SDRplay are well known for their range of popular SDR receivers which cover all the radio bands up to 2 GHz. They have now just announced a new SDRplay nRSP-ST model which can be networked and is intended for remote operation.

The SDRplay nRSP-ST is a 14-bit ADC wideband SDR receiver covers all frequencies from 1kHz to 2GHz, with no gaps. It can remotely monitor up to 10MHz of spectrum at a time from a choice of 3 antennas.

The release price is in the region of $500 which is about double some of their other current models. It is expected that it will be available to purchase towards the end of 2024.

Analysis... This is an interesting development from SDRplay and mirrors some of the trends with other manufacturers of HF transceivers catering for remote operation.

In the case of the new SDRplay nRSP-ST receiver, all someone would need to do is to provide power, a network connection and antennas at the remote location. The concept of remote receivers isn't new but in most cases, a remote computer/PC is required as well as a receiver. In the case of the new SDRplay model, no remote PC is required.

The new SDRplay nRSP-ST would seem to have the same radio features as the RSPdx-R2 model except that it can be connected to a network for remote control.

One particular nice feature is that it has three antenna ports which allows for dedicated antennas to be connected at the remote end. Without this, it would be a case of having just one antenna port to cover all the radio bands below 2 GHz.

It's not hard to imagine how this remote receiver might be of interest to some people. Many live in towns and cities in noisy RF environments and the thought of having a remote receiver located somewhere quiet in say a holiday home or a friends or relatives house is attractive.

This would seem to be an all in one box solution for a problem and I'd imagine people will find all kinds of uses for it.

At about €500, it's not cheap for a receiver but it's also a new release with first adopters likely to be paying the premium price.

Links...

1) Page on SDRplay site for the new remote receiver... https://www.sdrplay.com/nrspst/

2) SDRplay press release... https://www.sdrplay.com/wp-content/uploads/2024/09/nRSP-STPressReleaseSeptFinal2024.pdf

Addendum...

It was not all that long ago that I tried unsuccessfully once again to make an AM radio receiver from discreet components and an LM386 low voltage power amplifier.

Well, apparently the fifth time is a charm…because THIS ONE WORKS!!!

As you can see in the video and pics below, there are only really a handful of components. Two ICs, the LM386 and a LM741 op-amp. That is a 10-365pF capacitor on the left in parallel with a homemade coil. A 10K potentiometer in the middle. An 8-ohm 0.5W loudspeaker. An 1N34A germanium diode. Some electrolytic caps and some resistors. A alligator-clamped a few feet of wire to one end of the inductor as an antenna. The LM741 requires both a positive voltage and a negative voltage, in this case +9V and -9V to function.

I think in making this particular receiver work, it was in part the careful construction of the tuning coil. I have a much better understanding now of LC circuits that I previously did. I knew how to utilize a inductor and LC frequency formulas (and online calculators!) to achieve a coil that along with a variable capacitor could be tapped to achieve resonant frequencies within the AM broadcast band (540kHz to 1700kHz). I used 26 gauge magnet wire wrapped 70 times around a 1 5/16″ PVC pipe. The enamel was scratched off in between the two pieces of electrical tape, trying to expose the outer surface without causing shorts between adjacent coils. A wiper was constructed from a bent piece of 14awg solid wire, and grounded. One end of the coil was also grounded (using the same screw as the 14awg wiper, which was then grounded on the breadboard with the rest of the circuit). The other end of the coil attached to the antenna, and also back into the circuit on the breadboard. The antenna and coil are directly connected to the germanium diode.

The variable capacitor (left) is in parallel with the tuning coil. The leftmost IC is the LM741 which requires the dual positive and negative voltage sources. After crossing the germanium diode, the now rectified signal passes through the LM741 op-amp. This feeds through a 10K potentiometer that can increase the volume of the signal as it enters the LM386 which further amplifies the audio before passing it into the loudspeaker.

Unfortunately, this circuit is copyrighted. It is from one of Forrest M. Mims III’s “Engineer’s Mini Notebook” of prior Radio Shack fame. In particular it is part of Volume II of the four volume set, called “Science and Communication Circuits & Projects”. Volume I, “Timer, Op Amp & Optoelectronic Circuits and Projects” is also handy in getting the dual power supplies correct. I ordered the entire compendium from Solarbotics. As always, I receive absolutely no payment or products for this website whatsoever; this is an entirely non-monetized personal endeavor so don’t think I care if you visit that site or not. These books just seem very hard to find these days, and I was just lucky to stumble upon this company that had them for sale. I may not be the only one that feels that way.

Finally! A working DIY receiver! There is still a lot of experimentation to do with this one. Like will a Schottky diode work instead of germanium unobtanium? I will let you know what I find out!

You are always on my mind.

KM1NDY

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Every now and then I decide it’s time to homebrew a receiver. You may remember my attempt back here. Or even way back here. They never work. So this even more complicated, 3 transistor, 2 diodes, and audio amplifier IC definitely did not work. Again. Well sort of. Technically it is actually a receiver. Just not what I was hoping for.

See the electrolytic capacitor I am pointing out down below? And the resistor that is in series with it? If I touch either with my fingertip, while the circuit is live, radio stations play through the loudspeaker. These components form a loop from pin 8 to pin 1 of the LM386; these pins are the “gain” pins of this low voltage audio amplifier chip.

This is a lot like what happens back in my last receiver build attempt, except for this one, I needed to touch the potentiometer in order to pick up stations. I’ll repost the video from that build below so you know what I mean. Essentially I could remove the entire rest of the circuit and as long as I powered up the LM386 and touched the top of the potentiometer, I could hear a station through the loudspeaker.

I am not through debugging this current circuit or I would go into more detail about it. In fact, in preparing this blog, I can see I left one end of a capacitor floating. The cap in the arrow below should be sitting between pin 3 and pin 4 of the IC. Pin 3 is correct, but then you can tell instead of hitting the ground rail of the chip at pin 4, the other end of the capacitor is just freely hanging out in its own row in the breadboard. To be fixed! And if I make any headway, I’ll write up a more complete description of the circuit.

