pigeon background: testing on the air 🕊️

I’m sure a lot of folks out there will misunderstand my trademark tone and informality as carelessness, sloppiness and lack of “true engineering” – and you know what, I’ll give you one thing – it’s an easy mistake to make. I’m not a professional RF engineer, and I have no educational background that would indicate that I am a responsible grownup. That doesn’t change that it is a mistake, though. I’d like to make it as clear as I can – I give many shits, have treated all of my RF work with an incredible amount of seriousness, have been at this because I care deeply, and have, and continue to make every possible attempt to adhere closely to the letter and spirit of the service i’m operating in. I’ve been at this over the course of years because I am trying to learn something new, and have taken great pains to work through textbooks, instrumenting and validating every step to the best practical extent to which I can (given a hobby budget).

To get ahead of the (justified concern) I will inevitably receive when I include videos of over-the-air transmissions about how I am “encrypting” data (I’m not, just to tl;dr that one), or how I need to document the mode first, I decided to sketch out some details on how I’ve been testing this “over the air” in a way that doesn’t break the spirit or letter of the FCC code of federal regulations, or of the Amateur Radio service first. If you’re going to be doing similar at home, I’m sure you can navigate this yourself – but here’s a rough sketch of what my workflow is like.

Also, good news, these posts will definitely be enough to demodulate pigeon transmissions, and I plan to fully document it in a way that makes sense to operate “in the open” if I ever wish to try it outside of my lab.

A long while ago, I bought a Ramsey faraday box for testing my SDR transmit codepaths without needing to worry about emissions escaping my office. All of my work on pigeon was designed for, and exclusively tested on, amateur radio frequencies, even when inside my faraday box. Pigeon transmissions carry an ITU coordinated amateur radio callsign as part of the layer 2 header, and I’ve taken great pains to not put encrypted traffic over the transport – even inside my faraday box.

On a practical level, according to my best and most recent measurements, the highest configurable power output (across all frequencies I tested) of the SDR I’ve been using to test with is around 18 dBm, or 0.067 watts (67 milliwatts). Turns out this is also the highest power output of any of my SDRs. In reality, my actual power output is much lower, given my antenna selection (not ideal), frequency (out of band for the antenna), gain levels (far below max output), signal bandwidth and configuration. I tend to believe the chances of pigeon transmissions at the power level I’ve been testing with, even transmitted in the open inside my office, are basically zero. Nevertheless, I still try to do my development work “clean” and treat it like it would be.

While transmitting, I monitor the spectrum from just outside the faraday box, to be absolutely sure that I’m not transmitting “on the air”. If any signals were detected (due to, say, the lid not being closed), transmission would have been stopped immediately.

I would urge anyone experimenting with this at home to think through how to avoid interference, and how to learn in a way that complies with the FCC’s regulations.

A great way to do that with well known and documented emissions (such as FM, FSK or PSK modes) is to get licensed – within the US, a technician amateur radio license and good judgemnt will enable you to to experiment fairly freely with using software defined radios as transmitters.

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