipment, which is capable of providing a greater drive level than the amplifiers in small consumer products such as Walkmans/radios/MP3 players etc –

OK, the big test. How do all these different headsets/headphones/earpieces/earbuds stack up against each other? Firstly, allow me to say that I haven’t yet performed extensive testing with many very weak stations, so take these preliminary results with a pinch of salt. Also, at this point, I still have some significant improvements to make to my antenna, by lengthening the outside portion of it from 45 feet to about 75 feet. This will very possibly yield a far more noticeable improvement than which headset I choose to use will.

That said, here are my initial impressions.

As expected, the AKG 240’s are the least sensitive, though not by as large a margin as I expected. They was, at a rough guess, about a 6 dB difference between them and most of the other headsets which, surprisingly, all seemed to be roughly the same in sensitivity. However, the AKG’s have a wonderfully flat audio response, and AM radio sounds great on them. They’re really good for listening to stations that are moderately strong, or greater.

The piezo earpiece was sensitive, though the sound was very restricted and tinny and, because it’s just one earpiece as opposed to two, doesn’t sound as loud as the other units as it’s only in one ear. Wiring two in series or parallel might help with volume, though the frequency response will still be very tinny. Plus, it kept falling out of my ear, which was annoying.

The Baldwin Type C, Western Electric 509-W, homemade sound-powered headset, Sony Walkman MDR-W14 headset, and C Crane earbuds all seemed to be quite sensitive, about as sensitive as each other, and about the same volume on weak to moderate strength signals. On strong signals, the Baldwins and sound-powered headset weren’t quite as loud as the others due, presumably, to the physical limitations imposed by the balanced armatures.

My initial impression was that, if anything, the Sony Walkman MDR-W14 headset was very slightly more sensitive than any of the others tested here. However, I’m not sure if that’s really the case, or has more to do with the fact that sound is transferred more efficiently to the ear because of the way that the headset earpieces sit in the ear canal. Being earbuds, the C Cranes also sit in the ear canal, but the Sony headset has a slight edge over them. However, I believe this is mainly because the Sony set has more response over the whole audible frequency range, while the C Crane earbuds purposely have their frequency response shaped to favor voice, with a sharp drop-off above about 7-8KHz.

The Baldwins and Western Electric 509-W’s both have somewhat restricted frequency response; the WE’s have slightly more bottom end.

Both the C Crane earbuds and Sony MDR-W14 headset sound a lot louder on strong signals, yet are also sensitive on weak signals. Of the two, the Sony has the widest frequency response. For most types of listening, my favorite headset, of all of them, is the Sony. My sense is that it would also be good for listening for weak DX stations. It’s possible that a headset with less bottom end might increase intelligibility and copy on the very weakest of stations. It’s for this reason that I’m thinking it’s worth keeping several headsets to hand, just as many serious listeners keep a selection of diode detectors on hand, for the most challenging of DX catches. Interestingly, my initial take on these headsets aligns with what Al Klase has said, namely that in his experience, modern earbuds, even the cheap ones, are about as sensitive as sound-powered headsets.

I find all of this a bit frustrating, because I went to quite a lot of expense, time, and trouble, only to discover that my favorite headset to listen to my new crystal set on, was a cheap Sony Walkman model that I already owned! I am not too surprised by this finding, as I had already read Al’s remarks, but needed to find out for myself. I could have saved myself quite a lot of money. On the other hand, I do enjoy owning a few pairs of vintage and antique headphones. The Baldwins and WE 509-W’s both occupy significant places in radio headset history, and my mint Baldies are museum grade. There’s a definite pride of ownership at play here.

Bear in mind that with vintage headsets, there can be variation in their performance, especially if they’ve been treated poorly throughout their long life. Sound-powered headsets often received rough treatment while in service. If you have a set that are in poor physical shape, they may have received a lot of knocks during their life that degraded their performance. In other words, my very brief test drive of these different sound-producing devices was preliminary at best. Nevertheless, things are looking good for the combination of matching transformer and a modern headset/earbuds using neodymium disc magnets and lightweight components.

It turns out, after all those vintage headsets, and a homebuilt one, that my favorite way to listen to this crackin’ little crystal set is on a pair of Sony Walkman MDR-W14 headphones. They have great fidelity, are the loudest on strong stations, and appear to be sensitive as well. I do wonder if a pair of US Navy Decktalkers (the famed RCA “Big Cans”) would beat them on very weak signals, but Al Klase appears to have a pair of those, and still said that modern earbuds are about as sensitive as anything else he has used. Who’d have thought! Crystal set enthusiasts – what are your experiences?

Growing up as the youngest of 4 boys, I was well positioned to receive all the hand-me-downs. Although that might sound as if I just ended up with second-rate stuff, that was not the case at all. I inherited a lot of great things from my older brothers. I couldn’t have cared less that they’d had them before me. The stash consisted of all sorts of board games, Dinky toys, and books, as well as something that would fuel my imagination for many years to come – a crystal set. My very own crystal set! Manufactured by Ivalek, this little beauty sat in it’s white plastic case by my bedside, delivering quality programming from the BBC 24/7 –

In truth, in the big world of crystal sets, this little mass-manufactured set wasn’t a very good performer at all. In fact, I’d go as far as to say it was pretty awful, having the following schematic, which I don’t think should ever be used for anything other than a teaching tool, but not a practical build –

The above is almost the simplest crystal set you can build. The Ivalek also has a switch that switches in extra inductance for the long wave band, but we’ll ignore that. The main problems with the above schematic are –

1. The antenna is not impedance-matched to the coil, so it will load it down, reducing Q and therefore, selectivity.
2. The diode is not impedance-matched to the tank, loading down the tank, and also affecting Q, and selectivity.
3. The headphones may not be matched to the diode affecting – yes, you guessed it, circuit Q and, therefore, selectivity.

A lot of simple “toy” crystal sets that were marketed to kids in the 50’s, 60’s, and 70’s employed this simple schematic and, as a result, we all got the idea that crystal sets were fun, but not very good, and not to be considered as a “serious” receiver for extended periods of listening. In my case, making things worse, was that I thought the telephone earpiece I was using was high impedance. 8 year-old me had no idea the impedance was actually closer to 150 ohms. The end result of all of this was that my crystal set had very broad tuning indeed. On the other hand, it was very loud, because we lived only a few miles from the BBC Droitwich transmitters. The longwave transmission, on 200KHz at the time, was 500KW in power, and covered most of the UK. The medium wave signals, though not quite as powerful, were certainly not QRP either.

The lackluster performance of my Ivalek crystal set didn’t put me off. I just thought it was the neatest thing that I could leave it on all the time, and would never have to change the batteries. Plus, I had plenty of chances to listen surreptitiously under the covers at night, when I was supposed to be sleeping!

I also had this magnificently compelling book –

The Boys Book of Crystal Sets contained construction details for 12 different sets of varying complexity and, presumably, performance. The young me spent hours and hours gazing at all the articles and schematics, and thinking how grand it would be if I had an air-spaced variable capacitor or two, some litz wire to wind a coil with, and an empty tobacco tin or chassis, to build my crystal receiver in. I’d be the king of the hill! But an 8 year-old boy living in the countryside in the early 1970’s didn’t have the means to procure such elite and specific luxuries, so I settled for reading each article a couple of hundred times, and day-dreaming.

About 10 years ago, I came across a web-site belonging to Jim Frederick W4LF. Jim is a big fan of Cushman scooters, which are uniquely American motor scooters that boast an earnest following of enthusiasts. The Cushman Scooter company was formed in Lincoln, Nebraska in 1903, and produced their last scooters in 1965. The majority of Jim’s site is given over to discussion of these machines. As you might guess, it wasn’t these that interested me, but a single page on Jim’s site that is dedicated to another of his interests – little radios and crystal sets. On this page Jim shows pictures, with brief descriptions, of the neat little radio receivers he has built over the years. They include regenerative receivers, crystal receivers, and some amplified crystal receivers. I loved not only the fact that he was using double-tuned circuits with, in many cases, adjustable capacitive coupling between the tank and the detector for greater control over the selectivity of the circuit. I also really appreciated the attention paid to the casing and overall appearance of the final product. These were some really appealing little receivers!

Recently, I had the honor of assembling a pre-production beta build of the 3rd generation of Jim W4LF’s Hobbydyne Crystal Set Receiver kit. I won’t go into details of the build here, but the kit should be available soon, at this site. If it is not yet active, save it in your bookmarks, and check back later. When the site is up and running, it will be the place to get more info on this kit.

I’m very happy with the end result, and I hope you’ll agree that it is a very good-looking little crystal set, with it’s African mahogany base, and garolite front panel –

From top left to top right, the knobs are – a rotary switch to add extra capacitance to the antenna tuning capacitor, for help in tuning different lengths of antenna, the variocoupler, which controls the coupling between the coils, and also the selectivity and, on the far right, the Hobbydyne selectivity enhancement control. The circuit of this set is heavily based on, and very similar to Jim’s original Hobbydyne circuit, which was featured in Dave Ingram’s column in the Nov 2005 issue of CQ Magazine.

The brass binding posts on the left are for antenna and ground connections, and those on the right are for the headphones.

The headphones are a set of Western Electric 509-W’s. They’re about 100 years old and work well, if you’re looking for a set that will plug directly into a crystal receiver without the need for impedance matching.

As soon as I had constructed Jim’s Hobbydyne kit, I started looking for two things in quick succession –

1. A good set of high impedance headphones and
2. An impedance matchbox, to experiment with different types of headsets/headphones.

While planning the construction of an impedance matchbox, to match a variety of crystal set detectors with a range of headphone impedances, I also kept an eye out on eBay for headphones, and the necessary parts to build a sound-powered headset. That is the subject of a whole new post, which will come after this one. The construction of just one crystal set, which you’d think would be a simple affair, while not quite turning into a rabbit hole, was certainly becoming quite involved!