One of my most successful and useful builds is in action down below. This is the KM1NDY Voltage Converter that I designed out from scratch that uses 78xx series of linear voltage regulators in TO-220 packaging. The voltages are interchangeable, and for this receiver attempt I used a 5-volt 7805 chip. The power source is a 12 volt LiFePo battery. This system includes a replaceable fuse as well, as an attempt to minimize any potentially dangerous currents from reaching me when I accidentally short something out. This little device is actually quite handy! If I make another one, I’ll need to put a switch on it though.

Ok, now close your eyes if you are going to be squeamish, but it is probably too late. I just wanted to show the bloodshed that this ham radio hobby causes me. This cute little pattern of blood bubbles is what occurs when you send the pins of an IC socket deep into your finger. Don’t fret! I am okay!

So, yet another failure. But there is still some debugging left to do, so I won’t write off the entire project just yet. And there are some important mental successes. The first is that I can now start to see the various stages of a receiver circuit. They are making much more sense to me now. And I can see how you can work on each stage as a separate entity. I am already concerned that my antenna and tuning capacitor are not working properly. Or that there is not enough amplification at the RF amplifier stage. I have figured out inadvertently that the components of the audio amplifier stage work. I needed to substitute diodes, so am I not demodulating the AM properly? And I am understanding bit by bit how and what to probe, and with what instrument, to see what is working, and what is not.

Though not so secretly, I can’t wait for the day when I post a receiver build and it is actually a success. But I have always known that failure is not trying and failing. Failure is not trying at all.

That’s that.

KM1NDY

Ham Radio With K0PIR

Both the Icom IC-7610 and the Elecraft K3s are high-performance amateur radio transceivers, and they both have their strengths. The choice between them depends on your specific preferences, needs, and operating style. Here are some key points to consider: Icom...

The post Icom 7610 Vs Elecraft K3S: Which Is The Better SSB Receiver? appeared first on Ham Radio with K0PIR - Icom 7300 and 7610 SDR Transceivers and now Elecraft!.

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 the mid 90s I decided to throw together what I called a "Bat Listener" - a simple receiver used to convert ultrasonic sound down to the audible range.

Figure 1: The exterior of the ultrasonic receiver, complete with fancy labeling! Click on the image for a larger version.

Two types of circuits:

There are two common ways to convert a higher (ultrasonic) signal to the audible range, whether this is done using analog or DSP (Digital Signal Processing) techniques.

Frequency division

There are several ways to do this, the simplest being the "divider" type which digitally converts ultrasonic frequencies to audible by integer division of the input to a lower frequency.

The problem with this simple approach is that it does not preserve the amplitude (loudness) of the original sound since it must take the input signal, amplify/convert it to a series of logic-level pulses - which loses any amplitude reference - and do a brute-force digital division.  Additionally, if there are multiple signals present, for the most part only the strongest one will be converted down.

Clearly, one cannot "tune" this type of circuit:  A signal at 40 kHz will always be divided down by a fixed integer amount,  Let's say that the circuit digitally divides by 32:  That 40 kHz signal will be at 1.25 kHz.

Additionally, the direct "A-B" frequency differences between ultrasonic signals is lost, instead being "(A-B)/N" where "N" is the number of divisions.  In other words, the relative frequency differences between signals is not preserved.

Heterodyne conversion

The other way to do this is to convert the frequency.  In this technique, two signals - the ultrasonic to be converted - and another generated by the device (the "local" oscillator) are mixed together.  The result is an arithmetic shift in frequency.

The biggest advantages of this method are the fact that that not only are the differences in frequency preserved (e.g. two tones 1 kHz apart at ultrasonic will appear as two tones 1 kHz apart at audio) but the relative amplitudes (loudnesses) of the received signals are preserved as well.

Frequency conversion:

I chose to build a heterodyning receiver to convert the input frequency to a lower one.  This can preserve the amplitude and frequency relationships  - plus it is fully tunable, allowing one to choose the frequency range to convert to audible sounds - and since it is a simple conversion, multiple signals present will also be preserved.

When it comes to frequency conversion, there are two ways:  The simplest - direct conversion - would involve mixing a variable oscillator with the incoming signal and filtering/amplifying the resulting audio.  This has the advantage of being the easiest, and it is the method described in this article:

     April, 2006 QST article, A Home-made Ultrasonic Power Line Arc Detector - link)

While I could have easily built something like this a decade before the above article was published, as I'm sometimes wont to do I decided to make it a bit more complicated, constructing a superheterodyne converter.

While a direct-conversion receive simply mixes an oscillator with the desired signal to cause a frequency conversion, a superheterodyne receiver operates like a conventional AM or FM radio:  The desired signal is first converted to an IF (Intermediate Frequency) - and this IF is then converted to audio.  The advantage of the superheterodyne scheme is that filtering may be applied at the IF to limit the receive bandwidth - and since the IF is fixed, its width remains constant over the tuning range, just like that in a conventional radio/receiver.

Circuit description

Figure 2: Schematic diagram of the superheterodyne ultrasonic receiver. See text for a circuit description. Click on image for a larger version.

As noted above, this circuit is more complicated than it needs to be, so make of it what you will!

VCO:

The heart of the unit is U1, the VCO (Voltage Controlled Oscillator) which uses the venerable CD4046 PLL chip.  Often used for frequency synthesis, we are using (only) the oscillator portion, which provides a linearly-tuned and fairly stable frequency source, adjusted by the voltage applied via R101 (and scaling resistor R102).  The values were chosen to provide an approximate frequency range of 125 to 185 kHz (more on this later) to allow tuning of audio signals from (ostensibly) 0 to about 60 kHz.  The actual tuning range is closer to 115-190 kHz as a bit of extra margin for the frequency range.

The only critical component here is C101 which should be a frequency-stable capacitor.  I used a polystyrene capacitor, but an NP0 (a.k.a. C0G) or silver-mica could be used, instead.  When I reverse-engineered this device, I noted that the marked capacitance value was unreadable, but back-of-the-envelope calculates indicate that a value of "about 150pf" should be in the ballpark.

