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.

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!

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.

Back in May of this year, Sheldon N6JJA began sending me information and details of his version of N1BYT’s WBR regenerative receiver. It went through several iterations, before ending up at the final version as shown here. Even this version is still a work in progress – as all good products of experimentation are. Sheldon took the original WBR circuit, as described by N1BYT, and made a few changes. Firstly, he added a preselector. Regens are well-known for having poor strong signal performance. A pre-selector can’t help with very strong signals close to the received frequency, but it may well help to protect the fragile front end of a regen from very strong signals away from the frequency the regen is tuned to. Secondly, a tunable preselector is a very handy tool for anyone who is experimenting with simple receivers, whether direct conversion, regenerative, or simple superhets. It could be well worth building this as a separate unit, for future use and experimentation. Of great interest also, are the changes that Sheldon made to the classic WBR circuit. He describes them in some detail in this article. It makes for a good read, and may encourage you to try building it for yourself. If you do, please let me know how it goes, as I haven’t built this version of the circuit yet.

Sheldon, amazingly, found enough time away from his very busy silicon valley job, and responsible position as a first-rate cat dad, to write up this project as a complete article. Rather than attempt to interpret his words and re-write them in my own style, I’d rather that you get all this great information straight from the cat-dad’s mouth so, without further ado –

(Oh, one more thing – check out the way he draws his schematics. Accurate and beautiful!)

 

How it started (by Sheldon N6JJA)

In 1957 I was in second grade in a small town in rural Illinois. Our “library” was a bookmobile that came through once every two months. But even that long ago I was completely in love with all things radio and electronic, so when Alfred P. Morgan’s book The Boy’s First Book of Radio and Electronics appeared I checked it out and devoured it over the next two months. For me, the centerpiece of the book was a design for a simple, one-tube regenerative receiver. My desire to build such a thing knew no bounds, but a lack of money and parts made it a non-starter at that time. I became a ham in 1965 and my attention turned to more modern equipment and kits to build things, but I never got over that old regen. Fast forward eight more years and I’d gotten my hands on my first “boat anchor,” an SP-600-JX7. Alas, I was once again in love, but the radio wasn’t mine to keep, and I again resolved, “Someday…”

Oddly enough, that’s all part of how and why the WBR-Oscar came to be. Over the past few years I’ve been buying and restoring a variety of “boat anchors,” and now have a lot more than I can keep. And that radio-crazy second-grader wound up with a Ph.D. in electrical engineering and a huge junk box and some decent test equipment. So, somewhat naturally, my first major project last year was to build something to help those old radios perform with a little more pizazz. It started as a wide-range antenna coupler, then added a preamplifier that became also a preselector, then added an audio amplifier, then a DSP filter…well, you get the idea. One of our cats, Oscar, helped me with all of this, making sure that now parts got included that hadn’t been checked for obedience to the laws of physics, particularly gravity. As you can see in Figure 1 below, Oscar became the name of that “helper” unit. Oscar sits proudly atop his namesake.

 

I considered, for my next project, that one-tube regenerative receiver. But then, along the way, I came across an article from QST from August 2001 for a regen that I’d started to build but never finished. (Life gets in the way sometimes.) The article was by Dan Wissell, N1BYT, and titled “The WBR Receiver.” (Citation at the end of the article.) Rummaging through my collection of parts I decided that this was the right project at the right time. Mr. Morgan’s radio would wait a bit longer, but my desire to build a regen was finally going to be fulfilled.

Over the years a lot of these receivers have been built, and the results have been mostly good, it seems, even spawning a lot of designs based on the WBR detectors but without the WBR tank circuit. Lately, thanks to the ongoing blog of Dave Richards, AA7EE, interest in this design has been renewed and some of the design’s deficiencies noted and, to a certain extent, addressed by experimenters. Now, however, buoyed up by the information in Dave Richards’ blog and some other QEX articles, I decided that it was time to put my own spin on things and see how far I could push the design. The result is the WBR-O receiver, and it now covers, fairly easily, 6 to 15.6 MHz with a single tank circuit, making it now a true “40-30-20” meter amateur and SWL radio. As an additional feature, the “O” (for Oscar) part refers to a preselector/preamplifier that both isolates the input of the WBR circuit and adds front-end selectivity and some amplification. The preselector/preamplifier actually tunes from 80 to 10 meters (in two bands), includes widely adjustable gain, and can easily be built as a stand-alone for folks who only want that part. The photo in Figure 2 shows the pair as built.

The design you see in the photo is actually the culmination of months of work and numerous revisions and tweaks. Some things helped, some didn’t, but I learned a great deal along the way. I also, on purpose, used several “lousy” construction techniques to convince myself that, simply, “If I can build this, anyone can build this.”

This article is broken into two parts. First comes the “Oscar” preselector/preamplifier. As I said, I intended it to be either part of the overall receiver or used as a standalone where desired. The second part deals with the WBR upgrades. Both designs were built using the same techniques and I’ve tested both and found that—especially in concert—they do about as well as some of my boat anchors! So if your soldering iron is ready, I’ll start by describing “Oscar.”

