Category Archives: How To

Belka-DX: Installing the new speaker and battery pack from Mobimax

Last month, Mobimax announced a new speaker option for the Belka-DX DSP receiver. This speaker is slightly different from the original Belka-DX speaker in that it has a full-size battery pack and fold-out legs to prop up this pocket-sized receiver.

Mobimax sent one of these speakers to me to install and evaluate at no cost to me–I received it last week and installed it yesterday.

Installation

The installation couldn’t have been more simple: the only tool needed is a small Phillips-Head screwdriver. Note that my Belka-DX already had the original speaker option installed.

All I needed to do was remove the lower two screws on both sides of the Belka chassis.

After doing this, the bottom section of the chassis simply pulls out (do this slowly since there are both battery and speaker jumpers).

Next, I unplugged the speaker and battery jumpers from the original speaker option.

Installing the new speaker section was simply a matter of plugging in the speaker and battery jumpers (each plug is a different size so they can’t be confused), then attaching the new pack to the back of the Belka-DX using the same four screws that had been removed.

The whole process might have taken four or five minutes (mainly because I took photos!).

How does it play?

Since I can’t really do a side-by-side comparison with the original speaker and this one, I simply listened to the original speaker tuned to WWV, WRMI, and the Voice of Greece for a while before installing the new speaker.

Both speakers are obviously very small as the Belka-DX is the most compact shortwave portable I’ve ever laid hands on.

Audio quality

I believe the original speaker has better audio fidelity, likely due to the fact it uses the body of the Belka-DX as an enclosure or resonance chamber. The new speaker has a dedicated enclosure, but it’s maybe 40% the size of the Belka-DX body.

In the end, though? Neither speaker will give you the audio fidelity of a traditional portable. The original speaker is just slightly better than the new one. With the Belka-DX, I see the speaker as a wonderful convenience, but frankly, I reach for earphones or headphones if I want to do DXing or proper broadcast listening.

Battery

The new speaker option allows for a full size battery pack in the Belka-DX. This is probably the biggest selling point of the new speaker. The original speaker option fits both the speaker and a smaller LiIon battery pack on the bottom plate of the radio.

The original speaker and smaller battery pack (top section of this photo)

Since the new speaker option adds a dedicated speaker section, it opens up the full real estate of the bottom plate for a full size battery again.

I should also add that the new speaker section matches the original Belka-DX enclosure and speaker in that it’s incredibly durable. Frankly, it feels military-grade and over-engineered. I love it.

Fold-out legs

I really like the fold-out legs on the new speaker. They actually have two indented sections that click into place as you fold them out. This allows for two different stable viewing angles. I prefer having them folded out all the way.

Size

The new speaker option adds a bit of weight and bulk to the Belka-DX.

Again: we’re talking about a wee little radio here, so I can’t imagine someone complaining about the size or weight. The new speaker makes the radio slightly deeper or thicker if you look at it from the side or profile. Frankly, it’s a negligible amount, but worth noting.

Should you buy it?

In my opinion, the main reasons to buy the new speaker option are to take advantage of the longer play time from the full size internal battery and to gain the two fold-out feet. The Belka-DX is so efficient that even the smaller battery pack in the original speaker option will power this radio for many hours without recharge.

Still, if these two factors are important to you, this is a no-brainer.

I would simply pick the speaker option that best suits your needs.

I must say again that it’s a real pleasure evaluating products that are engineered to the degree of the Belka-DX (and Belka-DSP) and both speaker options. These feel like they’re built to last a lifetime and could really take a beating in my various radio packs and kits.

Many thanks again to Mobimax for dispatching one of these for my evaluation.

Click here to check out the new speaker option from Mobimax.

Click here to read about the original speaker option and its installation.

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Guest Post: Indoor Noise and Ferrites, Part1

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Many thanks to SWLing Post contributor, TomL, who shares the following guest post:

Indoor Noise and Ferrites, Part 1

by TomL

My magnet wire loop antenna on the porch reminded me to revisit aspects about my noisy Condo that I still needed to understand. Some RF noise I could control if I could find the right kind of information that is understandable to a non-engineer like me. There is a lot written about the general problem of noise and radio listening, for instance this ARRL article with web links to research – www.arrl.org/radio-frequency-interference-rfi, but I needed to get more specific about my particular environment.

I had tried some common clamp-on TDK ferrites I had obtained from eBay a long time ago but they only seemed to work a little bit. I have since found out these are probably the ones which are widely used on home stereo system connections used to reduce noise on those systems. There must be a better way.

The more I researched topics, like a portable “Loop on Ground” antenna, or, using RF chokes on the magnet wire loop, it dawned on my feeble, misguided brain that I was wrongly thinking about how to use ferrite material. For one thing, the material used to suppress RF noise is made with a certain “mix” of elements, like Manganese-Zinc, that electrically “resists” a specified frequency range. Fair-Rite has a useful Material Data Sheets web page which lists the Types of ferrite material. For dealing with noise (at the Source causing the problem), I needed to use the right kind of “Suppression” materials and proper placement. So, it (partly) made sense why the TDK snap-on ferrites might not fully work to reduce certain noise coming from my computer screens, LED lights, USB devices, and cheap Chinese-made power adapters.

A very good paper is by Jim Brown (K9YC) of Audio Systems Group entitled, “Understanding How Ferrites Can Prevent and Eliminate RF Interference to Audio Systems [PDF]”. There is a longer paper speaking directly to Amateur Radio folks, but the Audio version is simpler and it uses some of the same graphs and ideas. I was drawn to the very detailed Impedance measurements of many different “Types” of ferrite material used for different noise mitigation. I remember the traumatic pain of my college experience trying mightily to understand the Van Vlack Materials Science text book to no avail. But Jim’s paper reminded me of the importance of using the correct type of ferrite material and in an optimal way that reacts favorably in the target frequency range to solve a particular noise problem. So, what are my problem areas?

Shortwave Noise

Loop antennas have been what I have experimented with the most. They do not pick up as much man-made noise (QRM) and they have a space saving footprint. Fortunately, there is a wooden porch where these things have been tried. I had successfully built a broadband amplified “ferrite sleeve loop” (FSL) in the past. It was useful for a while but it fell into disrepair and also the Condo building has steadily increased in noise output. The amplifier was just amplifying the noise after a while. I also tried phasing two antennas but found the ever increasing noise cloud was coming from all directions and I could not null it out. I even tried a “mini-whip” from eBay but that just produced a wall of noise.

I recently tested AirSpy’s YouLoop written about before, and the results were good. However, it seemed obvious to me that it was too small as a passive loop to capture shortwave signals strongly enough without resorting to another amplifier attached at the antenna and would not improve the signal/noise ratio. My current solution is a unamplified stealth magnet wire loop about 32 feet in circumference. In that article, I mention things like common mode RF chokes at both ends of the antenna connection, horizontal polarization, and basically accepting that only the stronger shortwave signals will be received in a predictable manner. I think for now, this is about all I can do for shortwave and mediumwave noise, as far as my own Condo-generated noise. Neighborhood noise is a different topic.

