Beginning with Software Defined Radio

Observation and exploration with gqrx for Mac

I bought a Nooelec NESDR Smart, which is a USB dongle that allow you to connect an antenna to your computer so you can listen to RF (radio frequency) signals over the air.

It’s really cool but also daunting for new users without much experience or understanding of RF technology. I’m on Mac, so I finally settled on gqrx as my app to interface with the SDR dongle. The main app view looks like this:

The above screenshot appears to depict a broadcast FM station at 96.185 mHz.

Tuning into broadcast FM stations is the easy part of tinkering with SDR, whatever app you’re using. It’s all the stuff outside of that where it gets complicated.

Check, for example, this 2014 Radio Spectrum Allocations chart for Canada:

Depending on what band you’re looking at, the whole thing gets pretty complicated — and fast.

I’ve used a little bit as a guide the RadioReference site, Canada section, to help me figure out some broad strokes of what areas to go hunting for what kinds of signals (ie, what frequencies in mHz to try monitoring). Things like local police, for example, though I haven’t had huge success with that. But as a result of my experiments, I’ve come up with a sort of generalized “listening protocol” as I work my way blindly through the mass of RF frequencies out there.


Since I only rarely know what a signal is, I’ve started simply taking notes on what I observe, and in some cases recording samples of signals I hear.

Generally, what I will do is choose a specific frequency band that I’ve heard works on SDR (you can’t get all bands on an unmodified SDR unit), like I read that 400–500 mHz is supposed to contain walkie-talkie traffic. Now, it’s tough when you read something like that online, as the same band might be allocated differently depending on the country you live in. I don’t even know if that’s valid for Canada, but I gave it a shot and starting at 400 mHz incremented 1 mHz at a time upwards and simply wrote down what I observed in the app.

For example, at 406.536, exactly at the stroke of 5pm, I heard a man’s voice in French say “dix-sept heure” (five o’clock).

At 406.588 400 mhz (NFM — narrow-band FM), I recorded the following signal:

Now, I don’t actually know what this signal is, nor “what it means” in the grand scheme of things. I simply noted it, described it, and recorded it. On a piano, I also tested until I found the two initial notes or tones (e.g., frequencies), which begin the segment, F → C (F3 → C3, I think). [Tone generator, to test and determine freq.]

Again, I have almost no idea of what the above frequency is. Sounds like “data” of some kind, but I’m not advanced enough to know what exactly. Or even to begin looking. However, I believe there is great utility in using this kind of observational method and careful listening to uncover patterns in just about *any* domain of knowledge, but especially in RF. I figure, with enough practice and research, I’ll be able to hear a signal, check the band its on, and eventually with a certain degree of accuracy figure out what’s going on.

It may seem like a kind of “why bother” activity, but I’ve set myself to learn all about RF technology, and so far it’s really fun for me. It’s a way to participate in a sort of “secret internet” of signals that surround us every day without having to go through my wifi router, my internet service provider, etc. Without being exposed to ads and trolls and fake news and bad news. To just experience the raw signals and hopefully, eventually, to understand them.

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