I have a habit of over-estimating how involved I’m going to become in pursuits when still in the beginning stages. For instance, when digital photography was really starting to take hold, in the early-mid 2000’s, I decided to get back into photography, which had first grabbed my interests as a teenager. This time though, I was an adult with more disposable income. I went a bit hog-wild, buying a nice camera and a whole slew of lenses, accessories, and even some studio lighting equipment (not the cheap kind either). Although I had a lot of fun with all that gear, I realized over the next few years that I had overbought, and spent a fair bit of time selling all the photo accessories that were surplus to my needs, eventually distilling them down to only the essentials. All this is a prequel to me saying that if you want a crystal set which performs well but, being realistic, you’re not going to want to eke out the very last drop of high performance from it, you might be happy with connecting a set of high impedance headphones directly to the output of your new crystal set, and leaving it at that. I wanted to be able to test out a variety of different types of headset, headphones, and earbuds, so an option that offered a variety of output impedances was definitely on the cards for me. A selection of different input impedances would also allow experimentation with other crystal sets in the future.

Crystal set builders have used various methods to match the diode detectors on their receivers to headphones over the years, with a variety of transformers being used. Darryl Boyd’s very informative site at crystalradio.net has a section devoted to detector to headphone impedance matching, with a number of approaches detailed. One of the most recent solutions has been the very useful Bogen T725. More recently, an auto-transformer has come onto the market that is wound specifically for the needs of crystal set builders. The KPB-02 auto-transformer has both inputs and outputs (on the same terminals, being an auto-transformer) of 200KΩ, 100KΩ, 40KΩ, 20KΩ, 10KΩ, 5KΩ. 2.5KΩ, 1.5KΩ, 800Ω, 500Ω, 300Ω, 150Ω, 64Ω, 32Ω, 16Ω, 8Ω, and 4Ω. It was custom-made for our needs and, as such, has input and output impedances that satisfy any possible need a crystal set builder could conceivably want. Look for the KPB-02 on eBay, being sold by seller mkmak222. This auto-transformer formed the basis for my all-purpose crystal set impedance matchbox.

On the input of the box is a Benny, consisting of a 0.1µF capacitor and a 500K audio taper potentiometer. The Benny is named after Ben Tongue, who wrote a series of detailed technical articles on the subject of crystal sets, which, taken together, probably represent the most detailed analysis of this type of receiver architecture ever published. Ben is no longer with us, but you can find his articles here. You might want to save them all just in case one day, they are no longer hosted anywhere online. He talks about the Benny in article 01. Very briefly, it helps to reduce audio distortion on strong signals, by equalizing the DC and AC audio loads on the diode detector.

Both rotary switches are 12 position types but on the first one, on the input side, I only used 7 positions. Some switches have an adjustable stop, so that the switch will only rotate to the number of positions set by the user. You’re highly unlikely to encounter a detector with an impedance lower than 2.5K, so there is little point in going any lower. The last two positions, of 5K and 2.5K, are included in case a device such as a MOSFET is used as a detector; with diode detectors, the impedance is going to be somewhere in the 200K to 10K range.

There is more potential for variety when it comes to the output impedances, so you’ll probably find yourself using all of the positions of a 12 position switch. There are 20 position switches available, which would allow a builder to make all of the 17 impedance taps available. However, they are a bit pricey. On top of that, the one I found didn’t have a lug to prevent a loose switch from rotating, and I like to make use of those.

In my matchbox, all of the taps from 100K down to 1.5K were utilized. I found that range covered all of the vintage high impedance ‘phones I tried out, as well as the piezo earpieces in my collection. Kevin Smith, when building his impedance matchbox, divided his ranges a little differently from mine. His ranges were 100K down to 10K for magnetic and piezo ‘phones, 1.5K down to 300 for sound-powered headsets, and 32 to 8 for modern low impedance ‘phones and earbuds. The sound-powered headset that I put together turned out to have an AC impedance at 1KHz of about 3K, so the middle impedance range in the hundreds of ohms wasn’t needed. When describing his impedance matchbox, Kevin talks about the lack of a need for exact impedance matching, due to a listener’s inability to distinguish much of a difference in volume when the mismatch creates a volume difference of 3dB or less. He calls it “the 3dB slop”. If you build your own matchbox you will notice, when stepping through the impedance taps, how for any given set of ‘phones, there are several switch positions that give acceptable and almost equal volume. Looking at the schematic above you can see how, if you did happen to have a headset with an impedance of 800 ohms, for example, the closest tap available at the switch, is 1.5K, which would still give an acceptable match. Likewise, if your headset had an impedance of 500 ohms, the 300 ohm position would be adequate.

The KPB-02 auto-transformer doesn’t have a built-in mounting bracket. I cut a strip of flexible plastic from the top of a small storage container and used to it mount the transformer to the lid of an ABS plastic project box –

In the next photo, you can see an earlier version of the matchbox, which utilized chunkier, more modern binding posts. They are the type often marketed for speaker connections. I discovered that it is not as easy to connect the bare wire ends of the metal tips often used on vintage headphones to them, as it is with the more traditional style of brass binding post. I also ended up changing some of the impedance taps from the ones shown in this photo. There are vinyl bumper feet on two sides of the box, so it can be used in two different orientations –

Although a single pair of binding posts are used for the input, more output options are provided, in the form of a 1/4″ jack wired for mono, and a 3.5mm jack wired for stereo headphones, with the velements placed in parallel, in addition to the binding posts for the metal pins on vintage headphones as well as bare wire ends –

The internal wiring in the first version of this matchbox, before the wiring to the impedance taps was changed a little –

On realizing that traditional brass binding posts were going to work better in this application, I took out the speaker posts, made a trip to Ace Hardware, bought the appropriate brass hardware, and fitted the impedance matchbox with 2 handsome sets of brass binding posts. I also changed the wiring to some of the impedance taps on the transformer. Note the new labeling –

Unfortunately, swapping the location of the binding posts necessitated a lengthening of the wiring, and made it a bit more untidy. Nevertheless, I wanted brass binding posts on this matchbox, and am glad I added them –

The brass nuts on these binding posts are called brass flanged knurled-head thumb nuts on the McMaster Carr site, though I got mine from my local Ace. The 3.5mm jack is wired so as to place the 2 elements in parallel. I did this so that they would be fed in phase. If you use the ring and tip connections to feed them in series, you will end up feeding them 180° out of phase, though I’m not sure if that makes a noticeable difference in practice. The 1/4″ jack is wired to the sleeve and the tip only, for mono jacks. Another change I made, was to place the binding posts on the end of the enclosure that faces the operator. In the previous version, they were at the back, causing the metal tips of vintage headphones that were connected to the binding posts, to foul the two jacks. Small ergonomic details like that make quite a difference to the usability of a piece of gear –

One thing I noticed, stepping through the different output impedance positions for a given headset, was that although nearby switch positions to the optimum one produced almost exactly the same volume, the tonal quality changed. If the switch position is set higher than the headset impedance, higher audio frequencies are favored. As you rotate the switch through the optimum position to impedances that are lower than optimum, more bass response is favored. This could possibly be used with very weak signals, as a switch position that favors higher frequencies could, if copy was very marginal, perhaps improve intelligibility enough to be able to ID a station.

An impedance matchbox is a useful piece of gear in an experimental crystal set receiving station. It makes constructing subsequent crystal sets easier, as the builder doesn’t have to keep replicating the audio circuitry after the detector diode.

In the next post, I plan to show you the various vintage as well as modern headphones and headsets that I tested with the Hobbydyne Crystal Set and impedance matchbox combination.

In the meantime, more information on Crystal Sets, and DX’ing with them, can be found in the following, as well as the various individual blogs on the subject (I’ll let you find those!) –

The Crystal Radio DX Group on Facebook – a group founded and run by Steve VE7SL. Intended specifically for discussion of crystal set DX listening events, as well as circuits and techniques specific to DX’ing with crystal sets.

The Crystal Set Radio Group on Facebook – a larger group, for general discussion of crystal sets. If you are a newcomer to the world of crystal sets, this would be a better group for you than the previous one.

The New Radio Board – intended as a new version of the now defunct (and much missed) Radio Board Forum, this board contains discussions of construction of several different types of receivers, including solid state radios, tube radios, and crystal sets. Links to the different topics can be accessed from the lists of hashtags.

A good introduction to the subject of crystal sets can be found in this engaging talk given by Al Klase to the New Jersey Amateur Radio in 2022, titled ,“Understanding and Building Crystal Radio Sets”. The graphics to go along with the talk are here.

I first tried WSPR out in 2009, with a Signalink USB interface attached to my FT-817 and PC. For anyone interested in QRP and QRPp, the process of being able to decode a signal that is up to about 34dB below the noise level is quite fascinating. Morse code, sent by way of CW, engages and tickles my brain in ways that other modes don’t. WSPR though (and other weak signal modes), has it handily beat in terms of it’s sheer ability to extract data from a signal that the human ear cannot even detect. A few years later, in 2018, I assembled an Ultimate 3S QRSS/WSPR beacon transmitter from QRP Labs for a ham friend. This project opened me to the appeal of a standalone WSPR beacon that, unlike my earlier foray into WSPR, didn’t require tying up my main station gear. The addition of a GPS unit, as well as setting the timing of the transmissions, could also automatically insert the Maidenhead grid locator – no need to manually program that, making it ideal for travel.