Part 1: Oscar’s Preselector/Preamplifier

Figure 3 (above) shows the circuit during testing on my bench. To the left I placed the coils, clustered around a small relay (Panasonic TQ2-12V). To the right is the amplifier portion of this circuit.

Figure 4 (below) shows the schematic.

Now before you say to yourself that it looks pretty complicated, let me point out how simple it really is. Each “band” has its own bandpass filter that it tuned by one or more Toshiba 1SV149 varicap diodes. The 1SV149 is a little gem that was developed for AM radios but is now obsolete. In spite of this, that diode is plentiful and inexpensive on the internet (I got mine from eBay, about 50 for $10, but Amazon sells them, too, as does Minikits.com.au.) An important item to note here is the value of the series isolation capacitors for the diodes, C1 and C3 in Fig. 4. I use 0.1 μF, 50V ceramics with an X7R stability rating. The large capacitance is actually a must; as the value goes down, the interaction between those caps and the rest of the circuit becomes a problem. The relay is something that I had in a drawer, and made the layout a bit easier. I’ve also built “Oscars” using rotary or toggle switches to switch bands, by the way, so while the relay is nice to have, it isn’t required. One thing to note, however, is that, with the relay, the overall design lends itself quite nicely to a remotely-tuned preselector that can be mounted right at your antenna. So far, nearly everything I’ve built along these lines has worked. The amplifier in Fig. 4 is based on a low-noise MMIC pair in a push-pull arrangement that keeps distortion and unwanted harmonics down a bit. The MMICs are Mini-Circuit Labs MAR-6SM+ devices. At a maximum of 16 mA per device they offer gain of about 20 dB and noise figure around 2 dB. Quite impressive for so simple a device. The 1:1 transformers are also from Mini-Circuits, their T1-1+. All of these components offer good technology for a relatively low price. The relay (from Digi-Key, for example) is $3.88 in small quantities. The MAR-6SM+ is $1.40 each, but the minimum is 20, so either consider a lifetime buy for $28 or split the batch with a friend. The T1-1+ is $3.25 in small quantities. If you’re up to it, go ahead and wind your own transformers on small type 43 ferrite toroids. A simple bifilar winding should work, and there are usually design guidelines in articles on baluns and transformers to help you decide on a target inductance. Actually, I found that building my first “Oscar” made me make sure I had enough parts—including transformers—to build more of them. The circuit has become somewhat my “go-to” front end for things. I should add that I’ve also built the amplifier section with a single MMIC and without the push-pull transformers, and it still works okay, if you want to minimize or simplify things. The circuit then looks more like the C5-U1-C6-R6 cluster in Fig. 4.

 

Table 1 (below) gives my winding data for the toroids, including those for the WBR receiver part. I like the website toroids.info to help me design the coils, then I use another best friend—a Peak LCR45 meter to verify results. I find that, even knowing that each time wire passes through a toroid counts as a “turn,” I wind up removing a turn or so once I measure things. Frankly, this whole project would be a lot more difficult without the LCR meter, and once you use something like this I suspect you’ll be hooked as well. The bandpass filters don’t do nearly as well if the coils aren’t either the correct values or reasonably well matched.

Good thing to remember: the MMICs and diodes are sensitive to ESD. Not horribly sensitive, but you will want to be careful, since it will save you headaches later on. Sometimes working on some aluminum foil or an inexpensive ESD mat is plenty, also making sure you touch the foil or mat before handling a part. Overall, in my own experience I’ve found these parts to be pretty robust.

Depending on the available PCB space you have, you might want to experiment with the general layout on perforated PCB material and then make a sketch or photo of your final design to guide you in construction. Part of the fun of this project is that we all tend to do things differently and put our own “fingerprint” in the final result. Remember, although I’ve done a lot of up-front work to guide you, what you build will be your own to be proud of.

My own usual RF build technique is to use 0.10” center perforated PCB material with plated through holes and cover one side with copper foil tape to make a ground plane. The boards I used in this build are 7 cm x 9 cm, and as you can see, I have plenty of room. (I should also add that the last revision of the WBR was done on a board about half the area.) Then I use an Exacto knife (or something like that) to carefully cut away portions of the ground plane where necessary. Figures 5 and 6 show this in more detail. Then I use tweezers and solid wire-wrap wire (28 or 30 gauge, stripped first as needed) for the interconnections. I solder components to the PCB and leave about ¼” of lead projecting on the underside of the board to provide for the interconnections. These are made using the tweezers (and maybe a magnifier) to wrap wire on one lead, anchor it with solder, and next do the same at the other end. (I call this “Compact Wiring.”) This keeps the wiring neat and compact and still provides an excellent RF ground. However, to be honest I’ve also used “dead bug” and even less glorious methods of construction (even using longer wires), and just about everything works as long as you’re careful.

Mounting the MMICs is the only thing where I needed to think hard about “how to do it.” In Fig. 6 you can see the MMICs, mounted on the ground-plane side (the “underside”) of the board, but mounted “upside down” with respect to the photo. I drilled a hole in the board for each MMIC, just large enough for it to rest in the hole, allowing the protruding pins to just touch the adjacent plated through holes and allow me to make good electrical connections.

