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Wednesday, 4 January 2017

Monitoring avionics communication with the VMA Simple Spectrum Analyser


I received the ADF4351 version and here is a quick test monitoring the avionics radio frequency band.

At 121.1 MHz, pilots are communicating with the tower of the nearby airport.

On 119.0 MHz, you can see a faint signal: at this frequency the airport is broadcasting the local weather report.

Because it can take some time to register any communication, it is nice to have the waterfall diagram, which logs all communication inside the selected span.

Unfortunately, the ADF4350/ADF4351 has a rather low resolution when it comes to signals with small bandwidth. A closer look reveals, for example, that the signal at 121.1 MHz does not look right: it is an amplitude modulated signal (AM) and should not have an inverted spike in the centre:

You can read more about the ADF4350/ADF4351 capabilities in the datasheet:

Therefore I would rather recommend a RTL2832U based dongle for usage with SDR#, if you are into amateur radio applications. If you have been living on the dark side of the moon and never heard about these, then take a look here:
  1. Airspy is the former "sdrsharp" website. They now sell the Airspy device, which is better than a regular RTL2832U based dongle, especially because it has higher resolution and greater frequency range. However, the base software, "sdrsharp" (or SDR#) is still free and compativle with RTL2832U based USB dongles).
  2. If you are looking to purchase such an RTL2832U USB dongle, AVOID the ones with an R820T tuner! They will constantly crash or freeze your computer. Currently, the best are the R820T2 tuners.
  3. Because these dongles are so cheap, there is no excuse to not purchase one! If you are reading this blog, then you either already use SDR# or you should download it now and get such a dongle!
  4. Of course, as everything in life, there is a big problem with SDR#: the spectrum is rendered through FFT (Fast Fourrier Transformation) and not SSA (Sweeped Spectrum Analysis). Because of that, you only get the frequency span of the device you are using (remember that the ADF4350/ADF4351 allow a span over 4GHz!):
    1. RTL2832U based dongles: up to 2.4MHz span
    2. Airspy: 10MHz span


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