Fast forward to the current day. I’ve recently become a bit more active on the bands, and decided that I wanted to “stop the rot” of my CW skills, which were slightly degrading due to lack of use. I signed up for an online CW course with the CW Academy, offered by CW Ops. I just completed their intermediate course, and enjoyed it immensely. The Intermediate course is designed to take ops from 10-20 wpm. I was already comfortably having conversational QSO’s at about 16-18 wpm. At CW Academy, the emphasis is on head-copying, so that you can converse without needing to write anything down other than the occasional piece of essential info (name, rig, etc.) This, they explain, is an important skill, if you are to increase your speed. I, along with most of the other students, found it surprisingly challenging to listen to short stories in code, and extract meaning from them without writing anything down. It helped that we had a fantastic advisor, in the form of Randy N1SP. Practice sessions in between our online Zoom sessions could be challenging, but the prospect of classes led by Randy were a great incentive. He made learning fun.

Along with my renewed interest in CW came interest in weak signal modes generally, as well as a slight stirring in the desire to build radio things again. Over the last 3 years, I’ve been putting time and effort into working on my camper van, which took energy and money away from amateur radio. Well, I’m gradually angling towards selling the campervan, which will free up some mojo for other pursuits. Anyone want to buy a 1993 Airstream B190, with 67K miles, 200w of solar on the roof, and a 2″ lift?

Back to radio. The Autumn 2022 issue of SPRAT contained an article by Paul VK3HN, detailing the WSPR beacon he had built using modified open source code from Harry at ZachTek and, of course, the JTEncode and Si5351 libraries from Jason NT7S (Jason’s libraries pop up everywhere). If you don’t have access to SPRAT, and even if you do, Paul describes his beacon on his blog here.

As long as you know how to upload a program to an Arduino, or flash firmware to a microprocessor (same thing), the barrier to entry to building a WSPR beacon is now quite low – even lower if you don’t build a PA stage, and take the ~10mW output from the Si5351 clock output directly to the LPF and the antenna. Here’s what I built –

The output is taken from the CLK 0 output of the Si5351 and feeds directly into the PA stage that Hans Summers uses in the QRP Labs Ultimate 3S QRSS/WSPR transmitter. I’ve built both the Ultimate 3S and QCX rigs, and liked the class E PA’s he used in both designs. Simple in design – and I also like the fact that, because the BS170 is a MOSFET that doesn’t suffer from thermal runaway, you can simply parallel them up for greater power, without the need for balancing. Details of how to wind the bifilar transformer can be found in the assembly manual for the Ultimate 3S on the QRP Labs website.

In his beacon, Paul runs the Si5351 at it’s default of 2mW output, and follows it with a W7ZOI-designed 2 stage PA from the pages of EMRFD . Due, I suppose, to sheer laziness, I wanted to keep the PA stage as simple as possible, so opted for higher output from the Si5351, and a single MOSFET, with very few supporting components, for the PA. Paul mentioned that in the earlier days of the Si5351 being available to experimenters, he heard some talk of higher phase noise and jitter from the Si5351 at higher output levels. Perhaps running it at a lower output level, and making up for that later, is a worthy strategy? To run the Si5351 at it’s maximum power of about 10mW out into 50 ohms, I found the following line in Paul’s modified code –

si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);

and inserted the following line after it –

si5351.drive_strength(SI5351_CLK0, SI5351_DRIVE_8MA);

This sets the chip to produce the maximum power at the CLK0 output.

The very first iteration of this project used a passive patch antenna, as I didn’t realize that the GPS module supported active antennas. The patch antenna, with it’s very short piece of coax, was quite difficult to implement in the diecast enclosure I had chosen for the project. I mounted it on top of the lid, with the main board mounted on the inside of the lid, and the coax passing through a hole in the top. When I took the lid off to work on the circuit, the antenna was shielded from satellites by the lid, which was inconvenient. Once I discovered that the GPS module supported active antennas, I installed one. I have no photos of the implementation with the passive antenna.

Here’s a view of the next version of the board, with the clock generator and Nano boards unplugged, to allow viewing of the wiring underneath. As usual, I have used Rex’s wonderful MePADS and MeSQUARES for the Manhattan pads, and strips of header to plug the Si5351 board, Nano, and LPF boards into. Operating on a different band just requires changing the output filter, and reprogramming the Nano via it’s ICSP header –

The first version of this build used a single 7805 voltage regulator, bolted straight onto the board for heatsinking. I had forgotten how very hot these 1 amp regulators get. The IC itself got very hot, as did much of the ground plane on the board to which it was bolted. Although not my best idea, it turned out to be dwarfed by a particularly poorly thought-out aspect of the layout –

It’s perhaps not immediately obvious from the above photo, but might become more apparent from this image –

That is the BS170 PA transistor mounted directly underneath the frequency synthesizer board. The problem, is that the PA transistor gets very warm. Warm air rises – and what is directly above? Yes indeed – the most frequency sensitive part of the whole circuit. What a fool, an oaf, a bumpkin, a buffoon, and a rube! When laying out the build, I was mainly concerned with fitting everything in, and not having a long wire between the output of the Si5351 and the PA. I’m not sure why, as a short length of RG-174 would have worked just fine. Nevertheless, slightly disheartened at my mistake, I forged on, and proceeded to attempt to calibrate the unit using Jason NT7S’ calibration script. I’ll spare you the long, dull version, and just say that I couldn’t get Jason’s script to work. My suspicions lay with either the cheap Nano board, or the cheap Si5351 board that I had bought from Amazon. Not pictured here, the first Si5351 board I tried was a direct clone of the Adafruit board, with a purple board instead of the Adafruit blue color, and without the Adafruit branding on it. I ditched Jason’s script, and went for a rough calibration by beating the output of the board against WWV, and making adjustments to the correction factor, until I was within a Hz or two of zero-beat.

I then uploaded VK3HN’s script to the Nano. The unit was indeed WSPR’ing but, despite the fact that I had calibrated it fairly accurately, quite a few of the WSPR transmissions were out of band by anything up to 100Hz. This didn’t seem right, so I tried calibrating the board again, only to find that each time I calibrated the board, I came up with a significantly different correction factor. Replacing it with a genuine Adafruit board solved the problems. Suddenly, Jason’s calibration routine worked beautifully, and the board began producing consistent, repeatable frequencies. All subsequent WSPR transmissions were in-band. The Si5351 board that I had purchased from HiLetgo was only about $3 less than the genuine article from Adafruit. In retrospect, it was not worth the trouble just to save a few bucks. Lesson learned. In contrast, their Nano boards are significantly cheaper than the “real” thing, and seem to work just fine.

My first foray into WSPR with this mini concoction was on 10M. Drift figures were nearly all -4’s, and I wasn’t getting anywhere near as many spots as I would have expected to get. Because nearly all the drift figures were -4’s, that indicated to me that many spots were very possibly being missed, due to a drift figure of higher than -4. Placing the board in a diecast enclosure with the top on helped. I was then getting more spots, but still all with drift figures of -3 and -4, with more -4’s than -3’s. I went down to 20M, where the drift figures were a little better, but still not good enough. From cold, the first few transmissions produced no spots. After an initial warm-up period of about 30 minutes, I was getting more -3’s, and even a few -2’s. Still not good enough.

One obvious change would be to relocate the PA to the opposite side of the board, away from the clock generator board. If I did that, it would be in another build completely so, for the time being, I concentrated on other ways to bring the drift down. Here’s what I did –

1. Mounted the 7805 regulator on the side of the diecast enclosure, to which it was bolted. I also added a 7808 regulator, thinking that it wouldn’t hurt to spread the heat generation between two devices, even though these parts are designed to run very warm.
   
2. Added a 1N4001 diode in series with the 12V DC input. As well as providing reverse polarity protection, the forward voltage drop of about 0.7V should help to spread the heat dissipation from the regulators out just a little more.
   
3. Secured the clock generator board with 2 nylon screws and a threaded nylon spacer. I had been waiting for the parts to arrive from Adafruit, so hadn’t done this earlier. (This kit of nylon screws and spacers should last a while!)
   
4. Although not a modification, one thing I did differently this time before testing out the beacon, was to screw down the top of the enclosure tightly, instead of just placing the top on.
   
   

After these changes, the difference was dramatic. No spots were picked up on the first transmission. On the second transmission (on a 50% transmission duty cycle), several spots were received, all with drifts of -4. Things improved with every cycle until, after about 45 minutes, all spots were -1’s and 0’s, with the very occasional -2. Much better, and very encouraging.

Some more shots of the board with the regulator removed, and replaced with 2 regulators in series (a 7808 and 7805), both bolted to the side of the enclosure. The enclosure is a bigger mass of metal that provides more effective dissipation of heat from the devices –

Here’s a view of the board with the Si5351 breakout board and Nano board unplugged, to show the wiring underneath –

Looking dead sexy in it’s diecast enclosure from Tayda –

In attempt to further improve the drift figures, I made a heatsink from a piece of brass strip, and epoxied the BS170 PA transistor to it with JB Weld. A clamp held the mighty little MOSFET in place while the epoxy set –

A pair of round-nose pliers were used to bend the leads. The leads on some of these parts are quite delicate, so I prefer to coax them round the bend, rather than foisting an abrupt 90 degree angle on them –

I am unsure of the dielectric properties of JB Weld so, to avoid any problems, made sure to keep the area around the leads free of epoxy –

I think this heatsink looks mighty spiffy. Brass is such an attractive alloy –

A close-up of the heatsink –

Unfortunately, with the heatsink fitted, the drift figures were worse. After about a 90 minute warm-up period, I was getting drift figures of mainly -2’s and -1’s. Removing the heatsink got me back to drift figures of mainly -1’s and 0’s, with the occasional 2, after warm-up. After 2 hours, the drift figures are equally split between -1’s and 0’s. All of the figures I have quoted are from 20M operation, by the way. A quite satisfactory result, I think, from a frequency generator board that is not temperature compensated.