Turning back to the schematic, you’ll notice that I use small voltage regulators to keep things stable and quiet. The LM317LZ is an inexpensive part (about 40 cents apiece from Digi-Key) that can handle up to 100 mA of load. U3 is used to provide the tuning voltage required, and U4 controls the gain of the MMIC amplifier by varying the voltage presented to the current-limiting resistors feeding the MMICs. I’ve taken to using these ICs in virtually every project. They simplify the design work and are very flexible and stable. Add to that, they offer some low-pass filtering effects that can reduce the hash from cheap power supplies. No miracles, but good engineering.

I recommend coaxial cable input and output for this circuit. I bought a bunch of PCB-mounted SMA connectors a while back, and you see them in Fig. 3. But there are so many different ways of making these connections—especially if you choose to do the switching via a panel-mounted switch—that no builder should be intimidated by the technology.

I used a 10-turn pot for the tuning and a single-turn pot for the gain. Both are linear in design and do not need to have a wattage rating over 1/4W. But a word about choosing potentiometers. I’ve used cheap ones from the Pacific Rim and higher quality ones from US suppliers. In my experience, especially in the tuning pots you tend to get what you pay for, although the cheaper ones can be used if you are careful how hot they get during soldering. It seems that the inner workings of the less expensive ones are more susceptible to heat and can “quit tuning” if you do a lot of “cut-and-try.” If you do buy the cheaper ones, buy several.

Results

Figure 7 shows my measured results, using a Rigol DSA815-TG spectrum analyzer and subtracting the tracking generator output level. That means that the circuitry was tested with a good, solid 50 ohms input and output, but the circuit still works well in my shack with antennas not very well matched. The bandpass curves were taken with a “gain” setting of about 10 dB. As you can see, the curves are sharper on the low end of each band, but there’s also more attenuation there, about 4 dB less on the low band, and about 5 dB on the high band. The sidebar on “Designing your own Oscar bandpass filters” talks a bit about this, but once you start designing for Q values above about 10 these effects are normal. But a preselector that maintains Q over 10 over this range should produce a workable unit that provides some selectivity without having to constantly readjust things every time you change frequency by a couple of kHz.

 

 

 

 

 

 

At the bottom of Fig. 7 is the gain range curve at 14 MHz. With the design as I’ve built it, I can bias the MMICs to be “just barely there” and provide some signal reduction or to blast things with an additional 15 dB or more.

Now I chose to do this in 2 bands, but you may decide on 1 or 3 or anything else. The MMICs don’t care, as long as they think they’re seeing roughly 50 ohms at input and output. But be aware that the MMICs also will amplify anything from DC to 6 GHz! Using them without some form of bandpass in front will suck the amplification headroom from where you want it to someplace you don’t. And if you decide to make things simpler, using a single MMIC is a straight-through configuration works very well also. I’ve tried a lot of different configurations, so these are just my guidelines based on experience.

About all that remains is to hook this circuit up to a receiver and see what happens. Don’t be afraid to experiment. The fun of building something like this is that there’s a lot of room for changes, improvements, and growth. For instance, the gain section in the Oscar design is custom-made for a homebrew AGC circuit, and maybe someday I’ll give that whirl. (If you don’t beat me to it!) The next part of this article will expand on what I’ve written here to build the companion WBR receiver, but I’ll also refer back to this design to cover some of the construction guidelines I’ve mentioned here.

Part 2: The Upgraded WBR Receiver Design

As I mentioned in the beginning of this article, both the Oscar preselector/preamplifier and the upgraded WBR receiver are designed to work together or be built as stand-alone units. Figure 2 shows both units assembled with standoffs between them right before integration into a chassis that has been the final home for all the previous design iterations.

So what’s changed? Well, if you look at the schematic for the RF deck in Figure 8, it might seem that not much has evolved since N1BYT’s article back in 2001ⁱ. However, a lot of tests and calculations led to some significant changes. But rather than just list the changes, let me take you on a brief excursion as to why they were made.

Online blogs and chat lines have talked about different variations on the WBR theme for a while. Often there are a few things that everyone seems to agree on, like the fact that AM sensitivity is very low and that the near-zero input impedance is needed to block strong signals, and that it’s difficult to get more bandwidth out of the design, even if mechanical tuning capacitors are used. Sometimes it seems to be nitpicking, but I decided to dive in and see how much pizzazz I could give the basic design.

First, about the sensitivity. The WBR isn’t a “normal” regenerative detector design, and this gets overlooked sometimes. It’s actually a regenerative Q-multiplier with an infinite impedance detector (IID). When the Q-multiplier is oscillating, the available signals to the IID are quite a bit stronger than when the Q-multiplier is set just below oscillation threshold, as in for AM reception. But look at the IID part of the circuit. It’s actually a source-follower and thus offers little chance for any amplification. IIDs have been around in tube circuits for years, favored by AM aficionados for their excellent low-distortion detection characteristics. So when you’re trying to listen to AM signals, you’re going to need a fairly good audio preamplifier. That’s Q3 in Fig. 8. I’ve looked at a bunch of WBR-like circuits online, and there are several good (and not-so-good) preamplifiers to consider. In the end, I designed my own. It has pretty good gain (about 120-150) and doesn’t use parts that aren’t easily available. I should mention that the value of C12 is important, though not critical. I use a rather large capacitor there, 100 μF, and that really helps keep the gain up over a nice audio range. Probably anything over, say, 47 μF should work.

