Filter Your Pi And Be A Responsible Pirate

At this point it’s pretty well-known that you can tack a long wire to the Raspberry Pi’s GPIO, install some software, and you’ve got yourself the worlds easiest pirate FM radio station. We say that it’s a “pirate” station because, despite being ridiculously easy to do, broadcasting on these frequencies without a license is illegal. Even if you had a license, the Raspberry Pi with a dangling bit of wire will be spewing out all kinds of unintentional noise, making it a no-go for any legitimate purposes.

Unfiltered output of Pi broadcasting on 107.3 MHz

In an effort to address that issue, [Naich] has written up a couple posts on his blog which not only discuss why the Pi is such a poor transmitter, but shows how you can build a filter to help improve the situation. You’ll still be a lawless pirate if you’re transmitting on FM stations with your Pi, but you won’t be a filthy lawless pirate.

In the first post, [Naich] shows us exactly what’s coming out of the wire antenna when the Pi is broadcasting some tunes on the default 107.3 MHz, and it ain’t pretty. The Pi is blasting out signals up and down the spectrum from 50 MHz to 800 MHz, and incredibly, these harmonics are in some cases stronger than the intentional broadcast. Definitely not an ideal transmitter.

[Naich] then goes on to show how you can build a DIY filter “hat” for the Pi that not only cuts down a lot of the undesirable chatter, but even boosts the intended signal a bit. The design is surprisingly simple, only costs a few bucks in components, and conveniently is powered directly from the Pi’s GPIO. It even gives you a proper antenna jack instead of a bare wire wound around a header pin.

We’ve seen plenty of projects utilizing the Raspberry Pi FM transmission hack, and while this mod still doesn’t make it perfect, it’s always nice to see an awesome hack made even better.

20 thoughts on “Filter Your Pi And Be A Responsible Pirate

  1. Well my goodness me, the mod sure don’t make it perfect. The filtered spectrum appears to be absolutely filthy too, appallingly so. No self respecting pirate would ever connect that thing to an antenna.

  2. What’s the power level? There are a bunch of small, cheap FM transmitters that you hookup to MP3 players, which was a quick and dirty way to get music to an old car stereo. They’re legal in many countries as long as their power level is low enough.

    “In the United States, Part 15 of the U.S. Federal Communications Commission rules specifies that no license is needed if FM transmitters have a Maximum Effective Radiated Power (ERP) of 0.01 microwatts or 250 µV/m measured at 3 meters.”

  3. i am not sure if the fcc is actively going after violators like they used to with the defunding of the fcc and net neutrality not being in the fcc jurisdiction and such.

    of course if you build a power supply or are using a faulty power supply that takes down the emergency comminucations like neon sign power supply drowns out communications

    the fcc probably would go after

  4. “You’ll still be a lawless pirate if you’re transmitting on FM stations with your Pi”

    Why do you insist on this? I don’t know where you are Tom but here in the States we have Part 15. Under Part 15 rules we can transmit on an empty frequency (if we can find one). The power limits are very low, meant mostly for personal use but legal part 15 broadcasters living in densely populated areas have managed to generate small local followings.

    Of course this means you have to really clean up that signal. Just how many decibels down are all the spurs that make it through this filter?

    Also, you have to find an empty frequency. That is getting difficult thanks to all the licensed LPFM repeater stations that the texan bible thumpers have planted all across our country polluting our airwaves with their nonsense.

  5. “You’ll still be a lawless pirate, but you won’t be a filthy lawless pirate.”

    Love it. Even the lawless electronics geeks have their honor.

    I don’t mess with RF much myself, but I sure do admire the look of those circuit boards with the copper tone of the big, broad ground plane and all the twisty inductors sticking up. The physics of RF seems to freeze the circuits in a retro style and I can’t get enough of it. Maybe it’s time to get that ol’ ham license.

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