I am not a lawyer or a radio engineer, I can search FCC things and even come up with some convincing understanding of laws, but I don’t have the proper test equipment or background to give this advice, so read it as a fiction novel.
Cool, legalese out of the way, I did work for one of the world’s largest suppliers of microwave system, cellular infrastructure, and worked for years in the production and testing of leaky radax, 50 and 75 ohm cables with both high density polyethylene foam and air dielectric, waveguide in cables ranging from 3/8″ to 6″ in diameter (and therefore covering the related frequencies of those cables).
Functionality to Duplicate
Initial cut was to have a way to play music, not by request and not by voice. Reminders go away, weather, Alexa brand farts. We lose all of that. I just wanted something to play music.
Easy! Get an FM Radio!
And I did. I quickly remembered that I just really hate advertising. The good news, however, is that I have a ton of absolutely public domain music I can listen to by popular composers from the 1600s. I also, oddly, had some parts laying around.
See, when I was ~15 and lived at my parent’s house — I ran a radio station. I had a 12vDC adapter to a cigarette lighter outlet. I also had one of those good old “AUX” cables to radio adapter. This was plenty to cover my parents house, I just set it to a frequency that wasn’t used and streamed music and it worked really well.
Cool — I have the end-to-end I needed, grab an old Raspberry Pi 1B I have doing nothing in a closet and give it a job to do.
Here’s the dongle I have. Sadly, it barely covered my house, with plenty of dead spots and poor audio quality.
Worse, is the FCC is pretty damn clear on you not stepping on radio bands that are licensed and used, and for some reason I think Clear Channel has great lawyers.
This dongle did a very poor job with selectivity — ability to specify a frequency, with a 20+ year old shitty rheostat hanging off the side and a band selector. Plus, it has poor thermal control and that is what I suspect is the reason that it started drifting frequencies over the one song I played over it.
My “completed” radio station, complete with zip-tied and 3D printed Pi1B case and dongle. Also pictured: Old Pentiums and a useless camera/bug detector.
To ensure compliance (in my mind), this is what I did:
- Used radiolocator.net to find a frequency that was not being used. This was very low because I’m right in the congested Dallas radio market. In fact, only 105.5 was available.
- Made sure the dongle I was using was Part 15 “certified” or “compliant” (more on this later)
- Made sure the signal didn’t extend beyond 200′ from the transmitter site — and boy, did it not
Well, that wasn’t going to work, but I thought how cool would it be to get my kids on a radio throughout the house or use their voices for various radio elements “You’re listening to 105.5, and I’m not old enough to drive, next up is Canon in D by Pachabel” (I can’t spell it because, while a good song, I really don’t know many public domain songs).
So, being sold on the idea, I took my Visa card to Amazon and clicked “Buy now” on a transmitter I figured was absolutely fine. Doing my research on Christmas light displays and other home radio stations, I landed on what should be a quite underpowered and overpriced unit: https://a.co/d/d1dE4Bu. It’s a “0.1W to 0.5W FM Transmitter” with all kinds of fancy terminology that makes me think it’ll control bandwidth and stay on frequency.
I was so friggin happy when my STILL ALEXA announced my package was delivered, so I finished painting my cabinets and grabbed it and hooked it up, set the frequency to the lower power of 0.1W, and set the frequency. I then hooked it up to music.
If you’re from the FCC, this is a fictitious story and I drew the above waterfall in Microsoft Paint.
You can see the vertical stripes of yellow/orange/red near the top and bottom. These are other radio stations near me. You can see the signal strength, and how those transmitters do not have terrible harmonics. Then you see the part where I hypothetically turned mine on for a few seconds. The RTL-SDR dongle didn’t even have wide enough bandwidth to detect the entire signal’s width. I can’t comment on the intensity, because my receiving antenna and transmitting antenna were around 7′ apart. But wow.
So anyways, that was a damn failure. So I did what I always do, put it back in the box, returned it, and decided to find a reason that will not have federal agents here. This unit was marked Part 15 and I took that to be that it was legal to operate as long as I ensured it wasn’t over 200′ from my house. Additionally, 200′ from my house is a few neighbors I’m good with and not much else — and they likely don’t use FM like nobody uses FM anymore.
You’ll notice this post says “Part 1”, that’s because it’s a series. You know when Terminator or RoboCop came out (because I’m old), there was never a Terminator 1. They didn’t know if there will be more — was it a box office hit? Did my project fail? Yes. Part 2 will follow eventually.
Also, this MS Paint was drawn within a faraday cage.
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