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As many of my readers and followers will already know, these days I mostly enjoy listening to shortwave radio via the outdoor spectrum captures I make in my local park. Although I have built a system that helps me deal with urban radio interference at home, some of the weaker signals still can’t make it through the indoor noise. Since I have a limited amount of time for making outdoor trips, capturing entire portions of the spectrum allows me to record a lot of shortwave signals simultaneously, which I can then explore individually at a later time. However, these trips still need to be carefully planned because the time of the day and the time of the year both affect long-distance signal propagation, and do so differently depending on the frequency range. For example, signals on the 16 meter band are usually at their strongest during the daylight hours, whereas the 31 meter band is at its busiest around sunrise and sunset. Because my current portable recording set-up allows me to capture only 10% (3 MHz) of the shortwave spectrum at any one time, I decided to carry out a systematic exploration of activity on the shortwave bands to help me time my outings so as to capture as many signals as possible during each trip.
Capturing the shortwave spectrum out in the field with a portable SDR set-up.
Luckily, I didn’t need to make any of my own measurements for this. For over a year, the wide-band WebSDR at the University of Twente has allowed its users to see what the shortwave spectrum has looked like over the past 24 hours in a single image. More recently, however, the creator of the service, Pieter-Tjerk de Boer PA3FWM, has opened up his spectrum image archives, so it is now possible to see the past conditions of the bands on any single day in the last two years. Intrigued by how band activity changes depending on the time of the year, I created a timelapse animation of these images by taking two from each calendar week and lining them up in sequence. With Pieter-Tjerk’s kind permission, I share this animation below.
First, a really fast version to illustrate the broad effects the time of the year has on peak activity times across the bands:
The X axis represents the frequency and the Y axis is the time of day, starting at the top. Conventional wisdom about band behaviour can be easily confirmed by watching this video: the 60m, 49m and 41m bands are mostly active after dark, with the 60m and the 49m bands being generally busier during the winter months. The 31m band is most active around sunset, but carries on all night until a few hours after sunrise. The 25m band is active during sunrise and for a few hours afterwards, and around sunset during the winter months, but carries on all night during the summer. Peak activity on the 22m and 19m bands is also clustered bi-modally around the morning and the evening hours, though somewhat closer to the middle of the day than on the 31m and the 25m bands. The 16m band is mostly active during the daylight hours and the 13m band is quiet throughout the year except for the occasional ham contest.
It almost seems as though someone positioned in the middle of the image’s right edge (corresponding to noon UTC) is shining two flashlight beams on the bands in a V-shaped pattern, and is changing the angle of this pattern depending on the time of the year: wider in the summer and narrower in winter. Here’s a slower version of the animation that shows some finer week-on-week changes:
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:
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.
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.
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.
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.