R103, connected to the "R1" pin of U1, sets the approximate center frequency range while R104, connected to the "R2" pin - sets the lowest frequency - which important, since we want to constrain the tuning to 125-185 kHz.  Additionally, the low end of the tuning range was further refined by R102 on the "ground" side of the tuning potentiometer, which sets the minimum voltage that may be applied to the "VCOIN" pin.

The VCO output, a square wave, is buffered by U2, a hex inverter, and several sections are used to provide both a VCO signal and its inverted version to drive the mixer.

While the 4000 series CMOS chips throughout this receiver will happily run from 3-15 volts, they are operated from a regulated 5 volt supply - mainly to improve frequency stability and to provide a nice, stable voltage for a few other low-level circuits and to provide isolation from the main battery supply which will vary a bit, particularly at higher receive volumes:  This variance, if it gets back into some earlier stages, could cause instability of the receiver in the form of "motorboating" or some other type of feedback.

BFO:

Another circuit is the BFO (Beat Frequency Oscillator) which is used to convert the IF signal back down to audio - both being processes that we'll discuss shortly.  This uses an inexpensive 500 kHz ceramic resonator to form an oscillator using one of the sections of U2C, the signal being buffered by U2B.  This signal is divided-by-two using U3A, one half of a 4013 dual flip-flop - and then divided by two again using U3B, yielding a stable 125 kHz signal.  As with the VCO, two phases of this signal (normal and inverse) are available, this time using the "Q" and "!Q" outputs of the 4013.

Input signal path:

J1, a disconnect-type 3.5mm stereo jack is wired so that an internally-mounted electret "capsule" microphone is connected by default.  This microphone element (M301) is of the "2 wire" type or electret microphone in which a bias voltage is applied to the same pin from which audio is drawn - this voltage being applied via R301 from the 5 volt regulated supply.  The specific make/model of this electret element is unknown as it was selected from a small collection to find the best performer at ultrasonic.

At some point in the future, I'll replace this with a more modern MEMs microphone as described in Another article:  Improving my ultrasonic sniffer for finding power line arcing by using MEMs microphones - link.

The signal from the microphone is applied to U4A which is wired as a unity-gain buffer.  For this, an LM833 is used, an inexpensive, low-noise dual op amp:  An LM358 or many other types may be used here as well - just make sure that it is is fairly low noise:  I'd avoid the use of the LM1458 here as it is quite noisy by comparison!

Section U4B amplifies the signal voltage by 10 (20 dB of power gain) and this signal is applied via R305 to a simple L/C high-pass filter consisting of C303, C304, L301 and L302 the latter two components being inexpensive 18 milliHenry inductors.  Certainly, an R/C-based high-pass filter could have been constructed using U4B, but I chose not to do that for some reason.

Figure 3: Inside the ultrasonic receiver, constructed on prototype board and having been modified several times over the years. Click on the image for a larger version.

In simulation, the C303/C304/L301/L302 filter has a -3dB roll-off of about  23 kHz, it's down by 10dB at about 19.5 kHz, by 20dB at about 16 kHz and by 40 dB at 9 kHz and with the values shown, it's flat to within 1 dB between about 24 and 100 kHz.

The output of the filter is amplified by U5B - and then even more by U5A (which has a bit of roll-off from C307) to yield a whole lot of gain.  It's very possible that I over-did the gain here, but unless the signal source is quite close, there is no clipping observed on the output of U5A.

Its worth noting that a mid-supply voltage is created using R309/R310 to provide a "virtual ground" for the op amps and to maintain stability, it is heavily filtered by C306 and C302, each located near the respective op amp shown on the diagram.

Mixer and band-pass filter:

It is this next section that may seem unfamiliar to some - the use of a CMOS analog switch as a signal mixer.  For this, a CD4066 is used which consists of four separate analog switches.  The filtered and amplified ultrasonic input signal from U5A via C308 is applied to pins 2 and 10 of U6A/U6D.  When the respective signals on the control pins "VCO_A" and "VCO_B" go high, the switches are activated, and because VCO_A and VCO_B are inverts of each other, each of these switches is closed in turn.  The result of this is that the inputted signal is chopped up at the rate of the 125-185 kHz VCO and this produces two mixing products.  

For example, let's assume that there is a 40 kHz signal is present on the input that we wish to hear.  If the VCO is tuned 40 kHz above the 125 kHz IF (again, more on that momentarily) - to a frequency of 165 kHz - the switching action of U6A and U6D produces both the sum (165 + 40 = 205 kHz) and the difference (165 - 40 = 125 kHz).

T301 is a filter/transformer that passes only the 125 kHz signal - the difference signal in this case.  This transformer consists of two separate windings, each resonated using its internal capacitors and the externally-added 820 pF capacitors on each winding (e.g. C309/C310) to "pad" it down to 125 kHz.  This forms a fairly wide (8-10 kHz) filter that rejects signals outside the immediate vicinity of its 125 kHz frequency.  Because this filtering is at a fixed frequency, it does not vary with input tuning which means that its bandwidth is constant over frequency.

Of all of the components in this device, this transformer is unique:  It was originally a 262.5 kHz IF transformer from a 1970s/1980s Philco (Ford) AM-only car radio.  While I could have certainly used the original 262.5 kHz frequency - or even 250 kHz, when I built this I decided to pad it down to 125 kHz using C309/C310  - a frequency that is conveniently 1/4th of the 500 kHz resonator.

It's been so long since I built this, I don't recall why I didn't simply divide the 500 kHz by two and readjust that transformer to 250 kHz.  Practically speaking, I could have also up-converted to 455 kHz and used either transformers or ceramic filters from a modern AM radio as 455 kHz ceramic resonators were certainly available at the time - but I didn't do that.

Each half of T301 has a center tap and to this, a bias voltage is applied via R315 to assure that the voltage on these switches was in the middle of the supply range, away from the protection diodes on the 4066's I/O pins, which could cause clipping/distortion should they be allowed to conduct if the signal voltage got too near the ground or supply rails.  To prevent coupling between the two halves of the transformer via the center tap, R314/C311 was added, the resistor adding isolation with the capacitor bypassing the remainder of the signal.  Practically speaking, being able to adjust the bias voltage was unnecessary as a simple resistive voltage divider to set the bias at 2.5 volts (1/2 the supply voltage) would have been just fine.