Next, the “front end,” or lack of it. There are 2 points to make here. First, that’s why the Oscar preselector/preamplifier became part of my own design. Second, that 1” piece of wire in the original design by N1BYT helped “balance the Wheatstone Bridge,” but many builders have fiddled with that, adding a little inductance at that point. I tried that too, but finally got an idea that worked better. It’s a simple single-turn link immediately adjacent to the L1 center tap (which is connected to that 1 to 1.5 inch wire to ground). Feeding the 50-ohm sourced signal in through that tap turned out to be about a thousand times better than the “old” way. (See the addendum for technical details.)

I spent a lot of time working to get the design to work reliably over the roughly 10 MHz span that it finally achieved. The capacitance range of the Toshiba 1SV149 varicap diodes really did shine there, but early attempts to tune below about 8 MHz weren’t successful. In the end, though, what’s in the final design will probably tune below 5 MHz if I spent more time on it (and maybe even cover 80 to 30 meters or something like that). That’s where a lot of changes found their way into the design, but let me talk about them individually.

First, I was initially using garden variety (i.e., “flea market”) 2N3904 transistors for the Q-multiplier. Frustrated at the limited tuning range, I first increased the “gain” of the oscillator by increasing bias voltages from 5 volts to 12 volts. That helped, but not enough. In the end I found that the h_FE for that transistor (i.e., its DC gain) is vitally important. My weak oscillator used a 2N3904 with h_FE of about 85-90. I found a first-quality one with gain closer to 180. Aha! Much better performance. I got down to about 6.5 MHz at the low end.

But, to make a long story much shorter, I found an excellent, and even better, alternative. It’s the BC546CT from On Semiconductors. At about 10 cents apiece (about the same as the 2N3904) it offers the same basic qualities of the venerable 2N3904, but with an average (sample of 40) h_FE of 553! A batch of first-quality 2N3904s had an average h_FE of only 189. The pinout is the opposite of the 2N3904, but otherwise it was just a drop-in and now there’s no trouble with making a Q-multiplier that will oscillate easily just about anywhere, and I don’t have to measure h_FE endlessly to find a winner.

One experiment that also helped was to try different values of C2 and C3. You see, the feedback network in this oscillator (that’s the basis for the Q-multiplier) works fine for some values of C2 and C3, but as the frequency goes down, the losses in the feedback loop go up and can prevent oscillation. Dropping C2 and C3 to about a quarter (to 100 pF) of what I used initially (330 to 390 pF) did the trick. I’d recommend using small ceramic caps with an NPO or COG stability rating. Pushing this oscillator down further in frequency might entail re-tweaking those capacitor values, but that’s just part of the fun. One additional benefit from using the small capacitance is that the amount of regeneration bias required over the whole frequency span stays much more constant that when using the “older” values. Oh, and one last thing: in the original design the base of Q1 was biased using a more classic resistor pair, one carrying regeneration voltage, the other to ground. I removed the one connected to ground. What you see here works much better.

Now for my secret weapon. The little trimmer capacitor, C4, is unusually important. After staring at the original schematics for hours it occurred to me that the “balance” sought is almost impossible with the components used, no matter how carefully one measures things to force it to happen. Once I get a circuit oscillating, I tune to the low end of the frequency band and find where regeneration quits (you can hear it in headphones), even with maximum regeneration bias. Then I slowly tune C4 and make sure I have the headphone volume way down. Maximizing the oscillator strength with C4 is a set-once-and-forget adjustment, but it overcomes the last hurdles to giving this design its performance. Any trimmer that is small enough to fit and covers the 50-80 pF range should work. It just has to be near twice the value of C7 to be effective.

All That Bandwidth Presents a Problem

One thing became clear when operating this receiver. Even a 10-turn tuning pot made things dicey. I even found some Bourns “Digidial” counters on eBay and they helped, but not enough for this much bandwidth. I also played with several “bandspread” ideas using two tuning pots before deciding that instead of a bandspread control I could split the tuning range into, say, 6 pieces (I had a DP6T switch). This feature is included in Figure 9 along with the audio section built along with the RF deck. Again, I rely on the LM317LZ regulator, here in a paired arrangement, to set the ranges over which you can tune. Now signals are easier to separate, even with the smallish knobs on 10-turn counting dials. But taking this feature one step further (I encourage you to consider this), a builder might want to skip the “shortwave radio” aspect of the design and adjust the tuning ranges to cover only the 40, 30, and 20 meter bands, a very easy thing to do.

Table 2 lists the resistor choices I arrived at, both for the “shortwave” and “amateur-only” versions. I’d caution you to be prepared to build first and add resistors later. Your circuit might want resistance a little different from mine (and yes, small trimpots would make this a breeze).