I was already fairly satisfied with this result, but then things became better. The heat from the PA transistor was rising, and heating up the Si5351 board, forming a sort of crystal oven. Because of this, it occurred to me that if I were to adjust the bias on that BS170, it would affect the amount of heat the transistor gave off, and might also affect the drift figures reported by wsprnet.org. The transistor was currently providing about 200mW to the antenna. Although, by adjusting the bias, I could have coaxed some more power out of it – perhaps as much as 250mW, I didn’t want the transistor to run much hotter than it was already running. Likewise, I didn’t really want to run much less than 200mW. Fiddling around with the bias trimpot, I ended up with it in almost the same place as it was before. The transistor was probably putting out a mW or two more, but not much more. However, the difference in the drift was dramatic. Check out this wonderful result (still on 20M), achieved after a warm-up period of around 40 minutes or so –

These fantastic drift figures almost made me giddy! The only other thing I had changed, was to swap out the 4 oxide black panhead 4-40 machine screws on the sides of the diecast enclosure, for regular stainless steel machine screws. Perhaps they have slightly different thermal properties, but I think the main factor responsible for the improvement in drift, was the very slight change in the bias setting. I had haphazardly settled on a near-perfect bias setting, and created a very effective crystal oven! I did have a couple of other ideas I was going to try, namely placing foam over the Si5351 board, to insulate the Si5351 and crystal from air currents, and looking for a TCXO to replace the crystal in the Adafruit board. However, at this point, I don’t think it’s necessary. Running the beacon for another 6 hours, the results were much the same, though a single -2 and +4 drift figure did pop up. I think the +4 was an anomaly, probably caused by drift in the other station. This is a better result than I had hoped for. I’m ecstatic!

On 10M, it takes 3 hours to fully settle down, after which, drift figures are mainly -2’s and -1’s, with a few -3’s, the occasional 0, and the very occasional -4. However, I do notice that after running it all night, drift figures in the morning are a little worse, with a lot of -3’s and a few more 4’s. This suggests to me that the ambient temperature of the room might be playing a part.

Incidentally, changing bands only involves changing the band in the code, which requires simple changes to two lines in the sketch, plugging in a different LPF, and uploading the new sketch via the ICSP header on the Nano board in the WSPR beacon. As far as initial setup goes, before you upload Paul’s modified code, you will need to insert your callsign, and the power level in dBM. Mine puts out about 200mW, which is 23dBm. You can input your grid locator if you want, but the unit will calculate that from the GPS, once it has gotten a fix. Although I haven’t tested it yet, I assume that if the unit moves into a different grid square, it will report the new locator. (EDIT – Paul informs me that, although it would be easily possible to insert code that calculates the grid locator, his modified code doesn’t do that. I assumed it did, based on the fact that although I input my locator as CM87, wsprnet reports it as CM87ut. However, they are probably doing that based on their knowledge of my location. Looks as if I have something else to work on!)

By the way, when you’re changing bands, remember to also change the LPF. When assembling the LPF boards from QRP Labs, I always check the response curve on my NanoVNA. As an added testament to the fact that they do indeed work, I recently flashed the unit with firmware to change the operation from 80M to 20M and left it to run overnight. In the morning, there had been absolutely no spots. I was flummoxed, and even thought I might have fried the Nano board, until it dawned on me that I had not changed the LPF. The beacon was running on 20M, with an 80M LPF still plugged in. No wonder!

In the future, I may experiment with an Si5351 board that has a TCXO, in order to improve the drift figures on the higher HF bands. In the meantime though, I am deliriously happy with the performance on 20M (and presumably below). This project was inspired by VK3HN’s SPRAT article, and the realization that “throwing together” a few boards, and constructing a simple PA and LPF should be easy, and wouldn’t constitute a full-blown project. I have become somewhat shy of such lengthy endeavors these days. I wasn’t expecting it to turn into a cased-up and very serviceable WSPR beacon though. I tend to let it run in the evenings and overnight, when I’m not operating. That way, in the morning, I can check wsprnet to get an idea of what propagation is like. As many others have said, it’s a handy propagation tool. If you don’t want to build one, you can buy a ready-made WSPR beacon from Harry at Zachtek.

At the risk of posting too many pictures, here are a few more –

A cigar box from the local tobacconist, and some packing foam, makes a good storage box for my growing collection of QRP-Labs LPF’s and BPF’s. Only the LPF’s are used in this project. The BPF’s, in the front row, are for receivers (though they could be used in the early stages of transmitters, where only very low signal levels are involved) –

Definitely a successful project. Thank you Harry Zachrisson of ZachTek, Paul VK3HN, and Jason NT7S –

One last gloat. Look at these great drift figures. Pretty good for an Si5351 board without a TCXO! To date, this 200mW powerhouse has been spotted all over Europe, North, South, and Central America, Hawaii, Taiwan, Hong Kong, Australia, New Zealand, and several island clusters and nations in the middle of vast oceans. Exciting stuff!

Oh, and one last thing. Paul included an LCD display in his transmitter, which shows some extra useful information. The code will support it, and his blog shows how to connect the display. I think there is just enough space to fit a display into my unit. I didn’t chance it however, as I seem to have the thermal balance inside the case just right (for 20M and below, at least) and I don’t want to upset anything. My desire for a display isn’t strong enough to want to make any more changes. I’m fine with this, as I think of it as a set and forget kind of beacon. In the evenings, I plug it in, and forget about it until morning.

I haven’t been building much at all, for quite a long time now. However, the urge occasionally returns. When it does, it’s wonderful to have a small stash of parts on hand, so I can pull the soldering iron out and start building before the desire dissipates. I’ve been interested in beacons for a while, and this interest has followed a logical progression. I first noticed that my interest in talking with other hams over the air using phone (i.e. SSB) was waning. During this time, I would still check in daily with the Noontime Net on 40M (now on 7284 KHz). A quick check-in to a net was fine, and it was good to hear the other stations, many of whom were regulars, also check-in, and have the occasional quick conversation. In addition I noticed that my inclination towards CW QSO’s was also diminishing. I’m not exactly sure why. I enjoy talking with close friends, acquaintances, and neighbors in person, but my enthusiasm for chatting with relative strangers who I can’t see, and don’t know that well, all but disappeared. Too many QSO’s seem very cookie cuttter. Either that, or the conversation is about subjects that don’t interest me.

My main interest in amateur radio was fast becoming the medium of radio communication and the science behind it, rather than the content. I enjoyed quick contest exchanges, as it was a way of seeing where my signal was getting to. Some folk dislike contests, preferring to ragchew, but I find the average ragchew on the bands rather dull. Mike Rainey AA1TJ referred to brief QSO’s as being akin to high-fiving someone when you’re walking down the street. It’s an acknowledgement – a quick, “Hi! It’s good to see you. Talk later!” If you just built something, it’s good to know that it works, and a way to marvel in the mechanism of propagation that made it all possible.

This is why I like beacons. The ones I listen to send CW at regular speeds. I tend not to look for QRSS beacons, or data signals. I like CW that I can listen to and decode in my head, even if it’s just a few letters that are constantly repeated. I can put in the work to decode a very weak signal from a QRP (and often QRPp) beacon, figure out roughly where it is in some cases, and feel the satisfaction of having received a very weak signal, without listening to some chap talking about the model number of his transceiver, his latest medication, his political/social opinions, or whether he mowed the lawn and watered his plants today.

I’ve probably explained this before, but there are two main types of beacon I like to listen out for. The first are the so-called “unlicensed HF beacons” (for which read pirate). They tend to operate in the lower half of the HF spectrum. The lowest frequency one I know of is on 2097.3 KHz, is located somewhere in the southwest (many of them are in the SW deserts), and is relatively high power – probably in the range of 5-15W. It sends the letter A once every 10 seconds (approximately). There are a number clustered around 4096 KHz (a popular crystal frequency), quite a few around 8000 KHz, and others up to about 8500 KHz. A good one to listen out for, is the fairly new Desert Whooper, on 4095.65 KHz. It must be relatively high power, as I have heard it regularly in 6 Western states, on a recent campervan trip, with a portable receiver and set-top whip. It sends a whooping sound for a few minutes then, in CW, it sends the battery voltage, the outside temperature, the inside temperature, and a number that is related to the solar panel voltage. Then, if I remember correctly, it sends it’s ID a few times (DW, for Desert Whooper), and goes back to whooping for the next few minutes. The current list of known active unlicensed HF beacons is here, on the very excellent HF Underground forums –

https://www.hfunderground.com/board/index.php/topic,9478.0.html

I’m a bit too chicken to deploy one of these types of beacons, partly because I’m a licensed amateur. Also, how do you really know that your HF beacon is not operating on, or close to, some frequency that is used for critical communications? Certainly, there are published bandplans, but I’m not sure how much detail they really go into. On top of that, I just cannot bring myself to leave something that emits RF in a remote location such that I would have trouble getting back there quickly, were the beacon to malfunction. It’s a control thing, I suppose. I’m responsible for my stuff, and I like to be able to switch it on and off, and service it, at will. I know that the likelihood of a 100mW HF beacon interfering with critical communications is pretty unlikely but even so. Besides, as an amateur, there is a wide swath of spectrum I am allowed to use legally (though not for unmanned beacons below 28MHz).