On the "other" side of the transformer is the other half of U6 (e.g. U6B/U6C) - this time, clocked from the fixed 125 kHz oscillator.  From this, the signal - previously converted up to 125 kHz is now converted back down to audio.

Post-mixer amp/LPF:

The output of the down-converting mixer is applied to U7B via R316, a 1k resistor and a 0.001uF capacitor, both of which form a simple R/C low-pass filter to attenuate any high-frequency leakage signals from the mixer.  Because the mixing process itself is a bit lossy (about 25% efficient) as is transformer/filter T301, U7B boosts the signal by a factor of 10 (20dB) and then applies it to U7A, which is configured as a variable gain amplifier section.  The output of this is then boosted again by U8, an LM386 which is capable of driving headphones or even a small speaker.

A few comments about the design:

Originally, the circuit lacked U7 at all, but it was added when the gain of U8 (the audio amplifier), by itself, was found to be inadequate.  Since U7 was "patched" into place, this explains the odd gain distribution:  If I were rebuilding this from scratch, I'd certainly not need two post-mixer amplifier sections and I could have likely eliminated one full dual op-amp package.  As it is, I may add a "high/low" gain switch somewhere around U5 to allow reduction of the gain somewhat when in the presence of possibly-high ultrasonic signal levels to prevent clipping prior to the band-pass filter which would surely degrade overall performance.

If I were to build this again I would likely use a 455 kHz IF, instead.  While not as plentiful, 455 kHz ceramic resonators are available to use for the BFO as are either transformer or ceramic-based band-pass filters.  I would also likely reconfigure U4B or U5 to perform the high-pass filter function rather than using harder-to-find inductors.

Again, I built this unit in the mid 1990s and have since lost my original notes, but I do recall that I modified it a few times since, simply tacking changes onto the old circuit rather than completely revising it.

Use as a longwave receiver:

While primarily intended to "hear" ultrasonic sounds such as those produced by bats, insects, leaking pipes, arcing power lines, etc., it is just a longwave radio receiver connected to a microphone:  If one connects a few 10s of feet/meters of wire to to J1 - and provides an Earth/ground reference to its shield connection - one can easily tune in the high-power transmitters used for submarine communications (around 20-30 kHz) plus the WWVB time signal at 60 kHz.  This must, of course, be done away from man-made noise sources such as power lines.

Alternatively, I have used a loop of about 1 foot (25cm) diameter of a dozen or so turns of wire along with a 10uF capacitor in series (to optionally block DC from R301) and been able to hear such signals - even in suburbia - but with this arrangement you'll also likely hear plenty of similar signals from the myriad switching supplies that likely inhabit your house as well!

Final comments:

The reader should be under no illusion that this is an optimized circuit or that I would do it this way again:  It was assembled fairly quickly to suit a need and to test a few random ideas, just to see if they would work.  Will I rebuild it at some point?  I don't know - it works as it should, so I don't plan to re-make something that is currently fit for purpose.

While I've heard very few bats with this - probably due to the deficiencies of the electret microphone at ultrasonic frequencies (which explains the future switch to MEMS-type microphones) - I've used it to find powerline noise (arcs are noisy at ultrasonic) and to test longwave receive antennas.

This page stolen from ka7oei.blogspot.com

[End]

Figure 1: The packaged MEMs microphone, along with the ultrasonic receiver. Click on the image for a larger version.

Years ago - probably 20+ - I constructed a superheterodyne "Bat Listener" to eavesdrop on the goings-on of our winged Chiroptera friends.  (That receiver - the one depicted in Figure 1 - is described HERE.

In retrospect, this device is probably a lot more complicated than it need be as it up-converts from "audio" to a 125 kHz IF, using a modified 262.5 kHz Philco (Ford) car radio IF Can as the filtering element before being converted back down to audio.  This device has a built-in microphone, but it also has a jack for an external microphone, which comes in useful.

This device actually works pretty well for its intended purpose and, in a pinch, can even be used to listen to LF and VLF signals like the time station WWVB at 60 kHz and the powerful transmissions intended for submarines in the 20-40 kHz range if a simple wire is attached to the external microphone input, but I digress.

One of the weak points of this unit has always been the microphone.  To be sure, there exist the 40 kHz ultrasonic transducer modules:  These units used to be common in TV remote controls before the Infrared versions became common and you might still find them in the (now rare-ish) ultrasonic intrusion alarms.  While fairly sensitive, these units do have a "problem":  They are rather sharply resonant around their design frequency - which is typically somewhere around 40 kHz.  In other words, they aren't very good over much of the ultrasonic frequency range above or below 40 kHz.

It would seem that many commercial ultrasonic power mains arc detectors use these things (The MFJ-5008 seems to be an example of one of these) and there have been a few articles on how to make these devices (See the April, 2006 QST article, A Home-made Ultrasonic Power Line Arc Detector - link) but it, too, uses one of these "narrowband" 40 kHz transducers.

While certainly fit for purpose, I was more interested in something that could be used across the ultrasonic spectrum.  When I built my "bat listener" I fitted it with a "condensor" (electret) microphone, rummaging through and trying each of the units that I'd accumulated in my parts box at the time to find the one (make and model unknown) that seemed to be the most sensitive - but compared to a 40 kHz transducer, it was still somewhat "deaf".

This issue has nagged at me for years:  I occasionally break out the "bat listener" to (would you believe) listen for bats and other insects when camping, and it is useful if you have a suspected air leak in a compressed air system - plus it's sometimes just plain interesting to walk around the house and yard to hear what's happening at frequencies beyond human hearing - and it may also be used for finding arcing power lines as the QST article referenced above suggests.

In more recent years, an alternative to the electret microphone has appeared on the scene in the form of the MEMS (MicroElectricroMechanical System) microphone.  This class of devices are literally tiny mechanical devices embodied in silicon structures and they can range from oscillators to accelerometers to exotic tiny motors to (you guessed it) - microphones.  Their small size, which makes them the choice when space is at a premium, as in the case of a phone or web camera, also reduces the mass of the the mechanical portion that responds to variations in air pressure (e.g. sound) which can enable them to respond to frequencies from a few 10s of Hertz to well into the 10s of kHz.