VHF Noise

I then started to isolate which devices caused which kind of noise when listening to my outside amplified antennas for FM/VHF and UHF-TV transmissions. Many consumer Power adapters make a lot of noise from VLF up into UHF ranges. One thing I did right was to try a 10 pack of these little miracle “Wall Wart” toroids from Palomar Engineers. One by one, I put one of these small toroids (19mm inside diameter) on my home AC adapters as shown in the pictures, and the noises started disappearing. It does not explicitly say, but I believe it is Type 75 material which suppresses the noise generating AC adapter (at very low frequencies) when wrapped 8 – 12 times.

Most egregious of these was my CCrane FM2 transmitter. A strangled warbling sound kept emanating from the monitor closest to my laptop. Installing ferrites on the laptop and back of the monitor were not working. I moved the FM Transmitter and noticed a reduction in noise. So, I put one of these little toroids on the power input of the device and the noise disappeared. Apparently, it was picking up noise from the monitor (as well as its own power adapter) and rebroadcasting it to all my other radios! The strangled warbler is no more, I choked it (HaHa, sick bird joke).

While looking for the monitor noise, I put the eBay TDK ferrites on all the USB ports and HDMI ports. This has helped greatly on VHF and confirms my suspicion that these cheap TDK ferrites are indeed a common type of ferrite material. Some informative graphs can be seen in Jim Brown’s Audio paper mentioned before. One example might be Figure 22, which shows the #61 Series Resistance which peaks around 100 MHz when using a toroid with three “Turns”. More confused, I could not find a definition of a “Turn”. Eventually, in his longer paper to Amateur Radio operators, he defines it, “…is one more than the number of turns external to the cores”. Somewhere else he describes using many single snap-on ferrites being electrically equal to just one toroidal ferrite with multiple Turns. And interestingly, more Turns shifts the peak impedance substantially lower in frequency. So, using the graphs he supplies, one can target a noisy frequency range to try to suppress.

I then put 6 of the TDK ferrites on the VHF input to the AirSpy HF+. Some FM grunge was reduced and was thankful for that. The rest of the background noise truly seems to be coming from the outside picked up by the amplified antenna.

Also, I juggled a couple of the amplifiers around and now have separate VHF/FM and UHF/TV amplifiers which cleaned up the FM reception a little bit more – https://www.youtube.com/watch?v=zkDsy95et2w .

UHF TV Quality

On a whim, I put the balance of the TDK ferrites on the FM/TV splitter input cable, 10 in all. The FM reception did not improve but the Over The Air UHF TV reception Quality improved noticeably. My weakest TV station now has a stable Signal level and the Quality is pegged at 100%. This is a nice surprise since it means that now all local TV stations on UHF will come in cleanly without dropouts and I can view all digital subchannels. I was even able to rescan and added two more low-power stations never seen before. ?

LED lights

I have common LED lights hanging over a number of fish tanks and some grow lights over an indoor plant box and can hear this noise on upper shortwave and higher radio bands. In a future article, I will explore RF noise from lights as its own special topic. For instance, why do some LED lights generate RFI and how to know before buying (I am using BR30 spot bulbs from name brands)? Also, there is a new kind of LED “filament” light out now that uses much smaller LED’s on both sides of an aluminum strip, greatly reducing electromagnetic noise output (or do they??). More questions than answers.

I will explore creating my own customized AC power cord attached to the AC power strips of the LED lights. I will need to test this for safety and efficacy, so I will want to take some time to do this right. The hope is that, using Jim’s info, I will be able to create a broad spectrum RFI suppression AC power cord and cost less than $30 each cord. We’ll see.

Finally, I will look at “stacked” toroids using different mixes of ferrite Types, creating a custom RF suppression better than using just one Type of ferrite material, using AC cords as the main examples. For instance, the best set of graphs in Jim’s paper, in my opinion, are Figures 21 and 24 compared to each other. Something I did not know before is that one can not only use multiple turns on a single toroid to get a lower, peaked frequency response, but also stack multiple toroids of the same Type to get a smoother frequency response. Then on top of this, combine that set with other Types to create a customized frequency response curve.

Radios are quieter now. Those pesky grow lights are still a problem as well as the upstairs neighbor’s lights which seem to be on a timer, making FM reception noisy again after 5pm!

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How to give your Reciva WiFi radio a second life before the service closes on April 30, 2021

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The C.Crane CC Wifi

In November 2020, we learned that the Reciva radio station aggregator would be closing down permanently which would effectively render a large portion of WiFi radios on the market useless. This closure will affect a number of WiFi radio manufacturers, but two of the most notable are Grace Digital and C.Crane. I own one of each.

The Grace Digital Mondo

The comments section of my original post about the Reciva closure became the default discussion group for Reciva device owners who were trying to sort out options to keep their devices functional. That article (at time of posting) has nearly 200 comments alone.

There have been some very productive discussions about circumventing the Reciva aggregator before the announced closure on April 30, 2021. Since this information is buried in such a deep comment thread, I wanted to give it better visibility and search-ability by creating a dedicated post on this topic.

Ray Robinson, one of the contributors who has been actively helping owners, has very kindly written up a tutorial for us here and I’m most grateful.

Ray’s Guide to setting up your own “Reciva” WiFi webserver

Ray writes:

[T]he bad news is that Qualcom is shutting down the Reciva website on April 30th, and any Reciva-based Internet radios will no longer be able to tune stations from that aggregator after the shutdown.

The sort-of good news is that if you have a station link stored in a preset on your Internet radio, the preset should continue to work after April 30th, until such time in the future as the station needs to change the link for their webstream.

Because, the other part of the bad news is that most Internet radios don’t have any way of directly inputting or modifying a webstream, or storing a webstream manually in a preset. So, after April 30th, you would lose any ability to change or update any of the presets.

That’s where my work-around comes in. Internet radios do have the in-built ability to address and pull data from a webserver – that’s how they use the Reciva site in the first place. So what I have done is point my radio (a CCWiFi) to a ‘web server’ on my local network instead. This solution uses a Windows PC; there may be a comparable solution using a Mac or a Linux box, but I’m not familiar with either of those.

First, make sure the PC you are going to use is visible to other PC’s and devices on your local network (‘Network Discovery’ turned on, file sharing enabled, etc.).
Second, I recommend you give the PC a reserved internal IP address in your router. If you leave it with IP being assigned by DHCP, its IP address could change anytime it is rebooted, and then your wi-fi radio won’t be able to find it for the presets. In my router, I assigned 192.168.1.1-200 for DHCP, and then gave my PC the reserved address of 192.168.1.201, which ensures it always has that same address.

Third, enable IIS (Microsoft’s ‘Internet Information Services’) in Windows. This will create a local web server on the machine. In Control, Panel, go to Programs / Turn Windows features on or off. Click the box next to Internet Information Services and OK, and let Windows install that component.

