Recording the spectrum with portable SDR

Friday, April 17, 2015
Update (15/11/16): this article describes an older version of my portable SDR configuration. Jump to this post to see my current set-up.

As my friend Thomas Witherspoon frequently points out in his excellent blog, one of the great things about using a software defined radio package like SDR# is that one can record entire regions of the frequency space instead of a single station. Although I've always wanted to try this out, my home is plagued by tremendous electromagnetic interference and the challenge has always been finding the optimal equipment settings for a single station. Under such circumstances, capturing parts of the spectrum didn't really make sense. Now that I finally have a convenient portable SDR solution, I decided to give spectrum recording a go.

On the evening of 15/04/2015 I went out to a nearby park and set up my equipment as follows:

I used a thin SMA to BNC wire to reduce the strain on FunCube's RF front end connector.
Hiding the gadgets from prying eyes inside the carrier bag (all listening done via a pair Bluetooth headphones)
The feed line cable has a velcro strap which is great for attaching the former to a tree branch.
12m of wire were wrapped around the tree branches (two 6m wires connected to the balun in a dipole fashion)

I fired up SDR#, set the centre frequency to 12095 kHz and started recording the spectrum to disk. This is what it looked like:

FunCube Dongle Pro+ spectrum circa 1600 UTC on 15/4/2015 (click above to enlarge)
The frequency window of the captured spectrum was 192 kHz, which is the maximum that the FunCube Dongle Pro+ can provide. While this is only a small fraction of the entire shortwave spectrum, it it is still possible to simultaneously capture a fairly large number of radio signals within it. When I got home I replayed the spectrum data through SDR# several times to extract the following station recordings:

Voice of America (Somali service), 12055 kHz, 1600 UTC. The strongest signal in the above screenshot (unsurprising, as it happens to be transmitted from the closest geographical location). The SDR# / FunCube Dongle Pro+ combo did well not to produce audio artefacts given that the carrier peak occasionally inched close to 0db.

Radio Australia, 12065 kHz, 1559 UTC. SDR#'s passband tuning and synchronous detection were of invaluable help here because of VOA's blowtorch beam was destroying the lower sideband of Radio Australia's signal. A cheap portable radio almost certainly wouldn't cope in these circumstances!

NHK Radio, 12045 kHz, 1559 UTC. SDR# did a brilliant job of pulling this exceptionally weak station out from the static. However, once the nearby VOA powerhouse got going on 12055 kHz at 1600 UTC, it buried this signal almost completely, which you can hear towards the end of this short recording.

Voice of Korea, 12015 kHz, 1602 UTC. North Korea's international radio service in German. SDR# allows the user to select arbitrary audio filter widths, which was useful for this signal as its bandwidth was roughly 20kHz, making the sound come out in somewhat higher fidelity.

Radio Free Asia, 12075 kHz, 1600 UTC. Broadcasting from Tinian Island in the Pacific Ocean, the station comes through remarkably well. That is, until the Chinese start jamming it, which can be heard in the later part of the recording.

BBC World Service, 12095 kHz, 1559 UTC. BBC's signal is surprisingly crisp considering it is coming all the way from Madagascar.

Radio Farda, 12005 kHz, 1559 UTC broadcasting from Udon Thani in Thailand.

Radio Dialog, 12115 kHz, 1602 UTC broadcasting from Talata-Volondry in Madagascar (presumably airing the recording of their earlier Sunday morning program).

IBRA Radio, 12125 kHz, 1600 UTC broadcasting from Yerevan in Armenia.

Trans World Radio, 12160 kHz, 1602 UTC broadcasting from Tashkent in Uzbekistan.

All in all, I am really pleased with what I could pick up. If you have SDR# installed you can be the judge by downloading the spectrum recording itself. Unzip the file, select it after choosing "IQ file" as the input source in the application, and press play (make sure you tick "Swap IQ" and "Correct IQ" in radio options).

About the author

This blog is written by a shortwave radio enthusiast based in London, UK. You can follow him on Twitter at @LondonShortwave


  1. Thank you for posting the result of your recording. In what state did you record this? Mark KC2UES

    1. Hi Mark, this was recorded in London, UK. Cheers, LS