Figure 2: The MEMs microphone mounted and wired up.  The element is mounted "dead bug" by gluing its top side to the circuit board and small (#30) wires connect to the pads. Click on the image for a larger version.

Perusing the data sheets of devices found on the Mouser Electronics web site, I found what seemed to be (one of many) suitable candidates:  The Knowles SPU0410LR5H-QB.  This device, which is a version with an analog output, is about 3mm by 4mm, has a rated frequency response to at least 80 kHz - and it is pretty cheap:  US$ 0.79 each in single quantities at the time of this writing - and, in these days of erratic supply lines, it was available immediately as Mouser reported having more then 30k of them in stock.  

Importantly, this device had its "audio port" on the same side as the wiring - the intention being that it would get its sound through a hole in the circuit board, but this would also make it easier to wire up as described below.

The fact that this is a small, surface-mounted device may seem daunting to the home building - but don't be daunted:  Given the appropriate magnification device (I use a pair of "Geezer Goggles" that I got from Harbor Freight) and a fine-tipped soldering iron, it's perfectly reasonable to solder just a few fine (30 gauge) wires to a device this small.

Figure 3: The completed board, containing the circuit depicted in Figure 4, below.  The board with the microphone is on the left, and the attaching cable is seen in the upper-right. LED1, the one across the microphone element itself, was mounted on the bottom side of the board. Click on the image for a larger version.

First, I cut a small piece of circuit board material to use as a substrate and mounted at a right angle on a larger piece, as shown.  I then took the microphone and "Super Glued" it "dead bug" to the middle of this board (see Figure 2, above) leaving the side with the connections and sound port facing outwards.

With this simple operation, a very tiny part suddenly becomes a larger, easier-to-manage part - albeit with very closely-spaced wire connections.  Being careful with very thin solder not to get any solder or flux in the sound port, I first tinned the connections on the device itself (there are four pads - two grounds, a power and an audio) and then proceeded to use some #30 "wire wrap" wire to make flying lead connections to the device, using a slightly longer section of one to tie the two "grounds" together.  I could have just as easily used some tinned #30 enameled wire, instead, but I tend to keep the Kynar-covered wire wrap wire on-hand for this very purpose.  

With the flying leads and the piece of circuit board as a "breakout" device, I was then free to treat the MEMs microphone as a "normal sized" device and build an interface circuit onto the rest of the board.

In perusing the data sheet, I noted that the power supply voltage rating was 1.5-3.6 volts which was incompatible with the 5 volts of "phantom power" applied by my bat listener to the microphone jack to power a condensor (electret) microphone, but this was easily remedied using the circuit shown below:

Figure 4: The interface circuit used to adapt the MEMs microphone to the existing 5-volt electrect microphone circuit. Click on the image for a larger version.

Circuit description:

This circuit depends on there being power applied via the audio/microphone lead, as is commonly done for computer microphones.  Typically, this is done by biasing the audio line through a resistor (2.2-10k is common) from a 5 volt supply - and that is assumed to have been done here on the device to which this will be connected, as I did on my "bat listener".

DC is decoupled from the audio output of the microphone via C1.  In this circuit, I chose a 0.01uF capacitor as I wanted to reject audible frequencies (<10 kHz) to a reasonable extent - and this means that this capacitor value is way too small if you plan to use it as a "normal" microphone to listen well down into the lower audible range:  Something on the order of 1-10 uF would be appropriate if you do want audio response down to a few 10s or 100s of Hz.

A word of warning:  Do NOT use a ceramic capacitor for C1 as these can be microphonic in their own right.  I used a 0.01uF plastic capacitor (probably polyester) which is neither microphonic or prone to change capacitance wildly with temperature.

Resistor R1 (2.2k shown here, but anything from 2.2k to 4.7k would likely be just fine) decouples the audio from the DC and capacitor C2 removes that audio, providing a "clean" power source for the microphone.

Here, LED2 is used as a voltage limiter:  Being an "old fashioned" green panel indicator LED, its forward voltage is somewhere around 2 volts.  The use of an LED in this manner has the advantage that unlike a Zener, this type type of LED has a very sharp "knee" and practically no leakage current below its forward voltage - and it is much easier to find than a 2-2.5 volt Zener.  It's likely that about any LED would work here - including a more modern Gallium Nitride type (e.g. blue, white, super bright green) but I have not verified that they would properly clamp the voltage in the 1.5-3.6 volt range needed by the microphone.  (And no, there are not any detectable effects on the circuit from light impinging on the LEDs.)

LED1 is present to protect the microphone itself.  When it's plugged in, whatever voltage is present on the audio cable will be dumped into the microphone output as capacitor C1 is charged and it could damage it, particularly if the power source is 5 volts and the microphone's maximum rated voltage is just 3.6 volts.  This LED, which is the same type as LED2, will not normally conduct as the audio output from the microphone typically has a voltage of roughly half that of the supply, so LED1 will be completely "invisible" (in the electrical sense) in normal operation.

Figure 5: A spring, soldered to a wire connecting to the "ground" side of the circuit (also the microphone cable shield) used to make contact with the aluminum tubing. Click on the image for a larger version.

I mounted the board with the microphone in a piece of aluminum tubing that would fit the microphone mount of my parabolic dish (see below) and this not only provides protection for the microphone and circuitry, but also serves as an electrostatic shield, preventing energy - say, from a power line - getting into the circuitry.  To make this effective, the tubing itself is connected to the ground lead (cable shield) by soldering a wire to a metal spring and placing it in the end of the tubing as seen in Figure 5.

To secure things into place, a bit of "hot melt" glue was used, preventing the board from sliding out.  The connection to the receiver was made via a length of PTFE (Teflon) RG-316 coaxial cable - but shielded audio cable would have sufficed:  This cable is firmly attached to the board as seen in Figure 3 as a strain relief. 

