This blog is dedicated to shortwave radio listening in the 21st century: the art of tuning into the signals of radio stations that are transmitting from thousands of kilometres away without the use of the Internet. It can be viewed as a fun hobby but also as a way to maintain an alternative, independent means of obtaining information in our digital age. Posts cover topics including shortwave radio equipment and accessories, station profiles, program recordings and shortwave radio history.
This is a quick "how-to" post on making spectrum recordings with the latest
model of the ultra-portable Belka shortwave receiver, the Belka-DX.
A number of positive reviews of the previous model of this receiver have
already been posted online. The new version boasts greater sensitivity,
extended shortwave coverage and the ability to monitor I/Q – or radio
spectrum – data in real time.
A separate 3.5mm stereo port is used for outputting I/Q data in analogue
format. This seems an unusual design choice: nowadays, most SDRs send I/Q data
digitally over USB (e.g. see FunCube Dongle Pro+).To get SDR applications to
work with this data, Belka's I/Q output needs to be connected to the line input
of a regular PC sound card. The digital —> analogue —> digital
conversion chain will inevitably result in the addition of small amounts of
noise to the final spectrum output. However, one advantage of this design
choice is that it is possible to capture this data using a portable audio
recorder.
Belka DX I/Q output port
While Belka's I/Q sample rate is reportedly 192 kHz, few recorders are capable
of capturing audio at this rate (and none of them are cheap or particularly
compact), and the 96 kHz option is far more widely available. In practice,
recording at this rate means that roughly 48 khz on each side of the centre
frequency become truncated, but on shortwave, 96 khz can still pack several
broadcast stations (or, alternatively, a few dozen ham radio transmissions).
Additionally, DSP tools in SDR applications such as SDR# can be used to clean
up I/Q recordings in ways that are far superior to what's possible to achieve
with regular post-processing of audio recordings.
I made several field spectrum recordings with my Zoom H1, connecting Belka's
I/Q output to the recorder's line input and using the "WAV @ 96 kHz / 16 bit"
setting. I found that keeping the input level between -24dB and -12dB results
in sufficient gain for later analysis without overloading the recorder.
It's worth noting that the gain and the slight offset from the tuned centre
frequency seem to change depending on both the frequency and the chosen
demodulation mode.
Results
First up is the Voice of America recording I made on 06/11/20 in a London park.
To assess the quality of my I/Q capture method I recorded the audio output in
parallel using my Sony ICD-PX333. I set Belka's demodulation mode to "AM2", which
is its pseduo-synchronous AM detection setting. Below are the two
recordings, with the I/Q data demodulated in SDR#:
Belka's own demodulation is a little on the distorted side compared with SDR#'s I/Q demodulation. This is probably due to the specifics of its "AM2" setting.
Note that I'm using SDR#'s handy "invert spectrum" and "correct IQ" options,
and that I've modified the spectrum recording file names so that the centre
frequency is displayed correctly.
Below are two more examples of this I/Q capture method and their audio recording counterparts. For the latter, I set Belka's mode to LSB with 50 Hz lower and 4 kHz upper frequency cutoffs. I also used Youssef's excellent noise reduction plugin when demodulating I/Q.
Radio Rebelde on 12/11/20 at 07:01 UTC
Radio Rebelde was barely registering on Belka's SNR meter but the audio is perfectly intelligible, while SDR#'s noise reduction takes this intelligebility to an entirely new level.
Finally, here is a full I/Q capture of a CQ constest on 40m on 21/11/20, recorded in the same park location (click here to download the original WAV file). An
Over The Horizon Radar signal makes an unfortunate appearance during the first
20 minutes of the recording, causing severe interference. You will also notice a few hams ignoring the contest altogether and happily rag chewing!
Overall, this is by far my most portable shortwave spectrum capture combo: it
is handheld and absolutely no antenna set-up is required – just plug in the
supplied telescopic whip and you are good to go. The reduced bandwidth
compared to my AirSpy SDRs is a significant limitation, but it was only a few
years ago that I was making recordings with a FunCube Dongle Pro+ that only
provided double the above sample rate. It would be amazing if Belka's next
model included a microSD card slot and the ability to record the I/Q data
directly to it, but for now I am very happy with this way of making spectrum
recordings on the move.
Hello and Happy 2018 to all the readers of the London Shortwave blog!