The other type of beacons that I find quite fascinating to listen for, are the HiFERs. These are also unlicensed, though if you keep your power low enough, they are legal (in the US, at least). The FCC, for the 13553 – 13567 KHz ISM band, regulate the power level allowed in terms of the field strength a certain distance from the antenna. For those of us without a professional calibrated field strength meter (most of us, because they are expensive!) a couple of helpful gentlemen have expressed this in terms of power to various antennas. According to W1TAG’s calculations, you should be within, or close to, the allowed field strength with 4.6mW into a half-wave dipole, or 2.3mW into a quarter-wave groundplane. K6STI uses several different antenna installations over varying degrees of ground conductivity. Under certain conditions, he calculates that up to 473mW would be allowed. His calculations are at –

http://ham-radio.com/k6sti/hifer.htm

I’ll leave it up to you to decide what power level to use, but it sounds to me as if you’re pretty safe if you keep it under about 5mW. What? you think. Is it possible to even hear beacons at distance at those sorts of power levels? It certainly is. If you, like me, either have a poor antenna installation, or none at all, you can still hear some of these HiFERs by utilizing online SDR’s. One of my favorites is the KFS SDR, that uses several large antennas on a 150 foot cliff overlooking the Pacific Ocean, 6 miles south of Half Moon Bay, CA. This SDR has good ears! In the last few days, on this SDR, I have heard TON in Tonopah, Arizona, PCO in Pine, Colorado, and TSN in Tucson, Arizona.

Which brings me to this project. I wanted to build a HiFER beacon that would operate from a LiPoly battery charged by a small solar panel, for a compact installation that could be located outside on a hilltop, or in some desert area. These batteries are 4.2V at full charge, so I built up my beacon with a LP2950 5V regulator on board, figuring that if it worked at 5V, it would probably work at 4.2V. The regulator and it’s bypass capacitors are not shown in the following schematic, but there is a 100uF and a 0.047uF bypass cap on the input of the regulator. The bypassing on the output is shown in the schematic, and consists of a 0.047uF on pin 8 of the ATTiny85, and another 0.047uF on the +ve supply line side of the 8.2uH choke that feeds the collector of the PA transistor. A molded choke was used in this position.

13.56MHz crystals are easily available from many suppliers of electronic parts. I bought a bag of 20 on eBay. The RF chain is very straightforward, and is the same one seen in the transmit section of the Pixie transceiver. The output filtering is more than is necessary for this application. 2 poles of filtering would have sufficed, but it doesn’t hurt to add an extra pole. The 47pF capacitor across the center toroid forms a parallel tuned circuit with that inductor to increase rejection at the second harmonic. It’s a design by W3NQN. I took the values from the 20M LPF –

https://www.gqrp.com/technical2.htm

The ATTiny85 keys the oscillator. There is no chirp or instability caused by doing this. I like this approach, rather than keeping the oscillator running and keying the PA. I build most of these beacons for use around the house, and am usually in close proximity to them. If the oscillator is keyed, I don’t hear the constant backwave from the oscillator transistor. The code for keying, as in my Boris Beacon, was courtesy of Nick SV1DJG. I use pin 2 of the chip as the keying line, and had to change that in Nick’s sketch. The line –

#define ledPort 1

was changed to

#define ledPort 3

The feedback capacitors C1 and C2, as well as forming part of the feedback loop to maintain oscillation, help to pull the crystal, to determine the frequency of operation. Initially, C2 was 100pF, and there was no capacitor in the C1 position. The oscillator ran reliably, but it was on around 13561, which is too close to the center of the band where all the RF from the various ISM and RFID devices is. Take a look at 13560 ± 1 or 2 KHz on a waterfall display – on your own receiver if you have a good antenna, or a good online SDR. You’ll see why you wouldn’t want to run a QRPp beacon there. Too much RFI! However, above about 13562, and below around 13559.5, the band is quite clear. It’s a gift for very low power beacon enthusiasts. Where else in the HF spectrum can we legally operate an unmanned beacon that stands a chance of being heard? (Well apart from 10 meters, that is.)

I wanted to avoid frequencies that established and known beacons are operating on, as well as the 3 other devices in this band that I use at my house. I ended up with a 100pF cap for C2, and a 150pF one for C1, which gave me an operating frequency of 13557.49 KHz ± a few tens of Hz, depending on the ambient temperature.

With the above setup powered by the 5V regulator, I measured 0.913V across the 51 ohm resistor with the N5ESE RF probe and an EEVBlog Brymen BM235 DMM. The voltage drop across the diode in the probe was 0.234V, so this translates to a power of about 25mW. I removed the 51 ohm resistor and measured the power with a freshly calibrated WM-2 QRP Wattmeter. The reading was about 6 or 7mW. I am not sure why the discrepancy between the two readings, but if I’m able to get my hands on a DSO in the near future, I’ll be interested to see which of the two readings is the more accurate. I suspect it’s the wattmeter reading. If the power is closer to 25mW, it can easily be dropped with an attenuating pad, or a lower supply voltage from either a LiPoly battery, or a 3.3V regulator.

A successful project, I think. I may even press it into service one day!

When I built the VE7BPO DC Receiver Mainframe recently, it wasn’t intended to end up as a final finished project. The intention was more to have it as part of an experimental platform. The little box that contains the DBM, diplexer, and AF amplifier that make up the mainframe will most likely stay largely the same, now that they are built and boxed up. However, the outboard functions of local oscillator and antenna filtering can swapped around and changed at will. The mainframe includes a spot for an onboard plug-in bandpass filter. It was constructed so that the bandpass filters from QRP Labs could be plugged in, but this circuit section could be constructed from scratch, if desired. The first BPF I constructed was for 40M, and it did a fine job of removing many of the spurious responses I was experiencing with no antenna filtering in circuit. I purchased a 10-pack of these filter kits from QRP Labs, intending to, at some point, assemble most of them for listening to the amateur bands on this little receiver. That may happen, but I also wanted to listen in between the amateur bands. AM reception is not great on a direct conversion receiver, but there are quite a few non-ham SSB and CW signals to listen to outside the ham bands, and it would be good to be able to do that on this receiver. A tunable passive HF preselector seemed like a good way to get this particular show on the road.

The circuit is a very straightforward and standard double-tuned bandpass filter. Although my preferred variable capacitor of choice would have been an air-spaced component, I wanted to fit this into the same LMB Heeger 143 enclosure that I used for the other two receiver modules, the DC receiver mainframe, and the Si5351 “VFO”. I think it would be possible to find a suitable air-spaced part that would fit into this space but, for the sake of timeliness, I plumped for a polyvaricon.

GQRP Club member sales can supply the polyvaricons (to members) with a mounting kit that consists of 2 different lengths of mounting screw, to allow for different thicknesses of front panel. They come with a bolt and plastic spindle, for attaching a knob. Also supplied, are the inductors with adjustable ferrite cores. The inductors are Spectrum 5u3L types. The nominal inductance is 5.3µH, though it is adjustable over a fairly wide range. The L denotes that the secondary is a low impedance winding, suitable for matching to 50 ohm systems. If you are not a GQRP Club member, these coils are available from Spectrum Communications in the UK, who also sell on eBay. Some of the coils in this series are direct replacements for the Toko KANK series, which were popular with UK homebrewers in the past. It is possible to wind a similar coil on a toroid. I wanted these coils though, for the ability to easily adjust the inductance.

The 1pF capacitor that couples the two tuned circuits might seem rather low. In fact, it surprised me too. I began with higher values, of 47pF and then 39pF, but found that the coupling was too tight, and I ended up with 2 distinct peaks in the response, spaced far apart, with a large dip in between them. The final value of 1pF was far lower than I had expected. The only reason I can think of for this, is that the separate gangs in the polyvaricon are not as well isolated as they would be in a larger air-spaced part. I’m thinking that some coupling is happening inside the polyvaricon perhaps? A value of 1pF gave an acceptable response curve above about 8MHz. For the lower frequencies, an extra 10pF capacitor is switched in. Without it, the insertion loss at the peak of the response is a whopping 32dB. Switching in the extra 10pF reduced the insertion loss to 10.5dB which, although still a little high, is a lot better. The switch needs to be flipped to the high position over about 7 or 8MHz, otherwise the response is far too broad. With the switch in the “Lo” position on my preselector, at the maximum frequency setting (lowest capacitance), the peaks were spaced 5MHz apart, and the difference between the dip and the two peaks was 14dB – far too much. With the switch in the “Hi” position, at the highest frequency setting, which is 15.6MHz in my unit, the insertion loss at the peak is 4dB, the passband ripple 1.75dB, and the bandwidth at the -3dB points is 1.66MHz. At 3.5MHz, in the “Lo” position, the insertion loss at the peak is 10.27dB, the passband ripple a mere 0.83dB, and the bandwidth at the -3dB points 180KHz. Your first thought might be that 180KHz is not enough to cover the 80/75M band, but remember that this preselctor is tunable, so you can put the peak wherever you want it.

Some photos of this simple accessory –

The stack of modules that make up the complete receiver. From top to bottom, the preselector, the mainframe and, at the bottom, the VFO –

The stack as seen from the rear, showing the interconnections. It was starting to rain, and you can see some raindrops –

I recently acquired a NanoVNA, which was very useful for adjusting the trimmer capacitors and the inductors on this preselector, as well as for adjusting the fixed bandpass filters. Adjusting the trimcaps and inductors on the preselector is an exercIse in compromise, so it is very helpful to be able to see the effects of your adjustments almost in real time, as you make them. If you don’t have a NanoVNA, I imagine you have heard all about them. If not, there is a lot of information out there about them. Alan Wolke W2AEW has a fantastic YouTube channel, with several instructional videos on how to use a NanoVNA. A search of his channel for “NanoVNA” will yield many helpful videos. I have not yet watched it, but just found his introductory presentation to Fairlawn ARC on the subject of NanoVNA’s. Without going into too much detail, a NanoVNA can be used as a small and very portable antenna analyzer, and network analyzer. It can display the SWR curve for an antenna, over any frequency range you desire. Need to look at complex impedances? The NanoVNA has a Smith chart display too. It can also plot, in graphical form, the response curve of a filter. It is so useful to be able to build a lowpass or bandpass filter, and see the response curve, making adjustments easy. The majority of us regular hobbyists who couldn’t justify the purchase of a more advanced, and much more expensive network analyzer can now purchase a NanoVNA for somewhere between $50 and $150, depending what features you want. The original version has a 2.8″ screen, and is around $50-$60. The newer versions have a 4″ screen, which is much easier to read. Mine, the NanoVNA H-4 (the 4 denoting the screen size) was $93 on Amazon, delivered the next day. There is a more expensive version still, which has a metal case. I am doing fine with the plastic case so far. The very original version didn’t even have a fully enclosed case. I am happy to pay $30 or $40 more for a bigger screen and fully enclosed case. It’s much smaller and lighter than my old MFJ-259B, and does far more. Does anyone want my 259?