The parabola:

While the microphone is sensitive in its own right, its sensitivity can be noiselessly "amplified" many-fold by placing it at the focus of a parabolic dish.  I was fortunate to have obtained a Dan Gibson EPM model P-200 (minus the original microphone element or any electronics, but including the holder) at a swap meet, but the QST article linked above suggests other sources - and I have seen parabolic-based microphones on Amazon - often as semi-serious toys - as well.  Using the holder - the inner diameter of which was the basis for choosing the specific size of the aluminum tubing - the microphone was mounted at the focus of the dish.

Finding this focus can be a bit of a challenge without the proper equipment, so I set up a "test range".  At one end of my back yard I placed a 40 kHz transducer (of the sort noted in the QST article linked above) connected to a function generator set to 40 kHz:  I'm sure that a small speaker would have been sufficient to generate a signal.

Figure 6: The MEMs microphone, mounted in the aluminum tubing, at the focus of the parabolic dish, with attached cable. Click on the image for a larger version.

From across the yard - perhaps 30 feet (10 meters) away, I sighted the emitter through the dish, using its alignment dots and slid the microphone in and out until I had the best combination of the loudest signal, the sharpest aiming, and the "cleanest" pattern.  On this last point, I noted that if I focused too far in our out, the peak of the signal would become "blurry" (e.g. spread out) or, in some cases, I would get two peaks - one on either side of the "real" one, so the object was to have the single, loudest peak possible.  Once this was found, it was marked and a bit of heat-shrink tubing was put over the end of the aluminum tube, corresponding with that mark, to act as a "stop" to set the correct focus depth.

Again, refer to the QST article linked above for additional advice on where to obtain a suitable parabolic reflector, and hints on the mechanical construction.

Does it work?

The answer is yes.  From significant distances, I can hear the acoustic signature of switching power supplies (apparently, many of these have transformers that vibrate at their 30-60 kHz switching frequency) as well as the sounds of insects, and the hissing of the capillary valve of the neighbor's window air conditioner.

Importantly, I was able to verify that a power pole's hardware was, in fact, arcing slightly - although I wasn't able to determine which hardware, exactly was making the racket as it was quiet enough that it became inaudible when I stood far enough away from the (tall!) pole to get a better viewing angle.

When I get the chance, I will replace the capsule electret microphone built into the receiver itself with one of these MEMs units, but that's just one project on a rather long list!

This page stolen from ka7oei.blogspot.com

[End]

Original log book from 1971.

Previously, I had just used a spool of magnet wire strung around the ceiling of my attic bedroom. This was not a very good antenna. Just before school started in 1971, I bought a 25 foot roll of small speaker wire and unzipped it all. Soldering the two pieces together, this gave me 50 feet of antenna wire, which I strung out the window, across the garage and into a tree at the edge of the yard. 

I was eager to try this new antenna, so I proceeded to tune across the AM broadcast band that evening, and log each station I could. I started about 9 PM, and kept tuning until midnight. The next evening, I continued the process. 

Sep 3 log page.

Sep 4 log page.

I remember it distinctly. I would sit patiently and wait for the AM stations to ID, generally at the top or bottom of the hour. If I got lucky, they would ID sooner. 

Hard to believe that event was over fifty years ago. 

From the late 1950s until about 2012 there was a (mostly) annual event held in southeastern Utah that was unique to the local geography:  The Friendship Cruise.

The origins are approximately thus:  In the late 1950s, an airboat owner - probably from the town of Green River, Utah - decided to go down the Green River, through the confluence of the Green and Colorado rivers, and back up to the town of Moab.  Somehow, that ballooned into a flotilla in later years - with as many as 700 boats - in the 60s and 70s.  By the mid 90s, interest in this unique event seemed to have waned and by about 2012, it finally petered out.

Communications is important:

Figure 1: A high-Q 80 meter magnetic loop on one of the rescue boats Click on the image for a larger version

At some point, probably in the mid 1960s, amateur radio operators got involved, successfully closing the communications link using the 80 meter amateur band.  This tactic worked owing to the nature of 80 meters:  During the daytime, coverage is via skywave over a radius of about 200 miles (300km) and this high angle of radiation allowed coverage into and out of the deep canyons.  Furthermore, the same antennas that were small enough to be usable on boats, vehicles and temporary stations on this band were well-suited for radiation of RF energy at these steep angles.

For (literally!) decades, this system worked well, providing coverage not only anywhere on the river, but also to the nearby population centers (e.g. Salt Lake City) where other amateur radio operators could monitor and relay traffic as necessary and summon assistance via land line (telephone) if needed.  Because the boats were typically on the river only during the day, this seemed to be a good fit for the extant propagation.

While it worked well, it was subject to the vagaries of solar activity:  An unfortunately-timed solar flare would wipe out communications for hours at a time, and powering and installing a 100 watt class HF transceiver and antenna was rather awkward.  Occasionally, there was need to communicate after dark, and this was made difficult by the fact that 80 meters will go "long" after sunset - often requiring stations much farther away (e.g. in California or Nebraska) to relay to stations just a few 10s of miles away on the river!  Finally, it was a bit fatiguing to the radio and boat operators to have to listen to HF static all day long!

Enter VHF communications:

Figure 2: General coverage map of the course showing coverage of various sites. Click on the image for a larger version

By the time that the 1990s had come along, there was renewed interest in seeing if we could make use of VHF, on the boats, on the river.  The twist was that instead of direct communications between boats, we would try to relay signals from far above, on the plateaus farther away, and a few experiments were tried.  It 1996, I was on a boat on the river and took notes on what sites covered and where, trying nearby mountaintop repeaters and temporary stations set up at places near-ish the river courses themselves - the resulting map being presented in Figure 2.

Using the color-coded legend across the top and the markings on the map itself, one can see what sites covered where.  Included in this was the coverage from the 147.14 repeater near-ish Green River, Utah, the 146.76 repeater near Moab, and several other temporary sites atop the plateaus surrounding the river.  As can be seen, coverage was spotty and inconsistent over much of the route - with the exception of a site referred to as "Canyonlands Overlook" (abbreviated "Cyn Ovlk") which commanded a good view of the Colorado River side of the river course.  Clearly missing was reasonable coverage in the depths of the gorges along the lower parts of the Green River side - which started, more or less, where the coverage of the "Spring Canyon" (abbreviated "Spring Cyn") stopped.

