Close Encounters With The RTL-SDR

Standing on some mounds in the back yard, laptop in one arm, antenna pointed to the skies. WI5HER, Port Townsend, 2017

My RTL-SDR came in the mail a couple weeks ago. It’s a little $20 USB dongle with a long wire that connects to an antenna. And when I say antenna, I mean THIS is an antenna.

When unfolded, it’s almost as tall as a grown human and just ridiculously large enough to make your family or anyone in your immediate proximity cringe in embarrassment for your own sake.

Anyway, I was out on the Olympic Peninsula and after the fam had all settled to bed I went out on the back porch with my laptop and SDR gear and start scanning the frequencies. For less than the cost of taking a date to the movies, the rtl-sdr allows you to flip around the airwaves between 500 kHz to 1.7 GHz and visually see a ‘waterfall’ of incoming radio frequency traffic.

A typical waterfall

I started by first finding the local FM radio stations and from there chronicling and recording a whole slew of beeps and bloops all over the dial. I pushed the ear buds deeper into my noggin and smiled with delight!

Then, up around the 450 Mhz zone, I heard a voice descend from the æther.

At this point, I started to really flip out – I hadn’t expected to hear anyone talking in this range, and especially not someone seemingly reading what sounded like absurd poetry.

It was too good to be true. Was this a pirate broadcast? A “messenger” ?

Turns out, there’s a much more reasonable explanation. This is not poetry, per se, but something called the ‘Harvard Sentences‘.

A collection of sample phrases that are used for standardized testing of Voice over IP, cellular, and other telephone systems. They are phonetically balanced sentences that use specific phonemes at the same frequency they appear in English.

My understanding is that certain companies may play these sentences on a loop and then drive around and judge signal strength based on how well they can understand the words being spoken.

The sentences are strange, sometimes profound. I think they would make for some amazing “performance art” that would make Yoko Ono proud. Or a beautiful ritualized “spell” script.

Take for instance this list…  can’t you imagine trying to “accomplish” each of these steps in order to unlock some sort of otherworldly “achievement” ?

  1. The boy was there when the sun rose.

  2. A rod is used to catch pink salmon.

  3. The source of the huge river is the clear spring.

  4. Kick the ball straight and follow through.

  5. Help the woman get back to her feet.

  6. A pot of tea helps to pass the evening.

  7. Smoky fires lack flame and heat.

  8. The soft cushion broke the man’s fall.

  9. The salt breeze came across from the sea.

  10. The girl at the booth sold fifty bonds.

A full list can be found here.


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:

406.588 400 mHz Quebec, 26 March 2017

Listen to 406.588 400 mHz Quebec, 26 March 2017 | Clyp is the easiest way to record, upload and share audio. No account required.

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.