This post comes after a relatively long hiatus, as new work commitments starting last summer reduced the amount of time I could spend on shortwave radio listening. Longer working days and commute periods gradually encroached on the time slots that I had previously allocated for going to the park to record parts of the shortwave spectrum. (On the other hand, owing to continuously increasing urban RFI, my indoor reception conditions have deteriorated to the point of being essentially unsuitable for any serious radio listening).
However, just as I found myself under these new time constraints, my friend Thomas Witherspoon contacted me to tell me about his new initiative, The Spectrum Archive, and kindly invited me to become part of the team (which I gratefully accepted!). From the project website:
The Radio Spectrum Archive (RSA) allows listeners to experience radio history as it happened. It offers listeners the ability to tune through a radio band, listening not only to individual stations, but to all the stations in broad swathes of recordings, providing richly relevant radio context from the time.
[...]
The Spectrum Archive team actively creates new spectrum recordings and maintains existing spectrum recordings for current and future use by, among others, historians and researchers.
The opportunity to contribute to this unique project renewed the impetus for me to find a way to create outdoor shortwave spectrum captures on the go without having to make return trips home to drop off my relatively bulky recording equipment.
PocketCHIP and FunCube Dongle Pro+ (September 2017)
My first attempt at this was to use the PocketCHIP portable computer and the FunCube Dongle Pro+, together with the Sony AN LP-1 foldable active loop antenna:
The idea was to have a spectrum capture set-up that could fit into the side pocket of my laptop bag and be quickly deployed in any open space.
PocketCHIP runs Linux, for which there are plenty of SDR applications, however, because of CHIP's limited CPU capabilities it is difficult to get any of them to run glitch-free. As a result, it's possible to inspect the spectrum visually prior to starting the recording but it's impossible to do both at the same time and there is currently no way of monitoring the audio while the recording is underway.
On Linux, FunCube Dongle Pro+ can be accessed without any SDR application running in the background (a big advantage given PocketCHIP's limitations): the dongle uses the sound card I/O interface and a simple audio recording utility such as ecasound can be employed to record the I/Q data stream to disk. However, the main drawback of this dongle is that it can only capture 192 kHz at a time, making it impossible to record an entire shortwave band in one go. On the other hand, PocketCHIP's battery life is around five to six hours, far longer than that of any of my laptops or tablets.
GPD Win and AirSpy HF+ (November 2017)
In November I received a sample of AirSpy HF+ — a very sensitive, compact SDR, capable of recording an entire shortwave band in one go. I used it to form an alternative ultra-portable capture set-up: the antenna is still the Sony AN LP-1 compact loop but the PocketCHIP is replaced with a similarly-sized GPD Win: a fully functional 5-inch (!) Intel PC running Windows 10:
AirSpy HF+ is tightly integrated with SDR# — a Windows application — which ended up being the main rationale for getting one of the most portable Windows computers out there and putting it to the test. After a bit of tweaking and experimentation (which I'll describe in detail in one of my next posts) I managed to get it to work. GPD Win's CPU is capable of running SDR# without any problems, its battery lasts around 4 hours when recording the spectrum and the Sony loop antenna proved to be very sensitive indeed:
The above two recordings were made with the GPD Win / AirSpy HF+ / Sony loop combination on Christmas eve in a London park, and the quality easily rivals that of the captures I have made with my regular outdoor set-up under similar propagation conditions.
I hope the greater portability will allow me to continue making outdoor spectrum recordings under increased time pressure. Thanks for reading and wishing you all a great year with lots of interesting radio catches!
GPD Win, AirSpy HF+, Bonito GI300 isolator and the Sony AN LP-1 loop antenna preamp out in the field
Small size: At 8", it is much smaller than almost any laptop available on the market.
Price: at $169, it's cheap enough to be the dedicated device for this project. I suspect that many tablets under $100 — such as the HP Stream 7 — are in the same performance league as my two year old Toshiba, making it an even more attractive choice cost-wise.
Battery life: the tablet can capture the spectrum at 3MHz bandwidth for 2.5 hours on a single charge. None of the laptops I own would be able to do the same.
USB (5V) charging: this makes it possible to replenish the tablet's battery using a portable power bank, an in-car charger or a foldable solar panel — great for when you want to scan the bands while camping off the grid.
Why use AirSpy / SpyVerter instead of another SDR?
Low power consumption: the AirSpy/SpyVerter combination can run entirely off the USB power supplied by the tablet, requiring no additional power supply units.