 

Building It and Operating It

As you can see from the photos, the WBR section uses the same build technique (“Compact Wiring”) that the Oscar unit does. I encourage any builder to feel free to experiment with how the layout comes together and how the wiring gets done. What I’ve shown here is just my own way of doing things. Keeping the RF wiring reasonably short is a good goal, as is providing a good ground plane. Apart from that, all the versions I’ve built work (as long as I don’t forget a connection!), so there’s plenty of room for personal variations.

Figure 10 shows everything in my “WBR-O” box. You might note how the box has extra holes and knobs that are leftovers from all the previous versions. I included the ubiquitous 1k pot at the antenna input, but in hindsight, it isn’t needed when I use both circuits. I also decided to use a small 12 volt supply that fits in the underside of the chassis. It’s a good quality and lacks the persistent hash that some cheaper supplies produce. Right now I’m using a Delta PMT-12V35W1AA from Digi-Key. Small, quiet, safe, and at about $15 for a universal AC input it’s a bargain, really.

I’d recommend a 10-turn pot for the regeneration control. Regeneration at low frequencies is higher, dropping somewhat as you increase frequency. You’ll have to develop a feel for this adjustment. With the new biasing I’ve included, it might be possible to use a single-turn pot, but that’s just one more experiment for the future. The key thing when getting started with a regen is that you want as little regeneration as possible. I still have the bad habit of leaving it too high then wondering why things don’t sound right. Overbiasing the Q-multiplier just adds distortion, and even harmonics. Regeneration also varies the frequency a bit, so tuning in on a single CW station, for instance, will require a little practice, but in the end it becomes second nature.

Table 3 lists a few of the more critical parts and their Digi-Key part numbers. Although I use Digi-Key as a supplier in a lot of the electronic work I do for a living, they are also quite amenable to serving the needs of individuals as well. The same can be said for Mini-Circuits and Amidon. Prices are reasonable and you get to choose the quality you can afford.

All that being said, the final product was gratifying to use. Just about anything one of my other older high-quality radios can hear can be heard by this little gem. Of course, selectable selectivity, a noise blanker, and good AVC would help, but…that’ll come later, I think.

With all the changes I’ve made the one ingredient I hope I have added most of all is flexibility. The real possibilities of what this basic design can do have only been barely touched. I think it would be excellent, for instance, to see how small one can make the entire unit, so it fits into camping gear or such. Also, why not 2 oscillators (sharing the same tuning voltage) instead of one, and use the second to drive a small transmitter? Or a second oscillator to drive a frequency counter so you can actually see where you’re tuning? Or using what I’ve explored here on other designs, like AA7EE’s “Sproutie” regenerative? Or add some good audio filters? Or, as I said before, adding AVC via the gain control voltage on the Oscar circuit? Or…well, you get the point. This is definitely not a software defined radio, but an imagination defined radio, and, as Oscar would note, perched high above me on my equipment, the sky’s the limit.

—–

Sheldon Hutchison, N6JJA, has been a licensed amateur on and off since 1965 and currently holds an Extra ticket, a Ph.D. in Electrical Engineering (University of Illinois) and is an ordained Episcopal priest. He and his wife Eileen, KI6UZJ, live and work in the Silicon Valley with their cats, including Oscar and several other “helpers.” Dr. Hutchison works in the laser industry (while also a retired but active priest) and Eileen is employed in the Valley’s aerospace industry. An avid experimenter, Dr. Hutchison also enjoys restoring old “boat anchor” receivers, and currently—according to Oscar—needs to find homes for a few of them to give his helper more room to play.

 

ADDENDUM

File this under “Can’t leave well enough alone.”

There’s something in electromagnetics called “reciprocity.” Basically, it means that if signals get into an antenna or circuit efficiently, they get out just about as efficiently. Taking long walks at noon helped me find ways to use this phenomenon. For my “Oscar” receiver I found one. I’ve long used my spectrum analyzer, clipped to the low-impedance tap on L1, to indicate the health of the regenerative Q-multiplier by making it oscillate and observing the relative strength of the signal. Well, in the schematic below I experimented with adding a link right next to the low-impedance center tap of L1 as a way of improving coupling to the Q-multiplier from a 50-ohm source. Compared to the previous way of coupling to the center tap the signal measured on the spectrum analyzer shot up by 30 dB! Adding 2 turns is too much. Moving the link too far away from the tap makes oscillation much more difficult. And after adding the link, you’ll want to go back and re-tweak C4. The photo also shows the red wire link as I installed it.

Now Oscar’s even happier.

To many, this will be just another Si5351 VFO project, with nothing to distinguish it from the others. In fact, that’s exactly what it is. The “how to” of connecting an Arduino board to an Si5351 board, wiring up a display, and loading the firmware, is straightforward, and well established. To me though, it was a complete mystery. I have been very adept, my whole life, at studiously avoiding anything to do with digital electronics, computing, coding, and the like. When my friends in school were getting excited over Sinclair ZX81’s, BBC Micros, and Commodore 64’s, I was building a one-tube regen, an 80M DSB transceiver, listening to my British military R107 shortwave receiver, and talking to hams on the local repeater on my converted Pye tube VHF base station. I remember wandering into a Tandy store (what us Brits called Radio Shack) in the city of Worcester at some point in the late 70’s, and being greeted by the sight of a Tandy computer – probably a TRS-80 or something similar.