We are going to store our station webstream links on the PC in playlist files, which have the file extension of .pls. But first we have to tell IIS what to do with a .pls file, as it doesn’t know by default. (.m3u files will work as well, but I did it with .pls files, so I’ll detail how to use those.) We do this by adding a MIME type. Click the Windows start button, and search for IIS. The top result will be Internet Information Services (IIS) Manager. Click that. In the center of the panel that opens, click MIME Types and then ‘Open Feature’ at the top on the right. This will show you all the extensions IIS knows about. If you scroll down, you will see there isn’t one for .pls. So, we need to create it. At top right, click Add… In the panel that opens, enter a File name extension of .pls and a MIME type of application/pls+xml Then click OK and exit IIS.

If you now look in the root of the C: drive, you will see there is a folder called inetpub, with a subfolder called wwwroot. This is where we want to store the presets.

My CCWiFi has 99 presets, so I have put 99 files in this subfolder, named from Preset01.pls to Preset99.pls.

As an example, my first preset, Preset01.pls, is for Caroline Flashback. To create the .pls, open Notepad, and copy and paste the following:


[playlist]
NumberOfEntries=1
File1=http://sc2.radiocaroline.net:10558
Title1=Caroline Flashback
Length1=-1

Save the file, but change its extension from .txt to .pls.

Then, in Reciva, I need to store the entry in My Streams that will tell the CCWiFi to come and look at that file to know what to play. On the Reciva site in My Streams, I created a stream titled ’01 Caroline Flashback’ with a stream address of ‘http://192.168.1.201:80/Preset01.pls’ Remember, my PC has a reserved address of 201. If you use something different, then you will need to change the stream address accordingly.

Then, on the CCWiFi, go to My Stuff / MyStreams and select ’01 Caroline Flashback’. Reciva is telling the CCWiFi to go to my PC and look at the contents of Preset01.pls. This it does, and starts playing the stream. Then, it’s just a matter of storing that playing stream in preset 1 on the radio.

With that done, at any time in the future if I decide to change the contents of that .pls file, I can just store the details of any other station/stream, and the radio will play that instead without any reference back to Reciva.

I recommend you do that for all available presets on your Internet radio whether you are using them or not, even if they only contain duplicate entries for now, because that way you will maintain access to be able to use those presets in the future. And, you must do this before April 30th, when the Reciva site will shut down.

Actually obtaining the URL of a station’s webstream can be difficult; some stations are very helpful and provide them all on their website, while others seem to do their best to hide them. However, here in Los Angeles, I have found the webstream URL’s of all of our local AM and FM stations, plus the webstream URL’s of all North American SW stations, and all the UK stations as well (both BBC and commercial). I’d be happy to advise on that also, but it’s probably beyond the scope of this particular tutorial!

Thank you so much Ray, for taking the time to write up this tutorial.

If anyone is familiar with how to set up a similar webserver on MacOS or Linux, feel free to comment with details.

By the way Post readers: if the name Ray Robinson sounds familiar, it’s because he’s a weekly contributor to AWR Wavescan, and also a presenter on Radio Caroline Flashback!

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The “Signal Sweeper”: How to build a portable Wellbrook loop antenna

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Many thanks to SWLing Post contributor, Matt Blaze, for the following guest post:

The “Signal Sweeper”, a portable Wellbrook antenna setup

by Matt Blaze

Here’s a very simple construction project that’s really improved my travel shortwave and mediumwave listening experience.

When I go somewhere interesting (whether a day trip on my bike or a longer excursion to an exotic locale), the two things I’m sure to want with me are my camera gear and at least one good receiver. Fortunately, there are plenty of good quality shortwave receivers to choose from these days; the hard part is packing a suitably portable antenna that can do justice to the signals wherever it is I’m going.

I’ve long had a Wellbrook antenna on my roof at home. These wide-band amplified loops famously enjoy a reputation for excellent intermod and noise rejection, as well as an almost magical ability to pull in signals comparable to much larger traditional HF and MF receive antennas. A portable Wellbrook – something I could pack in my luggage that performs as well as the one on my roof, would be just ideal.

Fortunately, Wellbrook sells a “flex” version of their antenna intended for just this application, the model FLX1530LN. It’s essentially just the amplifier of their fixed-mount antennas, equipped with a pair of BNC connectors for you to attach a user-supplied ring of coaxial cable that serves as the antenna loop. This way, you don’t need to travel with the awkwardly large 1 meter diameter ring of aluminum tubing that makes up the normal Wellbrook. You can just bring a compact spool of coaxial cable and configure a loop out of it when you arrive at your destination.

The tricky part is how to actually form a stable loop out of coaxial cable without needing lot of unwieldy supporting hardware. In particularly, I wanted something that could be set up on a camera tripod to be freestanding and easily rotated wherever I happened to find myself wanting to play radio. The key would be finding or making some kind of mostly non-metalic support for the coaxial loop that could be folded down or collapsed to fit in my baggage or backpack for travel.

And then I found it: a humble 3-section telescoping broom handle sold on Amazon for about $15 that’s exactly the right size: the “O-Cedar Easywring Spin Mop Telescopic Replacement Handle“. It collapses to 22 inches (just short enough to fit in my suitcase), and extends to 48 inches (comfortably long enough for a one meter diameter loop).

Normally, a wire loop would need both vertical and horizontal supports in a cross configuration, but by using a reasonably stiff coaxial cable, I figured I could get away with just using the broom handle vertically. I found that LMR400 (the basic kind, not the “Ultraflex” version) holds its shape quite well in a one meter loop supported this way.

At this point, it was just a matter of the details of attaching and mounting everything together into a portable package.

A one meter diameter loop, which is the ideal size for the Wellbrook amp, can be made from 3.14 meters of cable (ask your middle-school math teacher). That’s about 10 feet for Americans like me. High precision is not required here, so I just cut 10 feet of LMR400.

The next step is to attach the middle of the cable to the top of the broom handle. The O-Cedar handle has a loop at the end for hanging it on a hook in your broom closet. It happens to be just the right diameter for LMR400, but not with BNC connectors attached. So you’ll have to thread the cable through before you crimp or solder on the with connectors. (See photo above). I used the Times Microwave crimp-on BNC connectors, which I had some extras of lying around. I also put some shrink wrap on the cable at either side of the broom loop, just to keep it from slipping out and becoming unbalanced, but that was probably unnecessary.

Now I needed a way to to attach the Wellbrook amplifier to the other end of the handle, as well as some way of mounting the whole thing to a camera tripod. My first thought involved a lot of duct tape. But I wanted something more permanent and reusable.

The key is something called an “L-Plate”, which is a piece of hardware intended to allow you to mount a camera to a tripod in either “landscape” or “portrait” mode. It’s basically two tripod dovetail mounts attached at a 90 degree angle. I used one that was in my junk box, but you can buy them new or used on eBay. I also needed a clamp to attach the L-plate to the broom handle. I used the Novoflex MiniClamp 26, which I got from B&H Photo. The clamp attaches to the inside of the L-plate with a captive screw. (See photos)

Next, I attached the amplifier to the other side of the L-plate using an ordinary screw-on hose clamp. Easy enough, and surprisingly sturdy.