Wideband performance: the two other SDRs I own that can be powered by the tablet alone are the FunCube Dongle Pro+ and SDRPlay RSP1. The FunCube dongle's maximum bandwidth is 192 kHz, while AirSpy is capable of pulling in up to 3MHz without maxing out the tablet's CPU. SDRPlay can provide a similar bandwidth, however, its performance leaves a lot to be desired compared to the other two SDRs. Simply put, the main problem with this radio is the large number of mixing/imaging artefacts at comparable sensitivity (signal to noise ratio) levels and spectrum bandwidth. I demonstrate this in the video below.
Bundled software: The other problem with SDRPlay is that the compatible software packages I have tried cannot write large (3MSPS) streams to disk reliably without buffer overruns on my tablet. In my evaluations, the Baseband Recorder plugin for SDR# is quite exceptional in this regard, and of course nowadays SDRPlay is not compatible with SDR#.
Why use a long wire antenna and not an active magnetic loop or a mini-whip?
Power consumption: the long wire dipole requires no additional power, unlike the alternatives.
Portability: an active loop antenna would require significant additional space; the same is true for a mini-whip antenna, although to a lesser degree.
Capturing the shortwave spectrum out in the field.
Radio interference is a major problem in big cities when it comes to indoor shortwave reception. One effective solution I have found is to head for the local park and engage in scanning the bands there. However, since my time for making such outdoor trips is limited, I would always feel like I am missing out on a lot of radio action by monitoring a single frequency, which is all you can do with a standard shortwave radio. There are so many signals out there — which one should I go for? This inspired me to put together a lightweight, portable set-up that would let me capture large chunks of the shortwave radio spectrum out in the field, which I could later explore in detail. After two years of experimenting with various Software Defined Radio (SDR) technologies I am pleased to report that I finally have a solution that works well for this purpose.
A good SDR can give the user access to large portions of the radio spectrum via a graphical user interface. The user can then either process a specified part of it in realtime or record the chosen spectrum window in its entirety onto disk and analyse it later with the supplied software. Here is a short video showing the playback of one of such spectrum captures I made in a London park in September 2016. Note the final part where I zoom out to show the entire recorded frequency range (covering two broadcast bands with one ham band in the middle!):
When I got home from the park, I was able to replay that part of the spectrum capture many times over while scanning the frequency space, which is how I was able to identify a weak signal from a very distant ham radio operator that I might have otherwise missed.
Below is the list of the components I have used to put together my "portable spectrum capture lab".
I bought this tablet in July 2014, based on the following criteria: the device had to have a reasonably powerful Intel processor, running the Windows 8 operating system. I believe that there are currently models on the market that are at least as powerful and are substantially cheaper (<$100).
Owing to its unique hardware design, the AirSpy SDR can monitor large parts of the radio spectrum (up to 10 MHz in bandwidth) while offering a high dynamic range and robustness to overloading, with almost no mixing/imaging products.
This additional device enables AirSpy to cover the shortwave bands (in fact, the entire frequency range between 0 khz and 30 MHz) and must be connected in-line between the AirSpy's front end and the antenna feed line, as follows:
Connection cables
Below is a small collection of cable accessories to connect the antenna to AirSpy/SpyVerter:
I use a three-terminal matched balun connected two 6 metre copper wires via its antenna terminals as a dipole antenna, and connect it to the SDR via the feed line terminal with the 3m BNC cable listed above. The balun (Wellbrook UMB130) is engineered in a way that prevents the radio noise current from the tablet (usually a significant source of interference) flowing into the receiving part of the antenna.
This foam-filled flight case comfortably houses all of the components. The parts 1 to 7 can remain assembled together, reducing the deployment time in the field.
I use this fast microSD card as the destination for my outdoor SDR recordings. The high transfer speed is critical - using slower microSD cards will result in large portions of the spectrum being dropped from the recordings. 64 Gigabytes can accommodate roughly one hour of spectrum data at 3 MHz bandwidth.
Windows tablets suffer from one major drawback: the touchscreen interface is usually inadequate for software that was designed for traditional computers with mice. A portable Bluetooth keyboard with a built-in trackpad solves this problem.
This small gadget turned out to be a very important part of the entire project. The Toshiba tablet has a rather unusual interference quirk that initially caused me hours of frustration. It turns out that significant amounts of radio noise are injected into the SDR when the tablet's external speakers are active. One way to fix this is to plug a pair of headphones into the tablet's line out jack, but this forces the listener to be glued to the device. The alternative is to pair the tablet with a Bluetooth audio receiving unit, such as the one listed above. It is worth noting that my other Windows tablet — a Dell Venue 8 — also suffers from this strange artefact.