This NanoVNA will be fantastic for tuning up antennas in the field. Here it is, connected to a little test rig I built up, for testing the QRP Labs BPF’s. I didn’t switch it on, as the display often doesn’t show too well in the daylight –

The unit can be configured to display up to 4 traces simultaneously, each one showing different characteristics of the circuit under test. Here it is, with two traces activated. One is showing the response curve of the 80M BPF. The other, which was left on accidentally, shows the SWR, which isn’t of interest here. The unit was set to sweep from 1.7MHz, the top of the AM broadcast band, up to 10MHz –

Just for fun, here’s a closer view of that BPF –

While on the subject of this BPF, I used different values of capacitance from the ones Hans Summers supplied, for the coupling capacitor. His filters for the lower HF bands are not designed to cover the entire band. The intended usage is for receivers for digital modes, for which a narrower bandwidth is perfectly acceptable. I used a higher value of coupling capacitor to get the bandwidth I wanted. The bandwidth of this filter, with a 113pF coupling capacitance (47p + 56p), is about 885KHz. It’s a little wider than I wanted, so the next step may be to try a slightly smaller value of coupling capacitance. Insertion loss at the peak of the response curve is 5dB. By contrast, the insertion loss of the 40M BPF is only 1.12dB at the peak of the response curve – a very acceptable figure. In the assembly instructions for the QRP Labs BPF’s, Hans quotes an insertion loss of just 1.27dB for a b/w of 465KHz with his filter. The figure of 5dB for my BPF seems a bit high. The insertion loss of 10.27dB for the preselector when tuned to 80M, seems way too big.

When using the receiver with the preselector, I jumper across the socket for BPF that is inside the receiver mainframe enclosure. Breadboard jumper leads work well for this. Interestingly, reception on 80M is much better using the preselector than the BPF. Although the insertion loss is greater, and I have to turn up the volume to compensate, the SNR is much better, making reception of stations when the band is noisy, much easier. With the internal BPF plugged in, the SNR is higher. It is the same when listening to 80M at night with no filter inline at all – a higher SNR. Reception on 40M is about the same with the BPF as it is with the preselector.

I have not yet used this little direct conversion receiver very extensively for general HF listening, but a few observations, based on my experience so far –

• The unfiltered output from the simple Si5351 is not perfect (surprise, surprise) and contains some spurious components, as well as the expected harmonics. The 40M band is largely clear. There is one fairly prominent one that is audible in the very bottom 300Hz of the band. There are a few others, at much lower levels, at a few points throughout the band, but they are masked by band noise when an antenna is connected. Outside the amateur bands, there are other spurii dotted throughout the HF spectrum. Annoyingly, there is a rather loud one at 10MHz, which makes reception of WWV troublesome. Future experiments could focus on reducing and/or eliminating these spurii, or looking at a different method of generating an LO signal.
• The preselector (or fixed bandpass filters, if used) is very effective at eliminating unwanted modulation products from AM BC band stations, as well as from spurii caused by harmonics of the LO mixing with RF signals from the antenna
• I’m happy with the mainframe circuitry. It is a good module for future DC receiver experiments. As it isn’t a single signal receiver, there is an automatic 3dB SNR disadvantage compared to a superhet or SDR. This is par for the course, however, and expected.

This is one of those projects that has been residing in my head for a long time, as something I wanted to build. I’ve always liked direct conversion receivers. With them, as with regens, I felt that they have been underestimated by many builders and hams as being novelty items. Their apparent simplicity can also be their greatest downfall. Because, in their basic form, they often have few components, they can be “thrown together quickly”, in an evening, by a novice builder. That, of course, is where the problems start. The high degree of audio amplification necessary in a DC receiver lends itself to microphony if certain types of coupling capacitors are used (ceramics are prime candidates, for example). Long, stray leads help to pickup hum, especially if they are in the earlier stages of the amplifier. Dead bug and Manhattan construction are very worthy methods of fabrication, but leads must be short and stout, especially in the parts of the circuit where it matters the most. Free-running LC VFO’s can add microphony if not solidly built. If the VFO is running on signal frequency and not adequately isolated from the later stages of the receiver, unwanted feedback loops can form.

For the above reasons (and more), some builders put a DC receiver together, twiddle around with it a bit, then toss it aside, thinking of it as merely a “fun project”. I think they can be more than that. In fact, I know they can be more than that, from experience, as does Todd (aka Professor Vasily Ivanenko), one of whose direct conversion receivers is the subject of this post.

As a kid, I spent countless hours gazing at a little direction conversion receiver project designed by R.H. Longden, in the June 1975 issue of Practical Wireless. It used a 40673 MOSFET as the mixer, and worked on both the 160M and 80M amateur bands. I never did build that receiver, but it wasn’t for lack of desire. I’m surprised I didn’t stare a hole right through the paper, so much time did I spend fixating on it! I was 11 years old when that issue came out, and I suspect the reason I didn’t build it, was partially lack of funds, but mainly lack of relevant experience on my part. It would have been a very involved and complex project for me at the time. Had I attempted to tackle it, I think there would have been a very high chance of it never working. Instead, I just gazed, and gazed, and dreamed about that little direct conversion receiver for top band and 80M –

Quite a few years later, in March 1983, a direct conversion DSB transceiver for either top band or 80M (your choice), was described in the pages of Ham Radio Today magazine by G4JST and G3WPO. A kit was available. By then, I was older, and a slightly better builder. I assembled the board, installed it in a case, and was overjoyed to discover that it actually worked! Paul G3UMV, who lived a mile down the road, heard me on 80 and, probably curious to see how a kid had made it onto 80M with a homemade rig, came over to ‘ave a gander at the rather messy creation that I had stuffed into an aluminum project case. The DSB80, as it was called, was based around a Mini Circuits SBL-1 diode ring mixer package. A free-running LC VFO, tuned by a polyvaricon, was coupled into one port of the DBM, while an antenna, via a double-tuned bandpass filter, fed the other input port. The IF output of the SBL-1 led to a simple diplexer, which fed a high gain audio amplifier. I had also constructed a simple active audio filter with 2 switchable bandwidths, to enhance the listening experience. I spent many happy hours tuning around and listening on 80M with the DSB80. It was this first experience that cemented my affinity for direct conversion receivers built with commercially available diode ring mixer packages. It just seemed so simple – you squirt RF into one port, a VFO into the other, and (after passing the result through a diplexer) amplify the heck out of the result. The seeming simplicity of the process of converting RF directly to baseband audio has held great appeal for me ever since. Unfortunately, that project didn’t survive. One day, in later adulthood, in my apartment in Hollywood, I reversed the polarity of the 12V DC supply and, discouraged at it’s subsequent refusal to work, tossed the whole thing away. Now, I cannot quite believe that I did that, but it was during a long period of inactivity on the ham bands, and complete lack of interest. If only I could go back, and not have thrown it into the dumpster of my apartment building! Hollywood is ridden with recent notable history. My little double sideband transceiver met it’s unfortunate end just 100 feet from the spot where Bobby Fuller, of The Bobby Fuller Four, was found dead in his car, in 1966, the subject of a mystery that is still unsolved to this day. The death of my little DSB rig was a lot less mysterious. To think that I heartlessly tossed an SBL-1 mixer into a dumpster, is a mark of how far I had strayed from my homebrewing roots, forged in a little village in England. Now, a few years later, in a city known for it’s sin and excess, I had cruelly ended the life of a stout and honest diode ring mixer. I suppose I should spare a thought for the polyvaricon but, well, you know – it was a polyvaricon! Some years later, I came across a fellow ham, Richard F5VJD (also G0BCT), who had also reversed the polarity of the 12V supply to his DSB80. Unlike me though, he hadn’t committed his rig to a sad and untimely end. He very graciously sent me his unit, which I revived, and installed in a new case.

Commercially manufactured diode ring mixer packages have the advantage of high dynamic range, over other mixer arrangements that use active devices. To me, an SBL-1, ADE-1 or similar, just looks like a virtually guaranteed-to-work DC receiver in a little box. It’s the heart of the receiver, all manufactured for you. You don’t have to bother with diode matching, or the overall symmetry of the circuit. It’s all done for you. Just add a VFO, diplexer, audio amp, and go!

A few years ago, a very generous friend gifted me an assortment of parts for experimenting and building with. Among them were some quality 3.3mH, 10mH, and 100mH inductors. I guessed that his intention was that I would, one day, use them in a diplexer. This is where Todd VE7BPO’s first QRP Homebuilder site comes into the story. On his information-packed site were details of what he called a “Popcorn Direct Conversion Receiver Mainframe” His popcorn approach, if I’m remembering this correctly, referred to the practice of employing moderately-priced, widely available parts, and using them to achieve good performance in his circuits. The mainframe moniker referred, I am guessing, to the fact that the circuit he described was the “meat” of a direct conversion receiver, requiring only the addition of an outboard VFO and a bandpass filter on the antenna input, for the frequency band of interest. The “mainframe” provides the rest of the circuitry.