And that’s it. To assemble the antenna, just extend the broom handle to about one meter, allowing for a roughly one meter diameter loop that’s as round as you can make it with the amplifier at the bottom. Then clamp the L-plate to the bottom of the handle so that the handle is just above the base of the plate, and attach to the tripod. (See the photos).

The Wellbrook is powered over the feedline with a 12VDC bias-T injector. So you need a clean source of 12 volts. I use a cheap Talent Cell battery pack (available on Amazon in various capacities). These actually deliver 11.1 VDC (3x 3.7V), rather than the 12V the Wellbrook calls for, but it works fine in practice. I can also use the same pack to power the radio and digital audio recorder.

In the photos, you can see the finished antenna setup on my roof, with my permanent base Wellbrook on the rotor in the background. The performance of the two antennas is quite comparable.

(Note that there’s an eBay seller that makes a somewhat similar travel loop. The performance is quite good under normal conditions, but it is a bit more subject to MW overload when near a transmitter site. So I prefer the Wellbrook, which is much less susceptible to overload, I’ve found.)

My usual complete travel setup is either a Reuter RDR Pocket C2 radio or a Sangean ATX-909X (recently upgraded to the X2 model). Both these radios work well with the Wellbrook. I use a Sound Devices Mixpre 3 to record airchecks in the field. In the photos, I’m on a rooftop DXpedition listening to Toronto traffic and weather from CFRX on 6070 kHz on a warm later winter afternoon.

The whole setup breaks down for travel pretty easily, and fits easily in my suitcase, backpack, or bike bag (see photos). I usually bring a larger tripod than this if I’m also taking my camera.

The Wellbrook setup has really made bringing a receiver into the field a lot easier and less uncertain. There’s no worry about finding trees or other supports for wires, and packing and unpacking is quick and easy. Have fun!

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Guest Post: Using Carrier Sleuth to Find the Fine Details of DX

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Many thanks to SWLing Post contributor, Nick Hall-Patch, for sharing the following guest post:

Using Carrier Sleuth to Find the Fine Details of DX

by Nick Hall-Patch

Introduction

Medium wave DXers are not all technical experts, but most of us understand that the amplitude modulated signals that we listen to are defined by a strong carrier frequency, surrounded on either side by a band of mirror image sideband frequencies, containing the audio information in the broadcast.

Most DXers’ traditional experience of carriers has been in using the BFO of a receiver, using USB or LSB mode, and hearing the decreasing audio tone approaching “zero beat” of the receiver’s internal carrier compared with the DX’s carrier frequency as one tuned past it. This was often used as a way of detecting that a signal was on the channel, but otherwise wasn’t strong enough to deliver audio. Subaudible heterodynes, regular pulsations imposed on the received audio from a DX station, could indicate that there was a second station hiding there, with a slightly different carrier frequency, And, complex pulsations, or even outright low-pitched tones could indicate three or more stations potentially available on a single channel.

With the advent of software defined radio (SDR) within the last 10 years or so, the DXer has also been able to see a graphical representation of the frequency spectrum of the carrier and its associated sidebands. (Figure 1) Note that the carrier usually remains stable in amplitude and frequency, unless there are variations introduced by propagation, but that the sidebands are extremely variable.

Figure 1

Figure 2

In addition, by looking at a finer resolution of the SDR’s waterfall display, one might see additional carriers on a channel that are producing heterodynes (audible or sub-audible) in the received audio (Figure 2). Generally speaking, a DX signal with a stronger carrier will be more likely to produce readable audio, although there are exceptions to that rule.

Initially, DXers wanted to discover the exact frequency of their DX, accurate to the nearest Hertz. Although only a small group of enthusiasts were interested, they have produced a number of IRCA Reprints (https://www.ircaonline.org and click the “Free IRCA Reprints” button) over the years under the topic of “precision frequency measurement” (e.g. T-005, T-027, T-031, T-079, T-090) describing their use of some reasonably sophisticated equipment for the day, such as frequency counters.

So, why would this information be at all important? In effect, the knowledge of the exact frequency of a carrier was used to provide a fingerprint for a specific radio station. Usually, this detail was used by DXers who were trying to track down new DX, and wanted to determine whether a noisy signal was actually something that had been heard before, so would not waste any more time with it. The process of finding this exact frequency has since been made much easier by being able to view the carrier graphically in SDR software, assuming that the SDR has been calibrated before being used to listen to and record the DX. Playing back the recorded files will also contain the details of the exact frequency observed at the time of recording. And, because the exact frequency of DX has become much easier to determine using SDRs, more and more DXers seem to be using this technique.

At present, Jaguar software for Perseus is the one being used by many to determine frequency resolution down to 0.1Hz, both in receiving and in playback. But, if you have recorded SDR files from hardware other than Perseus, it is possible to get that resolution also, using software called Carrier Sleuth, from Black Cat Systems, available for both Mac and Windows, at a cost of US$20.

This software will presently take as input, sets of RF I/Q files generated by SpectraVue, SdrDx, Perseus (which includes files recorded by Jaguar), Studio One / SDRUno, Elad, SDR Console, and HDSDR. It then outputs a single file with a .fft extension, that provides the user with a set of waterfalls, similar to those displayed by SDR programs. The user decides ahead of time which frequency or set of frequencies (including all 9kHz or all 10kHz channels) will be output, and these will be displayed as individual waterfalls. one for each chosen frequency. These waterfalls can be stepped through from low frequency to high frequency, or chosen individually from a drop down menu.

Let’s start by looking at a couple of output waterfalls and work out what can be done with them, then step back to find out how to generate them, and what other data is available from them. Finally, we’ll do a quick comparison with two other programs that can produce similar output, and discuss the limitations in all three programs.

Example outputs from Carrier Sleuth

An example showing the original intent of Carrier Sleuth, determining precise carrier frequencies, is shown in Figure 3, a waterfall from 1287kHz on the morning of 28 November 2020. At 1524UT, a woman mentions “HBC” and “Hokkaido” in the original recording, so, it’s JOHR, Sapporo. Although there are a number of vertical lines representing carriers in this graphic, only one has a strong coloration, indicating at least 25dB more strength than any other carrier at the time of the ID, and about 50dB more than the background level. The absolute values of time, signal strength, and carrier frequency precise to 0.1Hz, can be found by mousing over the desired point in the waterfall and then reading the numbers in the upper right corner of the display, (encircled in Figure 3). In this case, the receiver’s reference oscillator had been locked to an accurate 10MHz clock, disciplined by GPS, so the frequency indicated in the software is not just precise, but should also be accurate. Similar accuracy could be obtainable by the traditional method of calibrating the SDR to WWV on 10 or 15MHz.

Carrier Sleuth indicates 1287.0002kHz, within 0.1Hz of that observed by a contributor to the MWoffsets list about 7 weeks earlier (https://www.mwlist.org/mwoffset.php?khz=1287). If you look closely, there is a slight wobble on the frequency, but the display is precise enough that it can indicate that, despite the wobble, JOHR does not wander away from that frequency of 1287.0002kHz.

