Monitoring radio spectrum around you

Foundations of Amateur Radio

Previously I've talked about using radio spectrum monitoring to detect lightning to potentially protect your equipment. This triggered an email from Nick VA3NNW, who pointed out that AM demodulation of "one point twenty-one jiggawatts", is really a red-herring, and indeed it is. Nick went on to point at another approach, which nudged things along.

So, let's start with the herring, not sure about red, I prefer mine fresh, raw, and covered in finely cut raw onion. If you know, you know. If you don't, think sweet fairy-floss. For extra culinary points, pair it with fresh crusty white bread, preferably still warm from the bakery, and slather it in butter. Yum! Beats bacon any day.

Oh, before I forget, pickled herring, lovely, fish aside, in terms of flavour and texture, not in anyway related to fresh herring, just so you know.

Where were we, ah, uh radio.

Before jumping back in, "squelch" is a radio term, it translates to "subdue forcibly", in our case, we're using it to describe a process where radio signals below a certain level, or volume if you like, are suppressed, so you'll hear nothing until the level reaches above the specified level.

This is useful in all manner of scenarios where there's background noise. In amateur radio, squelch is used on VHF and UHF repeaters, but on HF it often means that weak signals don't get heard, keep that in mind next time you start diagnosing a silent radio, ask me how I know.

Anyway, my initial choice of "rtl-airband" as the tool of choice for lightning detection was based on the idea that I could use it to receive a chunk of bandwidth, then automatically apply the in-built squelch to make everything silent when there was nothing but local noise, then when lightning was afoot, noise would suddenly increase above the threshold imposed by the squelch, and I could detect that.

Nick pointed out that there's a better tool for this, "rtl_power", written by Kyle Keen in 2013, which can continuously monitor a whole lot more frequencies, collect them over a set period and intermittently output the power values, which can then be processed.

The tool allows you to specify which frequencies to monitor, how long for and at what resolution. This is extremely useful for exploring activity on a new band. You can use the data to create images of band activity and see what's going on.

I have previously used it to find intermittent interference. Note that it doesn't decode anything as such. It reports