Ideally, I would have liked to have broken a DC receiver into every single component stage, each one individually housed in it’s own case, connected to the other stages of the receiver via cables running between the various boxes. This would allow me to try different configurations and receiver stages, for the purposes of comparison. However, this would have resulted in more boxes and interconnecting cables than I wanted. Experimentation and optimization, though very worthy goals, were trumped by my desire to end up with a convenient and very usable receiver. I decided to build the mainframe with an ADE-1 mixer, and one of the better diplexers suggested by Todd. As it happens, the better diplexer that I chose didn’t work, for some reason. More on that later. I ended up with a less perfect, though very functional diplexer, which is shown in the circuit below. WordPress doesn’t seem to display images as large as it used to, which can make reading schematics a little problematic. I will show the whole schematic first then, for ease of reading, break it up into 3 separate and larger parts. If you’d like a larger version of the entire thing, drop me a line, either in the comments below, or via email (I’m good in QRZ) –

If you have any interest in building this receiver, I strongly recommend checking out the article on VE7BPO’s old website. His new QRP Homebuilder site is in a blog format, whereas the old one was a regular website. He had some issues with hosting, and took it down, though not before archiving it to a single PDF, and making it available to anyone who wished to host it for download on their sites. I won’t give the direct url here, but a quick Google search on Todd’s callsign should get you to the download link. If you have trouble finding it, drop me a line, and I can give you a download link to the file on my Dropbox account. It is well worth having a copy of Todd’s old site, to aid and inspire you in your homebrewing pursuits. Plus, his schematics are easier to read than mine.

Here’s the schematic broken up into 3 parts, which will hopefully make it a bit easier to follow. First, the antenna input circuit, double balanced mixer, and diplexer. Much of this section is block diagrams. If you don’t want to use the BPF from QRP Labs, you can build your own, using the circuit and component values on their site (link a bit later) –

OK, some notes and general guff about the above circuit. For a VFO, I used the Si5351 circuit I put together a couple of years ago. The ADE-1 is a level 7 mixer, meaning that it requires a drive from the local oscillator, of ~ +7dBM. I have read that the Si5351, at full output, develops +10dBM into 50 ohms. Unfortunately, my oscilloscope is not working too reliably, so I don’t have the means to measure this. I decided to incorporate a 3dB pad into the circuit to reduce the output from the Si5351 “VFO” to the +7dBM level (if indeed, it is putting out +10dBM). The pad resistors are soldered onto a header strip that plugs into the board, allowing the builder to easily change the level of attenuation, if required. Room for experimentation and modifications in the future.

The bandpass filter is one of the BPF kits from QRP Labs. These little bandpass filters plug into header strips on the DC mainframe board, making changing bands a simple matter of plugging in a new BPF board. The Si5351 VFO works all the way up to 160MHz according to the specs – and beyond, if you’re willing to accept that it is out of spec when in that territory. It would be interesting to see how this receiver does on the 6M and 2M bands. I imagine a preamp would help.

I didn’t take any photos of the board during construction, I’m afraid – only when it was finished. I began building, as I always do, with the final AF amp, and worked my way backwards. This is a good way to build, as it is easy to verify correct operation at every stage of construction. If you don’t have a signal generator and oscilloscope to inject a signal of known characteristics and amplitude, and verify that each stage is operating as expected, you can still do qualitative tests, with fingers, metal screwdrivers, and a general sense of what sort of sounds should be coming out of the speaker as each successive stage is added. As usual, I pressed Rex W1REX’s wonderful MePADS and MeSQUARES into service. To mount the BPF header strips to the board, I cut an 8 pin DIP MePAD in two, and used one half at each end of the BPF. A few of the Small MeSQUARES, known as Mini Stix, were used where needed.

The final AF amp is an LM386N-4 in it’s default low gain mode of 26dB, representing a voltage gain of 20. It’s a very pleasant part when used this way. Pins 1 and 8 are left gloriously undisturbed! I made 2 small changes to Todd’s circuit here. The first was the addition of a 10uF bypass capacitor from pin 7 to ground. If your power supply is clean, you may not need this. I noticed a reduction in general noise and hash on connecting the capacitor, so left it in. Some circuits show a 0.1 uF or similar value capacitor in this position, for RF bypass but, according to the datasheet, an audio bypass cap was clearly intended. There is a chart in the datasheet showing different degrees of power supply rejection, for different values of pin 7 bypass capacitors, from 0.5µF, to 50µF. You may not need it right now, but who knows what power supply you’ll be using in the future, or what environment you’ll be operating in? 10µF electrolytics are cheap, and easy to add. Even better, try it yourself. Build the amp, leaving out the bypass cap on pin 7. Connect a power supply, plug in some earbuds, and check the difference with and without the capacitor.

The second minor change I made to the final AF amp, was to ground pin 3 and use pin 2 as the input, instead of the other way round. I had read that doing this results in slightly lower distortion. However, I have now lost the source on this, and lack the ability to make such measurements. I am wary of blindly passing on unverified “knowledge” culled from the internet, so make of this what you will. Use whichever pin you like as the input, as it is unlikely to make much of a difference in this application.

After building the amp, connect it to a power supply and a speaker, or some earbuds (careful not to hurt your ears!) and touch the input pin with a wire that you are holding, the tip of a metal screwdriver, or similar. If you have experienced the sheer cacophony that results from doing this with an LM386 in high gain mode, you will be pleasantly surprised. You’ll still hear a mixture of hum and AM broadcast band stations, but at a much more genteel level, indicative of the lower gain. The LM386 is a much more seemly part when used in this way. You’ll also notice far less noise. Joy!

After building the 2nd preamp, you’ll get more of the same buzz, AM BCB noise, and other general extraverted nonsense, upon touching the input, but louder.

Next comes the low pass filter. The values of C1, C2, and C3 determine the bandwidth of the filter, though don’t expect anything other than a very gentle roll-off. Todd’s circuit specifies values of .047µF for CW, and .015µF for a wider SSB response. Wanting this receiver to be for general purpose ham band listening, as well as having the option to occasionally listen to SW BC stations, I decided to try compromise values of .022µF. I knew that the roll-off would be slow, so figured that this would still give me a wide enough response for SSB, and wouldn’t be too objectionable for AM SWBC stations. I needn’t have worried, as the roll-off is very gentle indeed! To illustrate this, I used the N0SS wideband noise generator to inject wideband noise into the antenna socket, and looked at the audio output with the help of Spectrogram. With .022µF parts in place at the C1, C2, and C3 positions, this is what the output from the speaker jack looked like – 

The vertical red markers are at 1,000Hz and 2500Hz. The response is down about 40dB at 10KHz, and only 20-25dB down at 6KHz. For a steeper roll-off, you could add more poles, or employ an active filter. You can read how I used 5532 op-amps to make some really nice, and effective filters for my Sproutie MKII regen. However, it is well worth considering the benefits of a wide response, namely, the ability to listen to a fairly wide swath of the band at one time. This is great for general listening on a speaker when you are doing other things in the shack. With CW, even if there are several signals in the passband, you can train your ear to hone in on one of them and ignore the others. If not keen on this idea, I’m thinking that a SCAF filter connected to the speaker jack, would provide a good way to achieve extra selectivity when needed. However, there are advantages to the wider bandwidth of a relatively unfiltered direct conversion receiver. The simple RC filter in this circuit cuts out the high pitch hiss that can make listening to these receivers so fatiguing. When tuned to 7030KHz, I can effectively hear anything that comes on the air between about 7022 and 7038KHz – a 16KHz bandwidth. It’s my own aural panoramic adapter! The higher pitched signals will be lower in amplitude, thanks to the filter, but you’ll know they are there, so you can retune if you want to listen to them. Nevertheless, if I were building this again, I think I’d use .01µF caps for C1, C2, and C3, and add an extra pole, with an extra 4.7K resistor and .01µFcapacitor.

Once you’ve built the low pass filter, touching the input should give you much the same sound from the speaker as when you touched the input of the 2nd preamp, but with a lot of the high end hiss muted. Onwards and upwards! Build the first preamp, and you’ll be rewarded with the same slightly muted sound, but more of it (i.e. louder). Congratulations – you have completed a very important part of this receiver, and now have an audio amp with lots of gain, and relatively low noise. With the AF gain pot turned to full, you will hear a fair amount of noise, but remember that this is an amplifier with a lot of gain. The best reminder of this will be when the receiver is completed. I absolutely wouldn’t recommend turning the volume pot up to full with no antenna connected (especially in the lower part of the HF spectrum), then connecting an antenna, as the band noise alone will blow your socks off. To reiterate, this amplifier has a lot of gain!

In Todd’s original article, which I will say again, I do recommend getting hold of by downloading the archive of his original site, he details several different diplexers, from which you can take your choice. Some are his design, while others are those of Wes W7ZOI. I chose the (A) diplexer, which was a design by Wes. It used 2 x 10mH inductors, and a couple of 2.2µF capacitors – 

A word here about capacitors. Audio folk aren’t keen on the use of electrolytics for coupling in audio circuits, and many prefer the use of poly-something capacitors, which have a much more linear audio response. By poly-something (a prof Vasily Ivanenko coined term), I mean polyester, polycarbonate, polypropylene, or similar. Mylar capacitors are polyester, so are applicable here. For the type of audio standards most of us hams have, you are probably fine using electrolytics for audio coupling. However, since I discovered that the polyester film capacitors from Tayda Electronics are very affordable, I use them for all audio coupling applications, as well as in audio filters.