Figure 3

But let’s face it, tracking carriers to such accuracy is a specialist interest (though admittedly, the medium wave DXing hobby is full of specialist interests, and this one is becoming more mainstream, at least among Jaguar users). However, if I played back a file from another morning, and found a strong carrier on a slightly different frequency from 1287.0002kHz, it might be an indication that some new Chinese DX was turning up, and that the recorded files would be worth a closer listen at that particular time.

Figure 4

In fact, I’ve found Carrier Sleuth to be useful in digging out long haul DX after it’s been recorded, as both trans-Arctic and trans-Pacific DX at my location in western Canada can be spotty at the best of times. This means spotty as in a “zero to zero in 60 seconds” sort of spotty, because a signal can literally fade up 10 or 15dB to a readable level in 20 seconds, perhaps with identifiable material, then disappear just as quickly. My best example so far this season was on 1593kHz, early in the UTC day of 16 November 2020, when a Romanian station on that channel paid a brief visit to my receiver in western Canada. An initial inkling of that showed up in a Carrier Sleuth waterfall, a blotch of dark red at 0358UT, and indicated by the yellow arrow in Figure 4; that caused me to go back to the recorded SDR files that had generated these traces.

The dark blotch indicates a 10dB rise and fall in signal strength including about 60 seconds of rough audio, which turned out to be the choral version of the Romanian national anthem (RCluj1593.wav). That one carrier and another one both started up at 0350UT, the listed sign-on time for Radio Cluj, which does indeed begin the broadcast day with that choral anthem. Which one of the Radio Cluj transmitters was heard is still an open question, due to the lack of carrier sleuths (computerized or otherwise) on the ground in Romania, but the more powerful one listed is a mere 15kw, so I will take either.

Finally, for those who have interest in radio propagation, the Carrier Sleuth displays can reveal some odd anomalies, for example, Figure 5 which displays both Radio Taiwan International (near 1557.000kHz on 28 November, but varies from day to day), and CNR2 (1557.004kHz) carriers as local sunrise at 1542UT approached in Victoria, BC.

Figure 5

The diffuseness of the carriers is striking, as is their tendency to shift higher in frequency at local sunrise. This doesn’t seem to be some strangeness in the original SDR recording, as there appear to be unaffected weak carriers on the channel. For comparison, Figure 3 shows the same recorded time and date, but on 1287kHz, and JOHR’s carrier is pretty stable, but there are others on that channel that show the shift higher in frequency around local sunrise. As one goes lower in frequency, these shifts became smaller and less common on each 9kHz channel, and disappear below about 1000kHz. On later mornings, however, the shifts could be found right down to the bottom of the MW band. Certainly, these observations are food for further thought.

Many of the parameters in Carrier Sleuth are adjustable by the user, for example, the sliders at the top of the screen can allow adjustment of the color palette to be more revealing of differences in signal strength. The passband shown is also easily changed, and in fact, setting the passband width to 400Hz, instead of my usual 50Hz , and creating another run of the program on 1557kHz, shows very clearly the sidebands of the “the Rumbler”, a possible jammer on the channel (Figure 6). Incidentally, a lot of the traces around 1557.000kHz in Figure 5 may well be part of “the Rumbler” signal as well, as filtering of the audio doesn’t seem to improve readability on the channel.

Although the examples here are taken from DXing overseas signals from western Canada, there is no reason why similar techniques may not be applied to domestic DXing, particularly during the daytime, when signals can be weak, but can fade up unpredictable for brief periods.

Figure 6

How to create these waterfall displays in Carrier Sleuth?

So, how can you get these displays for yourself? A “try before you buy” version of the program is available at http://blackcatsystems.com/software/medium_wave_carrier_display_app.html and both the website and the program itself contain a quite detailed set of instructions. However, the 25 cent tour can be summarized this way:

You start with a group of supported SDR data files, previously recorded, and use “Open I/Q data files” in the File drop down menu. Figure 7 shows the window that will open to allow you to choose any number of the files from your stored SDR files, by clicking the Add Files button circled in red. Then choose one of the options inside the green circle in Figure 7. They are explained in more detail in the help write up; note that the “Custom Channel” can be specified to considerably more precision than just integer kHz values, e.g. 1205.952 The rest of the settings you will probably adapt to your needs as you gain experience. Finally, set an output file name using the Set Output File button, and hit the “Process” button at the bottom of the window. There are a couple of colored bars in the upper right hand corner of the display that indicate progress, along with number of seconds left, although these are not always visible.

Figure 7

The generation of these waterfalls takes time. A computer with a faster CPU and more memory will speed things up. There is, however, an important limitation of the program. It is specified for 32-bit systems, and although it will run with no problem on 64-bit systems, individual input I/Q files are therefore restricted to 2GB or less. Many SDR users now choose to create larger files than this, and Carrier Sleuth will not handle them. Another possible limitation can occur when processing 32M FFTs, which are useful for delivering very fine frequency resolution of the carriers displayed. The program really requires in excess of 4GB of memory to handle the computation needed to deliver this fine a scale. Unfortunately, both the 2GB file size limitation and insufficient memory limitation deliver generic error messages, followed by program termination, which leaves the inexperienced user none the wiser about the true problem.

This might be a good place for a word about FFT size and Resolution Bandwidth (RBW). The FFT is a mathematical computation that takes as its input the samples of digital data that an SDR generates (or those samples that have been saved in recorded files), and generates a set of “bins”, which are individual numbers representing signal strength at a defined number of consecutive frequencies spaced across the full bandwidth being monitored by the SDR. You could think of these bins as a series of tiny consecutive RF filters, spread across the band, each delivering its own signal strength. As we are trying to look at fine scale differences in frequency when using a program like Carrier Sleuth, it is important that these little “RF filters”, or bins, each have a very narrow bandwidth. This value is called “Resolution Band Width” (RBW), and preferably should be a fraction of a Hertz to get displays such as those shown in Figures 3 through 5.

The “FFT Length” is the number of bins that the FFT display contains, and is equal to the number of I/Q samples (either from the SDR or recorded file) that are used for the input to its computation. The relationship between FFT Length, the bandwidth of the SDR or of the original recorded I/Q file, and the RBW is fairly simple:

Because the MW DXer is usually looking at data with 1MHz or more bandwidth, this equation tells us that to get a smaller than 1Hz RBW, we will need to have an FFT length of well over one million bins, so it would be wise to use an FFT length at least 8M(illion). If you are looking at a recorded file that is from an SDR using a lower bandwidth, then a lower FFT length will do the job to get a smaller RBW.

A downside of using a long FFT length is that the time resolution of the FFT becomes poorer, resulting in a display in Carrier Sleuth that will appear to be compressed from top to bottom compared with what was seen when recording the SDR file, and with correspondingly less response to fast changes in signal strength. However, using a 16M FFT Length on a recording of the MW band results in a time resolution of about 12 seconds, so it should not be a deal breaker for most.