Figure 3: The two TacTecs used for 2 meter reception, the voting controller (blue box) and the FT-470 used as the UHF link radio. Click on the image for a larger version.

The birth of a repeater:

During the next year I put together a system that I'd hoped would make the most of the situation.  Because of the remoteness of the site, accessible via a high-clearance Jeep road - and that we had to bring everything to live for a few days, it had to be relatively lightweight and compact - and I also wanted to avoid the use of any duplexers (large cavity filters) that would add bulk and - more importantly - losses to the system.  Taking advantage of a weekend to visit Panorama Point the next spring we determined that we could split the transmit and receive portions by about 0.56 miles (0.9km) apart, placing the receive antennas behind some local geographical features and using local topography to improve isolation.  The back-of-the-envelope calculations indicated that this amount of separation - and the rejection off the backs and sides of the beam antennas - would likely be sufficient to keep the receiver out of the transmitter.  The receive site - surrounded by three sides by vertical cliffs - also provided a commanding view of the terrain as can be seen in Figure 5, below.

Figure 4: GaAsFET preamplifier mounted right at the receive antenna to minimize losses. Click on the image for a larger version.

In addition to site separation and gain antennas, I decided to go overboard, adding mast-mounted GaAsFET preamplifiers, right at each antenna (Figure 4) and implementing a voting receiver scheme - something made much easier with the acquisition of two, identical RCA TacTec "high band" VHF transceivers.  These receivers were modified - clipping the power lead to the transmitter and adding a 3.5mm stereo plug to each radio to bring out both discriminator audio and the detector voltage from the squelch circuit.

A relatively simple PIC-based repeater controller was constructed, using a simple comparator to determine which receiver had the "best" signal, based on the detector voltage from the squelch circuit, and also using another set of comparators and onboard potentiometers to set the COS (squelch) setting for the receivers.  As it turned out, the front-panel squelch control adjusted the gain in front of the squelch detectors in the radios themselves, allowing each receiver to be "calibrated" from that control, allowing easy fine-tuning in the field.

To link the receiver site to the transmitter site, a single UHF channel was used and I modified my old Yaesu FT-470 handie-talkie to this task.  The mysterious rubber plug on the side of this radio was replaced with a 3.5mm jack, providing a direct connection to the modulation line of the UHF VCO while using the top panel 2.5mm external microphone jack for transmitter keying.  As it turns out, not only did this transmitter provide linking to the nearby transmitter site, but its UHF beam was pointed across the way, to another 2 meter repeater at Canyonland's Overlook that provided coverage on the Colorado River - providing what amounted to a linked repeater system.  A later addition was a CdS photocell on a grommet and a piece of "Velcro" strap allowed the detection receiver activity by "looking" at the front-panel LED to prevent the link transmitter from "doubling" (transmitting at the same time) and clobbering an ongoing transmission from the other repeater site.

Figure 5: The remote RX site, surrounded on 3 sides by sheer cliffs.  The mast has two 2 meter and one UHF link beam antenna.  The solar panels are just visible along the far right edge. Click on the image for a larger version.

The receive site itself was solar-powered, using lead-acid batteries to provide the energy when insufficient sun was available (e.g. heavy clouds, night).  In later years, the PIC controller was modified to not only read the battery voltage, but to regulate the solar panels' charging of the battery bank using a "bang-bang" type charger ^{(See note 1)} but also to report the battery voltage when it did its legal identification.  In this way, we could keep an "eye" on things without having to walk out to the receive site.

The two 2 meter and the 70cm link antennas were mounted on a single mast, the VHF antennas pointed in different directions to take advantage of the slight difference in physical location and in the hopes of providing diversity for  the weak signals from the depth of the canyons - which were all reflections and refractions.  As it turns out, despite the close proximity of the antennas, this worked quite well:  At the site, one could monitor the speakers on the receivers and watch the voting controller's LED and see and hear that this simple, compact arrangement was, in fact, very effective in reducing the number of weak-signal drop-outs caused by the myriad multipath.

In testing on the work bench, the measured 12dB SINAD sensitivity of each of the receivers (plus GaAsFET preamps) was on the order of 0.9 microvolts - far and away better than a typical receiver.  Later, I did the math (and wrote about it - see the link at the bottom of this article) and determined that it was likely that the absolute sensitivity of this receiver was limited by the thermal noise of the Earth itself and that it could not, in fact, be made any more sensitive.  This notion would appear to be borne out by a careful listening to the repeater in the presence of weak signals:  Very weak signals - near the receive system's noise floor - sounded quite different than what one might hear on a typical FM receive system near it's noise floor.  Instead of a "popcorn" type noise, signals seemed to gradually disappear into an aural cloud of steam.

The transmitter site:

Figure 6: The transmit site.  The tall (30 foot) mast and 2 meter transmit antenna is visible in the background with the UHF link antenna and the VHF "backup" TX antenna in the foreground. Click on the image for a larger version.

With so much effort having gone into maximizing receiver performance I decided to do the same on the transmit site in the years that this system was used.  For the first year, the transmitter was modest:  A Kenwood TM-733, on low power, driving a 50 watt RF amplifier into a vertical on a short mast.

The next year I decided to erect a taller mast and place atop it a 5 element beam, pointed in the general direction of "up river".  To boost my RF output power, I scavenged a pair of 110 watt RF amplifiers from some ancient Motorola Mocom 70 mobile radios (with some DC fans for cooling) and used two Wilkinson Power divider - one to split the input power and another to combine the outputs of the amplifier, yielding a bit over 200 watts of RF and about 1500 watts of ERP (Effective Radiated Power) - all without causing any measurable desensitization of the receiver system.  After a few days, one of these amplifiers failed, but the remaining 110 watt amplifier, now operating without the output combiner, happily chugged along.