“What does it do?” I asked the salesperson.

“What do you want it to do?” he replied.

This seemed like a strangely non-committal response. Maybe he didn’t know what it did, and was merely throwing the responsibility for finding out back on me? I don’t remember anything about him now, but perhaps he was some gangly teenager who knew little about the stuff he sold, and whose main thought was getting off work so he could go to the pub with his friends? That’s it! He was just trying to appear knowledgeable by giving me a non-answer! This suspicious reaction was quite representative of the way I thought about computers back then. Just as it’s hard, if not impossible, to get the measure of a person if they willfully refuse to reveal anything of themselves to others, so it seemed with computers. These expensive boxes just sat there, doing nothing, except waiting for instructions. Such a disappointing lack of character! How is one supposed to respect a person or an object that sits quietly in a corner, waiting to be told what do? How feckless! Dedicated hardware, however, was different. When you bought a radio receiver, you knew that, on twiddling a few knobs and flicking a few switches, it would receive radio signals. A burglar alarm would alert you to the presence of burglars (well in theory, anyway), and those remote control cars that RS sold by the gazillion were guaranteed to quietly drive your family nuts in the days after Christmas before work, and school, resumed. Computers, on the other hand, promised everything but actually did nothing, until you told them what to do – and even then, there were a myriad of ways in which they could obstinately refuse to comply with your wishes. Not for me!

And so it was that, throughout my adult years, I deprived myself of exposure to things digital. I am not proud of my incurious nature about many things – though, when I am interested in something, I exhaust myself with the sheer intensity of focus. It’s an odd type of blinkered approach to the world that leaves others confused. I can’t say I blame them. The projects I built ran off anything from a few volts, up to 15-20V or even more. 9V batteries worked fine (we called ’em PP3’s), as did the old 12V lead acid battery that would no longer power the family lawn mower, but did a sterling job of powering the radio gear in my bedroom. My circuits weren’t picky about voltage, but these new-fangled digital chips just wanted to see 5V. Really? What kind of a voltage was that? They came with a surfeit of incomprehensible nomenclature too. Words that sounded like something John Lennon would have made up for a song post-1965. Words like NAND. You know, it wasn’t so much that this stuff wasn’t interesting – it was simply that I was really into building little radio receivers, and didn’t see how this digital stuff could help me (I wasn’t very imaginative). I had a small stash of ferrite rods, variable capacitors, resistors, and transistors, and some 9V battery snaps and with that, I had all that I needed. These were the days when loading a program onto a computer meant playing an audio cassette into the “line in” jack of your computer. I just didn’t see how any of that world full of DOS, NAND gates, tiny amounts of RAM, and the like, as well as really weird voltages like 5V, could possibly apply to me. Yes – I was that closed-minded. It’s not hard to see how a few years later, when we all graduated from University, my colleagues went on to successful careers with big technology companies, designing integrated circuits, and building the backbone of the internet, while I moved to Los Angeles and promptly became a DJ

A few years ago, a friend generously gifted me a Bare Bones Arduino board. I didn’t know what it was. It had header pins sticking out of it but, at that point, I didn’t really know what header pins were, or how to connect to them. I looked at it, and wondered what to do with it. What did it do? How did it do it? What was I supposed to connect to it? I placed it carefully in a box along with some other electronic things that confused my simplistic analog mind, and carried on with my life. Every now and again, I’d take it out of the box, blink at it a few times, and put it back. I knew that Arduino was the new big thing, and something that was going to play a big part in ham radio homebrewing in the coming years but I guess that, with my toroids and air-spaced variable capacitors, I wasn’t ready for it yet. Not that my experience level in this arena was completely non-existent. I had taken part in the beta tests of the Etherkit CC-20 and later, the planned CC1 series of QRP transceivers. These experiences had taught me that I could solder SMT devices, and even replace an SMT ATMega328P with nothing more than a soldering iron, soldering wick, flux, and a sharp blade. It was a small revelation to learn that I could do this stuff. Jason NT7S very patiently walked me through the process of flashing the firmware onto the ATMega328 via the ICSP header mounted on the board. This was a first for me, and quite exciting to gain a new skill, which proved handy when I built the SPT “Sproutie” Beacon, and needed to flash firmware onto the ATtiny13 in that little transmitter.

Then, recently, I took the Bare Bones Arduino board out of the box which had been it’s home for a few years and, this time, something clicked. “Goshdarnit” I thought, “I’m going to make an LED blink. If others can do it, so can I!

I spent a few days and nights with the LED blinking and pulsating at various rates, as I loaded different sketches, and adjusted the parameters. As fun as flashing lights are to a simple lad like me, it wasn’t the reason I wanted to resurrect this little Arduino board from it’s relaxing life in storage. I had an Etherkit Si5351 Breakout Board that needed to have life breathed into it. I wanted to generate RF, by golly!