Producing signal strength plots

A further specialist activity for some DXers is recording signal strength on specific channels, and then displaying the progress of signal strength versus time, often to indicate when openings have occurred in the past (say, at transmitter sunset), and perhaps allowing one to predict such openings in the future. But, the world has come a long way from the noting down of S-meter readings at regular time intervals, both in deriving signal strength and in plotting the results. Read on for an example.

Figure 8

Carrier Sleuth recently added the capability of creating files containing signal strength versus time for specified frequencies, and, depending on the size of RBW, to deliver that signal strength as observed in a passband as narrow as 0.05Hz, or as wide as 10Hz. The program extracts the signal strength information from one of the FFT files that it has already generated from a selection of SDR I/Q files. In Figure 5, two stations’ signals, from Radio Taiwan International, and from CNR2, were featured in the display. With roughly 4Hz difference between the two signals, it is easily possible with Carrier Sleuth to derive signal strength from each one, specifying a bandwidth of, say 1.2Hz, to account for the propagation induced drifts and smearing of the carriers, not to mention any drift in either the receiver or transmitter.

The program creates a .csv file (text with comma delimiters) of signal strength versus time for all the frequencies chosen from an individual FFT file, but does not plot them. There are several programs that can create plots from CSV files For example, an Excel plot generated from Figure 5 is in Figure 8, showing peaks in those signals that occurred both before and after local sunrise at 15:42UTC. Note that the user is not restricted to the signals found on just one of the waterfalls that are found in the FFT file, but can pick and choose dozens of signals found anywhere in those waterfalls. (Note also that one can choose locations on any waterfall where there is no signal trace, in order to provide a “background level versus time” in the finished plots, if desired)

The process used to generate this CSV file involves searching through the FFT waterfalls for signal traces that are likely candidates for adding to such a file. On the first candidate found, the user right clicks the mouse on the trace, at the exact frequency desired; this will bring up an editable window. The window will show the chosen frequency as well as any subsequent ones that will be chosen, then the overall selection is saved to a text file after editing, so that the user can move on to generating the CSV file.

That file is created by going to the File drop down menu, and choosing “Generate CSV File”, where the text file produced earlier can be chosen. Once that file is selected, the CSV file is immediately generated, and can then be manipulated separately as the user chooses.

Are there comparable programs?

Displaying waterfalls in SDR programs playing back their own files is nothing new, though not that many can do it at as fine a scale as Carrier Sleuth does, and most programs are not optimized to handle such a variety of input I/Q files.

One that does read a fair number of different kinds of SDR files is the SDR Console program; this includes Data File Analyser (64-bit only) which also can display carrier tracks to a high resolution, so let’s take a quick look at what Analyser does. If you are familiar with SDR Console, and are reasonably experienced with the way it handles your SDR or plays back files from your favored SDR software, then these online instructions https://www.sdr-radio.com/analyser will help you get started with Analyser

This program will input a group of SDR files, then display an equivalent to a single one of the waterfalls output by Carrier Sleuth, displaying the carrier traces in reverse order, with time running from bottom to top of the display. Figure 9 shows the equivalent of Carrier Sleuth’s display of the 1287kHz carrier traces shown in Figure 3. Analyser has a convenient sliding cross hair arrangement (shown in the yellow oval) to reveal time and frequency at any point in the display, but the actual signal power available at that point must be derived from the rough RGB scale along the left hand border. Analyser is apparently capable of about 0.02Hz resolution when reading from full bandwidth medium wave SDR files, but the default is to display exact frequency only to the nearest Hertz. The “Crosshairs” ribbon item has a drop down of “High-Resolution” which displays to the nearest milliHertz however, though that will be limited by the actual RBW of the generated display. The graphic display can be saved as a project after the initial generation of the signal traces, which allows the user to return to the display without having to generate it all over again, equivalent to opening one of Carrier Sleuth’s FFT files.

A useful facility in Analyser is the ability to click “Start” in the Playback segment of the ribbon above an Analyser display, then mouse over and click on a signal trace; this action will play back the audio for that channel in SDR Console, at that point in time.

It is possible to generate a signal strength plot of signal strength versus time for any individual frequency in the waterfall display, and to save that plot as a CSV file (“Signal History”). But, the signal strength is that found only in a +/- 0.5Hz passband around the chosen frequency, with no other possibilities. If you want to generate a plot for another frequency on the same waterfall, then you will need to run the process again, and if you want a plot for another frequency in the SDR files, then you need to generate another waterfall, which, depending on your computer’s capability, could take some time. On an i3 CPU-based netbook with 4GB of memory, it took 30 minutes to produce one frequency’s worth of traces from data files scanning three hours. On the same machine, Carrier Sleuth could deliver all 9kHz channels in 1hr20min from the 3 hours of files. However, it also took 1hr20min to play back just one channel in Carrier Sleuth, which is not so efficient. (further note: Nils Schiffhauer has developed a technique to speed up Data Analyser processing, by first using Console’s Data File Editor on full bandwidth MW recorded files; details will likely appear at https://dk8ok.org)

To conclude then, SDR Console’s Analyser will produce a display of a single channel faster than Carrier Sleuth will, and will play back the audio associated with that channel, while also having the capability to plot and record signal strength for a single given frequency within that display, but only on 64-bit computers. It can also handle SDR files larger than 2GB in size, and will run more quickly if a NVIDIA graphics card has been installed. Analyser is also strict about sequence of files. If there is the slightest gap between one file finishing, and the next file starting in time sequence, it regards that as a new set, that will need to be processed separately.

Where Carrier Sleuth is more useful is that once an FFT file has been generated, it is easy to quickly check multiple channels for interesting openings during the recorded time period. It can also provide very precise frequencies of carriers, and is able to generate a file of signal strengths versus time from multiple frequencies, including those frequencies that are separated by barely more than the RBW. For the MW band, that can be near 0.1Hz, often beyond the capability of transmitters to be that stable. See Figure 10, which shows signal strength traces from JOCB and HLQH both on 558kHz, and separated in frequency by 0.1Hz. At 1324UTC, JOCR dominates with men in Japanese, and at 1356UTC, the familiar woman in Korean dominates, indicating HLQH.

Figure 9

Figure 10

Incidentally, another program that seems to offer a similar functionality to Carrier Sleuth and SDR Console’s Analyser is, of course, Jaguar, which has made a point of displaying 0.1Hz readout resolution when using the Perseus SDR, and in playing back Perseus files, but…only Perseus. There is a capability called Hi-Res in Jaguar Pro that can be applied when playing back files; this also displays fine scale traces of frequency versus the passage of time. Steve VE6WZ, sent the example shown in Figure 11, zeroing in on his logging of DZAR-1026. As with Analyser, clicking on a certain point in the display plays back the audio at that time, but it is unclear at this point whether the display can be saved, or whether it is generated only for one individual channel, and then is lost.