Having built the (A) diplexer, it originally appeared to be working. When walking outside in the street, with the receiver lid off, it was picking up a huge amount of 50c/s hum, which I assumed was coming from the utility wires outside, and induced in the 10mH inductors in the (A) diplexer. This was happening when all stages in front of the diplexer hadn’t yet been built, so that the input of the diplexer wasn’t terminated. I took this as a good sign and continued building. Long story short, when I finished the receiver, it was as dead as a doornail. By a process of elimination (touching inputs and noticing when the noise stopped), I strongly suspected the diplexer. However, it had appeared to be at least passing an audio signal earlier in the build, which gave me pause. It was at this point that I re-read an old blog post from Rob AK6L, and found great succor in the fact that he had experienced problems with the (A) diplexer as well. Rob plumped for the less ideal, but still perfectly functional (C) diplexer, which is what I did too. I know I should have persevered, and figured out why the better diplexer wasn’t working but, at this point, I just wanted a receiver that worked, and so capitulated. In the picture of the board that follows, you can see the reworked section where I removed the old diplexer that took up more space, and replaced it with the more diminutive (C) diplexer. The red wire that emerges through a hole in the board at the input of the diplexer, is coming from the IF port of the DBM, which is pin 2 – 

I don’t lacquer my boards any more. It adds one extra stage to the process of building, that I am keen to bypass. Nowadays, when I get the urge to build, I don’t want to add too many extra steps that might diminish my ability to stick with a project to the end. It’s the same reason why I no longer build enclosures from PCB material, when the LMB Heeger 143 fits my needs perfectly. As it happens, I only scrubbed the above board with an old Scotch-Brite pad. I had forgotten that I had steel wool pads in the house. Had I used a steel wool pad, the board would have been a lot brighter. Oh well. It is still perfectly functional. Both the LM386 and ADE-1 mixer, are mounted on Rex’s 8-pin DIP PADS, by the way. In retrospect, I do wish I had scrubbed the board with a steel wool scrubbie, so that it would be brighter and prettier. Next time.

Once the DBM is installed, you can inject your local oscillator signal and start listening, to ensure that it works. The BPF will “clean up” the signal, but you’ll still hear plenty without it. You’ll just hear signals that are on other frequencies too, thanks to the harmonics of the LO mixing with RF from the antenna. If, like me, you’re using an Si5351 or similar device for the LO, you may experience mixer products from LO spurii too, as well as LO harmonics.

Once you’ve verified that your receiver works, you definitely want a bandpass filter on the antenna input, so you can be reasonably sure you are listening to signals within the passband of that filter, and little else. It is educational, and quite surprising, to hear how much cleaner the band sounds with a bandpass filter! Being in the SF Bay Area, there are quite a few AM stations close by, both strong and medium-powered. Without a bandpass filter, there are many specific frequencies throughout the HF spectrum on this receiver, where I can hear some of these stations. Bandpass filtering removes these unwanted mixer products very effectively. If you are constructing this receiver for a single band, you can build the BPF directly onto the main board – no need for plug-in filters. So far, I have built just one BPF, for the 40M band. A NanoVNA proved very useful for tuning it for optimum results. I may build BPF’s for other bands. However, because I really want access to all of the HF spectrum, it occurred to me that would take a lot of plug-in filters! I am now looking into building a passive, tunable pre-selector. Stay tuned* for details.

(* preferably with a high-Q tank circuit )

The receiver is housed in what has become a firm favorite, the LMB Heeger 143 plain aluminum case. Measuring 4″ x 4″ x 2″ high, it is stout and, with little vinyl bumpers on the bottom, stackable. Perfect for building up a little QRP and SWL station. I get them from eBay for $15.39 including shipping (+ tax). If I want one with a perforated cover, such as the enclosure used for the Si5351 VFO, I order those direct from the factory, as no-one else seems to stock them. You pay a bit more when ordering direct from LMB Heeger. They also have these enclosures in painted smooth grey finish, as well as a black non-smooth finish (almost like a crackle, I seem to remember). I am curious to know what the latter looks like, but the plain aluminum is a “classic homebrew” look, and leaves lots of options open for later finishing – if that ever happens –

The coax bringing the RF from the antenna socket to the input of the bandpass filter, is routed underneath the board. It comes up from below, through a hole drilled in the board, as can be seen in this next shot – 

I must admit that I am bugged about two things. Firstly, that I cannot find a free WordPress theme for this blog that has clean, uncluttered lines, and that also allows for larger images. Schematics especially, need more space in order to be clear, which is why I had to resort to breaking this one up. The second thing that is bugging me concerns this project specifically, and that is the fact that I didn’t scrub the board with a steel wool pad before building on it. This build is perfectly functional, and I am happy with it’s stability, and apparent reliability. I just wish the insides showed a little better. I need to get over this.

Perhaps it looks better in black and white………..

The front panel is simple, and very plain. Speaker/earphone jack on the left. AF gain control on the right. One of these days, I’ll get a Dymo or Brother label-maker, to complete the classic homebrew look. It will also help anyone who might inherit my homebrew efforts in the future, to know what they have, and which knob does what! – 

On the rear panel are, from left to right, the antenna jack (BNC), the VFO input jack (SMA), and two 12V DC connectors. They are connected in parallel, so that one power cable can supply the circuitry in this enclosure, and a short power cable can run from the other connector down to the VFO, mounted directly underneath – 

A shot from the rear, showing the interconnections between the Si5351 “VFO” on the bottom, and the receiver on the top. Having the mainframe up top makes it easier to pop off the cover to change bandpass filters – 

Each case is 4″ x 4″ x 2″ high, so the stack is 4″ x 4″ by a little over 4″ high. The Altoids tin and playing cards are for scale – 

This is a practical and useful little receiver. Many receivers of this type just drive headphones. For me, having a receiver that can easily drive a speaker makes a huge difference in the amount of time spent listening. Since building it a couple of weeks ago, I have listened every day, for virtually all the time I have been at home. I couldn’t have done that on headphones. Thank you Todd Gale!

Please note that the above photo is of my SW200, though Rod’s no doubt looks exactly the same.

 

A few months back, I received this note from Rod KQ6F –

In this post from May of last year, I detailed the construction of a 1mW solar-powered HiFER beacon. I named it the Boris Beacon, in tribute to my neighbor’s cat. The beacon was never mounted permanently outside. I kept it indoors, powered from a small solar panel in the window, and feeding an “antenna” of sorts, consisting of the original dipole wires folded up into two small bundles. Obviously, I had no serious intention of it being heard by anyone; I just liked having it come on every day when the sun came up, and transmitting until later in the day, when the light was too low to sustain operation.

Recently, another location became available in my house that seemed like a good place to install a beacon outside. The Boris Beacon was still in operation from inside my apartment. Moving it outside onto this first floor balcony and spreading the dipole legs would be a straightforward task. You’ll notice from the original post on this beacon that, in attempting to seal the holes where the leads entered the enclosure, I used Plastidip. It’s a rubbery solution that sprays on. It’s great for some applications, but not for this one, as I ended up getting the rubbery liquid all over the enclosure. I do like my projects not to look too messy, so for this new iteration of the Boris Beacon, I moved the circuit board into a new enclosure –

Here it is, close to it’s final installed position, on a first floor balcony (Edit – I just noticed, after a year, that I should have called it a second floor balcony. In the UK, where I haven’t lived since I was in my early 20’s, we call the second floor the first floor, and the first floor the ground floor. I guess old habits die hard!) –

In it’s final installed position. The solar panel is fixed to the top of the wooden railing with 2 wood screws, as is the beacon enclosure. The dipole is stretched out behind the wooden  fence at the top, and then trails down onto the balcony floor in one direction. In the other direction, it is attached at the other end to the wall of the house, so is partially elevated –

A close-up view, showing the silicone caulk around the entrance/exit holes. The underside of the lid has a foam weather sealing strip embedded in it, which can be seen in the original post, linked to at the beginning of this post –

I was unsure how impervious the little solar panel would be to the elements, so I caulked around the edges. If it fails, these kinds of low wattage panels are cheap and easily available anyway –

The panel I’m using is a small 1.8W one, intended for use as a 12V battery maintainer –

It is probably overkill, but I popped a silica gel packet in the enclosure, to mop up any excess humidity that might find it’s way inside. The dessicant turns pink when saturated, and is blue when dehydrated and ready for action –

Another view, with the gel packet flipped –

The beacon sends the letters “BRS” at 10wpm, with a break of 3 or 4 seconds between the end of one transmission and the beginning of the next, with a mighty power to the dipole of about 1mW. The frequency is a nominal 13556.9KHz (13.5569MHz), which varies either way by a few tens of Hz, depending on the outside ambient temperature. I will be overjoyed if anyone, anywhere hears it! There is no battery, so it transmits during daylight hours only. It comes on about half an hour after local sunrise, and goes off about half an hour before local sunset. I’ll update this with more accurate information, as I observe the on and off times over the next few days.

The Boris Beacon is definitely a successful project. I just need someone to hear it. Even one person will do! If we were allowed to run 100mW on this band then getting spots would be much easier. In fact, if the dipole were situated more up and in the clear, that would help too. As it is, 1mW into a compromise dipole will make this little beacon a super DX catch. I don’t know how long it will remain in operation, as the long-term future of my current living situation is in doubt. I suspect that it will be up and running for much of 2019 though. I will update this page if and when it goes off the air.

Reception reports greatly appreciated!

EDIT –

Almost a week later, and it seems to be faring well in the rain, although it’s early days –

Rain was pooling on top of the panel and although it’s supposed to be weatherproof, I’m not too sure how waterproof this panel really is –

I raised one end slightly, to help a bit –

Still no reports!

EDIT – As of Aug 2019, the BRS beacon is off the air, probably permanently. The space from which I was operating it from is no longer available. It was put to sleep, having received not one report. I put it down to two things. Firstly, it was active during a period of particularly poor HF propagation. Secondly, the power was around 1mW. Even so, I was hoping for at least one report. I think it would have been worthwhile to have reprogrammed the chip to send QRSS.