This next stage was where things started to come into focus, and it began to dawn on me that using one of these little breakout boards to generate a stable RF signal wasn’t all that hard at all – well, from the point of view of the end user, at least. Once I did a bit of reading up on how to control the Si5351, I was just a little gobsmacked. You mean all it needs in terms of data input is 2 connections? SDA (serial data) and SCL (serial clock)? That’s it? I made those 2 connections between the Arduino and Si5351 board, uploaded the Etherkit Si5351 example sketch, and almost fell off my chair when the Si5351 began emitting RF on the frequency I had entered into the sketch just before uploading it. It was a moment of realization – that this little board actually was a programmable oscillator. How incredibly neat! No more custom-cut crystals – for ~$10, you can get a board like this, and program it to the frequency of your desire (within it’s specified limits), and it replaces both the crystal and the oscillator. For a single frequency, once you have programmed it, it doesn’t even need a micro-controller connected to it.  Fantastic!

I suppose that was the moment at which my mind, which moves at the speed of molasses, “got” that a VFO with this board is really a micro-controller which, with the help of a rotary encoder, is re-programming the Si5351 “in real time” as the encoder knob is turned. Every single click of the encoder sends a new instruction to the Si5351, to step up or down, in an increment which the firmware has already specified. I was hot to trot, so began looking around for a basic Si5351 sketch. The word “sketch” reminds me of the Etch-A-Sketch which I never came close to mastering as a child. I must admit that I think this association slightly trivializes Arduino programs (which are written in a type of C) in my mind, but that is what they are called, so that is what I will call them.

What I was looking for was a sketch that would allow me to vary the frequency of the Etherkit Breakout Board continuously in the HF region, from at least 3 – 30MHz. I was only concerned with one of the 3 clock outputs. At this point, I simply wanted to use it as an HF signal generator for testing purposes, or to control a general coverage direct conversion receiver. Perhaps at some point, I’ll begin fiddling around with code, and learning how to modify it for my own purposes, but at this point, I wanted a sketch that I could upload to the programmer, and immediately be in business. I also wanted to use one of those tiny little OLED displays, due to the enclosure I was considering.  This was the point at which I found Thomas LA3PNA’s sketch entitled, “A simple VFO for the Si5351 for either LCD or OLED.” Perfect!

This was also the point at which I discovered that the ATMega168 in my little Arduino clone board didn’t have enough memory to hold Thomas’ VFO sketch. I considered purchasing a newer Arduino board or clone, but most of the ones I saw had more stuff on them than I needed or wanted, in terms of inputs/outputs and programming ability. All I wanted was the ATMega328P, and a 6-pin ICSP header to program it with. Then I remembered back to my time working on the Etherkit CC-20 beta, and how I had expertly fried the micro-controller. Jason sent me a replacement and, wisely, included a few extras, in case my prowess at destroying delicate chips were to reassert itself. I still had those little SMT ATMega328P’s lying around, as well as a supply of breakout boards to mount them on. Problem solved! Building something from parts on hand is so much more satisfying than purchasing a ready-made solution – at least, for the first time, it is.

I sat down to scribble out a schematic, and it was during this process that the realization hit, as to what an Arduino board is. What makes Arduino, well, Arduino, is not the board, but the software platform that supports it. Apologies for stating what is well known fact to many readers, but this had all been previously unknown to me. The board itself is really just a micro-controller, with the power supply and input/output options either suited to the tasks at hand or, in the case of a larger and more general purpose board, such as the Uno, many different such options, to make it as versatile as possible. Ths was fantastic, because what it meant was that all I needed to control the Si5351, was a micro-controller (ATMega328P), a 16MHz crystal with the two associated capacitors, a 5V power supply, and some 0.1uF capacitors for bypassing. Oh – and a 6-pin header for programming. The schematic for the VFO is simple because, as far as the hardware goes, everything happens inside the micro-controller and the the Si5351 (which are both internally complex). The rest of it happens in the firmware. As far as hardware goes, we’re simply tasked with the 21st century equivalent of assembling a crystal set.

Here’s what I came up with. There are an awful lot of unused pins but, for this purpose, there are a lot of pins we don’t need. Without thinking, I was about to connect the AREF pin to +5V, because that’s what I was seeing in the various schematics I was using as references, until it occurred to me what AREF stands for. It’s an Analog REFerence pin. This application uses only digital inputs and outputs, so figuring that I didn’t need an analog reference, I didn’t connect it –

While planning this little VFO, a number of questions were presenting themselves to me. The main one concerned the issue of both the Si5351 Breakout Board and the OLED display being connected to exactly the same SDA and SCL connections. The I2C protocol does allow for multiple devices on the same line, but my understanding was that if more than one device is employed, then the firmware needs to include the unique address of each device. Would Thomas’ sketch work from the get-go, I wondered? As it happened, it did and, as of writing this, I don’t know if this is because

a) the address of the OLED was included in the library definition for this little display, or

b) with a setup like this that only has 2 devices connected, the instructions for the OLED are ignored by the Si5351, and vice-versa.