Figure 11

+ + + + + + + + + + + +

Availability

Carrier Sleuth http://blackcatsystems.com/software/medium_wave_carrier_display_app.html

Analyser (SDR Console) https://www.sdr-radio.com/download

Jaguar http://jaguars.kapsi.fi/download/ (these are the Lite versions; to unlock the Pro version, purchase is needed)

(this article first appeared in International Radio Club of America’s DX Monitor)

Many thanks, Nick. This is amazing. What a brilliant tool to find nuances of a DX signal. I can’t help but marvel at the applications we enthusiasts have available today. Thank you for sharing!

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Rob compares horizontal and vertical SWL random wire antennas

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Many thanks to SWLing Post contributor, Rob Zingarelli, who shares the following guest post that originally appeared on his blog in October, 2020:

Shortwave Antenna: Vertical or Horizontal?

by Rob Zingarelli

This is a question that has circled around on the fringes of my consciousness for years now, but one that I’ve never quite found time to test. And it is a simple question: When using a random wire antenna with a portable shortwave receiver, is it better to string the wire vertically or horizontally, or does it even matter? Mostly this is a question when out camping, because arranging a 19′ wire vertically is usually a good bit more involved than just stringing it out along some nearby bushes.

Before going any farther, I want to point out that this is an exercise in ordinary backyard shortwave listening with relatively inexpensive equipment. There are many, many better-engineered and more costly solutions to the technical challenge of shortwave scanning, and this does not address any of those sophisticated approaches. This is for the person who opens up the box and wonders about the best way to hang the included long-wire auxiliary antenna.

Equipment: Tecsun PL-660 SW/AM/FM/Air Band receiver, with its included 19′ random-wire antenna. Internal battery power used.

Conditions & Time: Clear local weather. hamqsl.com’s nowcast of band conditions were fair from 3.5-14.35 MHz, and poor for higher frequencies, with SFI = 72, SN = 26, A = 5, K = 1. Time was 21:00-21:30 UTC, or 4-4:30 pm local CDT.

Procedure: Out in the backyard (typical residential neighborhood, well-spaced ~150′ between houses, above-ground power lines 125′ away), suspend random wire from ground to its full length. This was achieved using a length of paracord over a tree limb, with the tree trunk ~30′ from the radio’s location. With the PL-660’s antenna gain control set to “Normal” (i.e., the mid-setting of Local-Normal-DX) and the bandwidth set to narrow, use the receiver’s automatic scan function to see how many stations were received. Make notes of the number of transmissions detected, reception characteristics and quality, and any perceived noise levels. Re-orient the antenna to a low horizontal position, over two sawhorses approximately 3′ high (see picture), and repeat.

Sawhorses spaced ~17′ apart. Radio and notepad can be seen on ground in front of the near sawhorse.

Results: For the vertical antenna orientation, 32 stations were detected between 5959 – 15730 kHz. Nearly all were intelligible, with those at the lower end more steady and those a the higher end much more variable in strength. For the horizontal antenna orientation, 21 stations were detected between 9265 – 1570 kHz. Similar overall signal quality was heard for the received stations in either antenna orientation. More noise was noticeable at the lower frequencies between the stations for the vertical antenna orientation. However, this was significantly below the received signal levels, and not an issue in the overall listening quality.

Conclusions & Discussion: Suspending the wire antenna vertically worked better, especially at the lower frequencies. Getting a wire up 21’+ vertically is usually not as convenient as stringing it horizontally, but it may be worth the extra effort, depending on the location, campsite, nearby trees, etc. The overall conditions were typical for fall camping weather, with fair-to poor radio propagation conditions, so this result should be broadly applicable for how SW portables are often used. This result may change with propagation and radio noise conditions, both for atmospheric and local noise sources. Testing will continue as propagation conditions improve with solar cycle 25 getting underway.

——-

Addendum, 10/12/20: While writing this up yesterday evening, it occurred to me that I hadn’t tested the PL-660’s built-in whip antenna. This comparison is important, because sometimes the wire antenna is too cumbersome to deploy. So, how does the whip antenna compare?

Conditions & Time: Overall, very similar to yesterday. hamqsl.com reports fair conditions from 3.5–14.35 MHz, and poor for higher frequencies. SFI = 72, SN = 26, A = 3, K = 1. Same time of day as yesterday’s testing.

Procedure: Repeat of yesterday, with the whip antenna added to the test. The whip was oriented vertically.

Results: For the vertical 19′ wire, 31 stations were found by the auto-scan function between 2380 – 15770 kHZ. Electrical noise was low but audible in the 3 MHz region, fading to none at higher frequencies, and not a significant source of interference with any stations. For the horizontal wire, 15 stations were found between 9265 – 13630 kHz. Electrical noise was barely audible. With the whip in use only 1 station was found. Switching the antenna gain to its DX (most sensitive) setting, 6 stations were found.

Revised Conclusions: Adding to yesterday’s conclusions, the whip antenna functioned but was vastly inferior to the wire antenna in either configuration, even with the gain set to DX. Today’s results with the wire antenna were, unsurprisingly, very similar to yesterday’s, given that the ionospheric and weather conditions were nearly identical. Noise was not a factor in receiving for any of these antennas or configurations, but did noticeably increase for the vertical wire antenna.

Thank you for sharing this, Rob! It’s experiments like this that help us determine, especially, what antenna setups work at our own particular locations since RFI characteristics can vary so much. I’m guessing had your horizontal wire been elevated to even 20′ off the ground it might have produced better results, but sometimes this can be difficult to achieve. I like how you used the auto search function to determine the number of stations you could receive with each setup and it was a great addition to include the built-in telescoping whip.

Thank you again for sharing your results with us!

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How to build a PC keyer and AM modulator for the EMTX emergency transmitter

PC keyer and AM modulator: A 15-components versatile keyer and powerful PSU modulator for the EMTX (Emergency Transmitter)

by Kostas (SV3ORA)

Schematic of the keyer and modulator (on the left) for the EMTX. The EMTX schematic is shown as well on the right, to determine the connections to the keyer/modulator.

Introduction

My very successful emergency transmitter (EMTX) was only capable of CW or other slow speed ON/OFF keying modes. Then I thought, why not “give voice” to the design? CW is good, but it is half of the fun. If you could use your simple CW transmitter to send out your voice as well, this would be great. You could now chat comfortably on the nets or use any digital radio amateur mode and have much more fun. The simplest modulation you can apply to an existing CW transmitter, is the AM modulation. And whereas this is an old modulation, mostly abandoned by HAMs due to beeing inefficient, there are still AM nets on HF. But do not forget, AM can also be heard by SSB receivers by zero-beating the receiver to the AM carrier. So you could still use your simple AM transmitter to QSO with the SSB guys!

Along with the modulator, there is also a versatile keyer embedded to the circuit, so that the EMTX can be manually keyed with different ways or automatically keyed by audio tones from the PC. For more information on the keyer, keep reading.