Total cost: $610
Internal layout of the flight case
You'll see that I have stacked the SpyVerter enclosure on top of the AirSpy one. As the latter can get very hot, it is essential to leave a sufficiently large gap in the foam for ventilation. It's also worth leaving a small gap next to the tablet's power button to prevent Windows from accidentally going into standby mode.
Software configuration
The best software to use with the AirSpy/SpyVerter combination is SDR#. It offers an impressive collection of features that many software packages and conventional radios don't have, such as advanced noise reduction and synchronous detection with passband tuning. The following adjustments are required to make recording the spectrum a seamless experience:
Install the Baseband Recorder and File Player plugins
Baseband Recorder: this plugin enables efficient recording of very large spectrum (or "baseband") files. Download and decompress the plugin zip file. Copy the .dll files into the directory with the SDRSharp.exe executable. Open the MagicLine.txt file and copy the first line of text into Plugins.xml file, just before the "</sharpPlugins>" line.
File Player: this plugin enables the playback of recordings made with the Baseband Recorder plugin. Download and decompress the plugin zip file. Copy the .dll files into the directory with the SDRSharp.exe executable. Open the MagicLine.txt file and copy the first line of text into FrontEnds.xml file, just before the "</frontendPlugins>" line.
Configure Baseband Recorder
Open SDRSharp.exe and check that the program reports no errors when it loads.
Baseband Recorder configuration
In the plugin pane on the left, expand the Baseband Recorder tab and click "Configure". Change the File Format to WAV RF64 and make sure that the File length limit check box is not ticked. Click "Folder select" and choose the MicroSD card as the destination directory for the recordings.
Adjust AirSpy settings
Disclaimer: in this section I describe how I capture the maximum spectrum bandwidth that my tablet's CPU can handle. It involves operating SDR# in "debug mode" and exposes some internal functionality of AirSpy, which, if used incorrectly, can damage the radio. If you choose to copy my approach, please understand that you are doing so at your own risk and follow my instructions carefully to avoid voiding your AirSpy warranty.
Open SDRSharp.exe.Config file in Notepad. Look for "<add key="airspy.debug" value="0" />" line and change it to value="1".
Once the AirSpy and SpyVerter have been connected to the tablet, open SDR# and select AIRSPY in the Source tab. You will see the following configuration dialog.
AirSpy configuration
In the "Sample rate" field, type in "6 MSPS". For the "Decimation" option, choose "2". This setting will result in spectrum captures of 3 MHz bandwidth (although only 2.4 MHz of it will be shown on the waterfall display). To capture smaller chunks of the spectrum, increase the decimation value. Make sure the SpyVerter check box is ticked. Do not touch any of the fields or buttons under the "Address Value" line.
Make a short test recording
Press the play button in the top left corner and set the desired frequency.
In the Source tab, select the "Linearity" option. Keep increasing the Gain value by one position at a time until you notice that the radio signals suddenly become "saturated" (the waterfall display becomes full of artefacts and the signal you are listening to gets swamped with noise). Take the Gain value back down by two positions. This will ensure high sensitivity while preventing AirSpy from overloading.
In the Baseband Recorder tab, press "Record". While recording, do not change the radio frequency and do not move/drag the waterfall portion of the display. Stop the recording after a few minutes.
SDR# FilePlayer plugin
In the Source tab, change the input to "File Player" in the drop down menu. Click the Settings cogwheel button and select the spectrum recording file from the MicroSD card. A vertical band visualising the timeline of the spectrum capture will appear immediately to the right of the plugin pane. Click on the play button and select a radio signal to demodulate in the spectrum display. Listen to the audio carefully to make sure there are no dropouts or clicks: if so, your tablet and MicroSD card are capable of handing and storing the specified spectrum bandwidth.
Keep an eye on the gain
While making longer spectrum recordings, select a weak radio signal and keep monitoring its audio for signs of overloading. If the overloading does occur, reduce the Gain value further by one or two positions.
Some example spectrum captures
Shortwave for lunch. Playing back parts of the shortwave spectrum captured earlier in the park, inside a local cafe.
Below are some example videos in which I play back and explore the spectrum recordings I made during the trips to my local park.
Tropical and the 49m bands recorded outdoors on 03/07/16 at 0432 UTC. A good time of the day for listening to Latin America on shortwave.
This blog was started on a hot and sunny afternoon in July 2013, while enjoying listening to some distant signals with a Roberts R9962 shortwave radio.
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