I’d like to be able to describe the exact steps I took when setting up the sketch, but I have, lamentably, forgotten them. I do remember installing the UG8lib library in the Arduino IDE, which supports the commonly available OLED displays. I also remember, at some point, uncommenting a line that specifically refers to devices that have an SSD1306 driver chip. If you purchase a cheap monochrome 128 x 64 OLED, this is probably the driver chip your display will have. These little displays are available for <$3 including shipping. Deal!

Here’s the ATMega328P mounted on the breakout board –

The controller part of the circuit constructed, with it’s supporting components. No power supply yet, as during initial testing, it will be powered through the ICSP header –

And with the Etherkit Si5351 Breakout Board fitted. The I2C control lines and 5V supply line are connected underneath the Etherkit board –

It took a while to figure out how to mount the OLED to the front panel. The 4 mounting holes are sized for #2 screws. I thought of running 4 #2 screws from the front panel, straight through to the OLED, and spacing the display away from the panel with 4 #2 nuts. The nuts were so close to the glass covering the display though, that they could have cracked it while being tightened. In retrospect, stacked #2 washers might have worked, though long before thinking of that, I came up with this rather more complex solution. It involved a small piece of PCB material, drilled and cut to size. #2 shakeproof washers and 3/16″ x 3/16″ nylon spacers were also employed. Their use should be apparent in subsequent photos –

I seriously considered fabricating a PCB enclosure for this little VFO, even getting as far as cutting some of the main pieces. The primary reason for wanting to use a custom enclosure was that the other case I was considering (which I ended up using) was a little too high. As a result, the front panel had, in my opinion, too much empty space. A PCB enclosure, about 4″ x 4″ x 1.5″ high would have looked mighty spiffy. However, I didn’t have the mettle to go through with it. I just couldn’t get quite inspired enough to put all that extra work into making a custom enclosure, and fell back on my favorite ready-made enclosure, the 143 from LMB Heeger. It is 4″ x 4″ x 2″ high, and available in plain aluminum finish, smooth light grey paint, or a sort of wrinkled black finish. They are also available with either an undrilled cover, or a perforated cover. The encoder was connected using header. The main reason for this was that I wasn’t sure if the cheap Chinese encoder ($1.68 each, inc shipping) was up to the task, so wanted to facilitate easy replacement –

A close-up, showing a little more detail of the mounting of the OLED to the front panel. Like the encoder, the display was also connected using header, making installation, and any dismantling for repair or upgrade purposes, easier –

Amazingly, it worked!

With the perforated cover you could, if you wanted, add some internal LED’s, for a splash of light to brighten up the shack, and add some flair. Being frugal energy-wise, I left the LED’s out for the time being. As it stands, the VFO already consumes 86mA at ~12V, which is not an insignificant amount –

Although the hardware side of this little project is finished (or very close to it), I am still very much fiddling with the firmware. As well as using Thomas’ code, I have been trying out sketches from other folk too. I began by trying to find a sketch that would do exactly what I wanted it to do, and fast discovered that, at the very least, an ability to modify code was required. That lesson led to a desire to actually develop a more complete understanding of C, so that I can at least do intelligent re-writes, if not write my own from scratch. This is all a bit overwhelming, and I vacillate from having fun, to being very grumpy, and back again

Thank you Thomas LA3PNA for the sketch – and also to the many others whose code I have been borrowing, and will no doubt butcher. I view this little VFO very much as a learning platform, from a programming point of view. Also, a big thank you to Jason NT7S for the Etherkit Si5351 Breakout Board, and the very useful libraries, which are seeing much use from homebrewing hams.

PS – I just started reading “Beginning C For Arduino” by Jack Purdum. Great stuff.

UPDATE (Jan 30th 2021) – I made a few small changes to the sketch, to improve the display a little. I found a font to display the frequency, that is a little larger. It is in the U8G library, and is known as helvB18. I also removed the word “step” from in front of the step size, as I felt it was self-explanatory. The text needed to be moved around on the display a little to fit it in, and this was adjusted in the sketch.

Since building this little clock generator, I noticed that, although the sketch set the Si5351 to start up on 7030KHz, it would nearly always start up 100Hz higher. Unsure whether this is a software or hardware issue, I did the clumsy workaround of altering the sketch to start up on 7029.9KHz. As a result, it now starts up on 7030KHz. I’d like to find the reason for this discrepancy, and fix it at the source.

I also added a 5mm red LED inside the case, with a 1K resistor in series with the +ve lead, connected to the 12V DC power connector on the back. The LED lights up the whole inside of the case, though this is not so obvious in the daylight shot below – 

However, when used indoors, the red light is visible through all the perforations in the case, and looks great. It’s almost reminiscent of the old tube days. The red light effect actually looks a bit better in real life than in the photo. This Si5351 VFO/signal generator draws 105mA, of which 10mA is the LED –  

Although I’ve had quite a few thoughts about using it as a stable signal source for other projects, it’s very handy to have around the shack, just as it is. Calibrating receivers is a lot easier, with this accurate signal source. I didn’t run the calibration sketch but, through a process of trial and error found a correction integer to place in the sketch, that places the VFO to within a few Hz, as verified by beating the output against WWV.