The AM modulator

In the old days, the most common way to apply AM modulation was to modulate the high voltage to the plate of the tubes, using a transformer and a powerful audio amplifier. In low voltage solid state circuits, you can still do it using transformers, but you can also use series transistors instead of the transformer. All these things require many components and/or powerful AF amplifiers if one is to modulate higher power transmitters. This does not match the keep-it-simple design I am trying to achieve here.

So I thought of a simple trick with the use of the extremely common LM317 regulator, used as a modulated power supply. This modulator uses just a few common cheap components and it is able to achieve remarkably good modulation levels for it’s parts-count, just from line audio input. It juices every bit of the internal circuicity of the LM317, just look at where the base current of the 2N2222 comes from.

The AM modulator is a kind of novelty. Whereas there is nothing special in a modulated power supply, this circuit has some interesting properties. It is amazingly sensitive and it is able to provide lots of modulated current to any low power transmitter that it can feed. It can be easily driven by the line output of any laptop (around 20% volume) and provide a very good depth modulation to the transmitter. Charles Wenzel was kind enough to do a simulation on the circuit I developed, which is shown below.

His simulated circuit is a slight variation (for measurement purposes). The resistor to ground on the base stabilizes the bias and the ratio of R1 and R2 set the output voltage (0.6 volts across R2 gives about 8 volts across R1). He put in an emitter resistor just for good measure. Same for the series resistor from the source. Charles words, “I don’t know how believable these results are but it looks pretty darned good!”.

The circuit is being used as a current booster, the current being the supply to the transmitter and dependent on the voltage it produces. The LM317 always tries to keep 1.25V between it’s output pin and “adj” pin but where we benefit here is the current at the “adj” pin is very low, so it is easier to apply audio to it. Effectively, the error amplifier inside the voltage regulator is used as an additional amplifier stage. The output pin voltage varies according to the voltage on the “adj” pin so if we use it to bias the transistor we get negative feedback which improves the quality of the modulation. More output voltage = more bias current = lower output voltage. The result, is a very cheap, low components-count, very sensitive AM modulator that can supply lots of power to easily drive the transmitter and produce a clean and deep AM modulation!

The AM modulator bias is set with the 1M potentiometer. Depended on the bias level, the idle carrier on the EMTX can be set from about 0.5W all the way up to 8W. Needless to say that this modulator can modulate any similar power transmitter, not just the EMTX.

The keyer

If it is to modulate the EMTX from the PC, so as to use the different digital modes, there must be a way to key it also from the PC. This is why I decided to embed into the same circuit, a PC keyer which is triggered by the line audio of the PC, but also triggered manually (internal or external key). Keying by audio tones was decided, because modern PCs do not have LPT ports to trigger directly by DC. This keyer uses a reed relay to reliably, fastly and scilently key the EMTX, which is activated by a transistor. The base current for the transistor is derived from the audio signal after rectification. The incoming audio from the PC line passes through the mini audio transformer to increase its voltage, it is rectified and then charges the shunt capacitor to drive the base of the transistor. The keyer “speed” (decay) is determined by the shunt capacitor size. The circuit starts to trigger from about 50-60% of my sound card output signal level.

The relay used to key the EMTX, must be able to tolerate at least 1A of switching and carrying current. Note that the relay contacts switching current is not the same as the contacts carrying current. Reed relays are the best especially if you want long relay life, noiseless operation and very fast switching speeds, like the ones used in Hellshreiber. If you can’t find such a relay, you can use a reed switch capable of 1A of switching and carrying current and then place a suitable electromagnet close to it, so you can build the relay yourself. If you do so, find the best point where the reed switch responds to the electromagnet.

The keyer relay must be as close as possible to the emitter of the transistor used in the EMTX. The connectors at the back of the EMTX and the keyer/modulator have been physically placed so that when the two units are side by side, a very short link cable is required for this purpose. With the two devices placed close together, you can now use any length of cable for your manual external key, which is now connected to the “EXT” connector of the keyer/modulator.

The keyer does also have an internal mini straight key. I find this idea very nice, to avoid extra cables. It is not the most convenient key in the world, but it is there along with the transmitter every time you need it. By using a special panel switch from apem, I was able to triple this switch usage for the different modes of the keyer. The vinyl lever cap you see in the next picture, is the original part of the switch, to make it easier to key with your finger. But you may build such a part on your own, to fit on other switches types.

The switch is an ON-OFF-(ON momentary) switch type. In the default (middle) position, only the PC keying action is activated. In the top position (ON), the keyer is always active, which is useful for broadcasting audio (into a dummy load). The bottom (ON momentary) position, is the manual PTT action. This is used as a straight key on OOK operation, or as a PTT on AM voice operation. Simple and effective!

Initially, I used one channel of the PC sound card for triggering the keyer and also as an AF signal for the AM modulator, but this caused several problems of unreliable keying or distortion. So I decided to use a second separate AF input (KAF) to key the keyer. This second input, uses the other channel of the stereo sound card. With the addition of this input, there is no interaction between the keyer and the modulator. The AF levels that the keyer and the modulator require, can be set independently. Instead of adding more hardware for the purpose, I have chosen to set these levels by adjusting the volume and the balance of the sound card, which works great. Also, programs like Fldigi, have options for using one of the two channels of the stereo sound card as a keying interface (PTT channel), which makes the keying efen more reliable. When the program is in transmit mode, a continuous tone is heard on the PTT channel. This steady tone, is used by the keyer as a reliable keying signal, independent of the audio signal of the digital mode that modulates the modulator. This solution works very reliably for any mode. But if the program you are using does not have an option for a PTT channel, that is ok, as the keyer works reliably even without this feature. For voice communication or broadcasting music (into a dummy load) you just use the internal key switch as a PTT to handle these modes.

Results

Prior to building the keyer and the modulator in the same device, I had tested the circuits independently quite a few times, to ensure the results can be reproduced. The modulation quality and depth out of the AM modulator have to be listenned to be believed. I have not made any linearity measurements, I just trust my ears on this one. It works great on music as well as on voice. Apart from that, this is the most sensitive AM modulator I have ever built, requiring only a small fraction of the line level output of the PC sound card.

When modulated by this modulator, the EMTX shows no audible signs of FM modulation. I switched my receiver to SSB and I could perfectly zero beat the AM modulated music signal which stayed on frequency and it’s tone did not change during loud audio signal music. Switching back and forth from SSB to AM modulation on the receiver, I did not notice any difference in the audio quality, apart of course from the narrower bandwidth on SSB modulation, due to the narrower IF filter inside the receiver on SSB.

The AM/OOK switch is used to select the modulation applied to the EMTX. When the keyer is set to be triggered by audio from the PC, at the OOK position, the EMTX is just switched on and off by the audio tones applied to the keyer, or by the manual key, internal or external (connected to the “EXT” connector). At AM position, the EMTX is switched on by the audio signal applied to the KAF connector and at the same time AM modulated by whatever audio signal is applied to the AF connector. On voice communications, the momentary position of the internal key is used as a PTT. On music broadcasting (into a dummy load) the non-momentary position of the internal key is used to keep the keyer always active.