Fox Hunting With A Raspberry Pi

No, not a real fox! [KM4EFP] is a ham radio operator with a passion for fox hunting, which is an event where several radio operators attempt to find a broadcasting beacon (a “fox”) using radio direction finding techniques. [KM4EFP] has just built his own portable fox using a Raspberry Pi in a very well-built enclosure.

Since the fox could be outside for a while, the project was housed in a reasonably weatherproof ammunition case. A mount for an antenna was attached to the side, and it is hooked up to a GPIO pin on the Raspberry Pi. The entire device is powered by a 6000 mAh battery pack which allows the fox to broadcast long enough to be found.

The software running on the Raspberry Pi is very similar to the Pi FM transmitter program but it is specially made for ham radio broadcasting instead. Almost no extra hardware is needed to get the Pi broadcasting radio, as these software packages can drive the antenna directly from the GPIO pin. This is a great twist on the standard FM transmitter that ham radio enthusiasts everywhere can use to start finding those wily foxes!

11 thoughts on “Fox Hunting With A Raspberry Pi

  1. Very cool project. My ham radio club recently held two fox hunts and everyone had a lot of fun. This little device would have added an extra challenge beyond just finding the person transmitting.
    I might just add something like this to my to do list.

  2. From the summary

    ” A mount for an antenna was attached to the side, and it is hooked up to a GPIO pin on the Raspberry Pi.”
    “these software packages can drive the antenna directly from the GPIO pin.”

    No! Not a good idea! If you read the article he did NOT attache the antenna directly to the Raspi. He has a low pass filter in-between!

    Good filtering is important for any radio transmitter, ESPECIALLY one which is really just a GPIO being turned on and off really fast. That is a square wave. A square wave isn’t just one frequency. A perfect square wave would be a combination of one frequency and all of it’s odd harmonics up to infinity!

    Without proper filtering who knows what you will be interfering with, police, cellphone, tv, avionics, medical devices, military frequencies! In most countries that will get you into trouble. You can get big fines, lose your ham or other radio license and even go to jail!

    The power output is low enough that with your typical tiny piece of wire in the house sort of setup probably won’t cary far enough to get you into trouble. Still, even that isn’t something I would do more than just as a temporary demonstration. Once you take it outside however and actually attach a real antenna… use a filter!

    1. Correct. Furthermore, transmitting an unfiltered square wave is not only bad practice, it’s illegal under the FCC rules governing the amateur service. See part 97.307(d) and 97.307(e) for details, but the mean power of any spurious harmonics must be at least 43 dB below the fundamental when transmitting below 30MHz. That rule holds regardless of the transmitter power, even for a very low powered fox beacon. The rules become more complex above 30MHz.

  3. Thanks everyone. I’m glad you like my project and hope it helps others when making their own fox box. It was really exciting getting a front page featured article on HaD especially less than 24hrs after posting the project. Let me know how it works out for anyone using my script.

  4. Cool project, good article, please learn the difference between “broadcasting” (not permitted to radio amateurs) and “transmitting” (permitted, that’s what hams do).

  5. Great project, good article, Hackaday please learn the difference between “broadcasting” (not permitted to radio amateurs) and “transmitting” (permitted, that’s what hams do).

  6. Question: does anyone know if any of the RasPi’s GPIO traces have even remotely controlled impedance? If not (and I suspect not) then there may be signal reflections going on when toggling pins at these speeds. If so however, then I can think of a dozen new uses for the RasPi! Even if the pin headers have to be removed, a lot of fun could be had with signals at such speeds!

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