The next year I acquired a 300 watt Vocomm amplifier and was able to use it for the remainder of the times that the Friendship Cruise was held.  Requiring 50 watts of drive, I still had to use the 50 watt amplifier, driven by 5 watts from the TM-733 to attain the full RF output.  When keyed down, the entire transmitter system drew about 60 amps at 12 volts from the battery bank, requiring frequent topping-off by a generator and DC power supply that were brought along. ^{(See note 2)}

With that much transmit power, the antenna was held aloft by a 30 foot (9 meter) mast to keep it away from people - and to help clear the local terrain and its effects.  As can be seen in Figure 6, there was a second mast with the UHF link antenna and a "back-up" 2 meter antenna.  When we arrived at the site, the first order of business was usually to set up the receive site, but once back at camp, we used a radio in cross-band mode and the two antennas on this short mast to get it on the air, providing "reasonable" transmit coverage.  Because of the effort required to set up the tall mast, battery bank and power amplifier, we often waited until the next morning to complete the setup, bringing our radiated transmit power up to its full glory!

"Listening" on the link frequency, this transmitter not only relayed my own, nearby receive site, but also the "other" repeater at Canyonland's Overlook. 

How well did it work?

The Panorama Point repeater itself worked better than we could have hoped:  It was "reachable" nearly everywhere on either the Green or Colorado River - although some sections of the upper Green and Colorado had somewhat weaker signals, requiring a good antenna and 50 watt radio - comparable to a typical car mobile installation - for reliable coverage.  Unexpectedly, it also provided coverage into the town of Moab, as far north as Price, Utah and even down near Hite, Utah - both well outside its expected coverage range and well outside the expected pattern of the beam antennas.

I'm confident that if I'd simply plopped down a "store bought" repeater with a single antenna and cavities, its performance - particularly on receive - would have been very much inferior as the signals from the depths of the gorges on the upper Green River were very weak and "multipathy". ^{(See note 3)}

With about 2.5kW of ERP one would expect that this repeater would have been an "alligator" (all mouth, no ears) but this was not the case:  When users were operating from the more extreme fringe areas - as in a deep river gorge, using a 50 watt mobile radio - the transmitter and receiver seemed to be more or less evenly matched, and despite running this much power, we did not experience any detectable "desense" where the strong transmit signal would overload the receiver.  At least part of this was attributed to the receivers themselves:  The RCA TacTec receivers used only modest amounts of RF gain in their front ends and a passive diode-ring mixer.  I have little doubt that if we had used more "modern" receivers we would have experienced overloading and would have had to place notch cavities, tuned for the transmit frequency, between the GaAsFET preamps and the receivers.

As a system, the Panorama Point and Canyonlands Overlook repeaters completely replaced the need for HF gear on the boats in the last decade or so that the Friendship Cruise was held, providing nearly seamless coverage from start to finish.

 * * *

Note 1:   A "bang-bang" solar regulator simply connects the solar panels directly to the battery when the voltage is too low - say, 13.2 volts - and disconnects them again when it rises above about 13.7 volts.  The PIC software implemented a timer so that after a disconnect from the panel when the voltage was high, it would not reconnect for at least 30 seconds, preventing rapid cycling.  With an open-circuit voltage of around 15 volts for the panels used, this was a simple, safe and reasonably efficient approach that could simply not cause radio-frequency interference in the way many modern "MPPT" solar chargers (with their PWM switching) might.

Note 2:  In the later years, a pair of 40 amp switching power supplies were used at the transmitter site to charge the battery as quickly as possible.  Not unexpectedly, we could load the generator to only about 60% of its rated output, owing to the terrible power factor of these supplies caused by their simple capacitor inputs:  Power-factor corrected supplies were not cheap and readily available at that time.  Also in later years, a very low power (1 milliwatt) 2 meter transmitter was constructed, connected to the battery bank, that telemetered the battery voltage using MCW (Morse Code).  If the battery voltage got too low, this transmitter would activate a subaudible tone and a receiver that had been parked on this frequency, configured to detect that tone, would remain silent unless/until the voltage dropped below the threshold, alerting us to the need to start the generator.

Note 3:  "Multipath" is when a signal - likely due to obstructions - finds more than one way to the other end of the communications path via reflection and refraction - a condition that is the rule rather than the exception when trying to get signals in/out of the deep gorges along these rivers.  While these multiple signals can reinforce each other, they are equally likely to cancel each other out.  By having multiple receivers and antennas - even two antennas very close to each other - the probability is significantly higher that at least one of the receiver/antenna combinations will be able to hear such a signal.  Because of the nature of FM signals, one can generally infer its quality by analyzing the amount of noise on it:  By comparing the amount of noise on the same signal, from two different receiver/antenna combinations - and always selected the "better" signal - the probability is increased that the received transmission will suffer less degradation.

* * *

Additional (related) articles:

• "The Most Sensitive Repeater On Earth?" (link)  - An article with more technical details about the link margins associated with the receive system.
• "Clint's Friendship Cruise Page" (link) - Pages describing the system, a bit of history about the Friendship Cruise, and other details.

This page stolen from ka7oei.blogspot.com

[End]

 I had a past test on this device but didn't touched it for around two years. Was convinced that it was a software or serial port situation. In the end nothing more than the antenna coil wire broken beneath the enamel (at the coil side) and one other that I had fixed previously at the PCB side. Because I tested continuity from the PCB to the antenna terminals only at that time I didn't realized the second situation until returning to it this last week.

Bellow the device:

After that sorted it works receiving the DCF 77 transmiter time. It's just a little picky in the direction and placement since I'm currently on the actual range limits. I used "vocap" site for  the actual direction of the signal.

There is a long thread on this device (in German) here. Also has a link for the "RCCD" software if you don't have original DOS software diskette provided with the device, it also runs under "dosemu" in Linux:

A native build for Linux can be found here (thanks to Guntram). I tested under Linux Mint 32 Bit. Bellow the debug output.

In the format HHMMSS and then day of week date year (better visible on the source code).

For 64 Bit Linux Mint you will probably get when running the hopf binary: "No such file or directory" when running "./hopf -d /dev/ttyS0" , you need to install the following lib32 "sudo apt-get install lib32stdc++6"

Inside the box:

The main chip for the RF reception is marked U2900, I could not find the datasheet for it but did found one that could be similar in terms of connections, as far as I reverse engineering it; the U2775B :

..still not sure if the same or not. 

Anyhow, I'm happy that the device is confirmed working and will go now to the box of the finished stuff without immediate use...!

Have a nice day!