AI6YM's PicoFox: A Versatile New Open-Source Transmitter for Radio Direction Finding
The PicoFox, developed by amateur radio operator Justin (AI6YM), is an innovative little two-meter FM transmitter designed specifically for amateur radio direction finding. Compact, rechargeable, programmable, and open source, it offers hunt organizers an impressive range of possibilities—particularly the ability to transmit customized audio recordings.
From Experiment to Finished Product
The PicoFox began as an experiment to determine whether an inexpensive SI5351 programmable clock generator could produce a usable FM signal on the two-meter amateur radio band.
Working with an RP2040 microcontroller, Justin developed a compact transmitter and documented the process in his entertaining PicoFox Saga. The project involved plenty of challenges, including circuit-board design, power management, RF filtering, and the suppression of unwanted emissions.
The original design eventually evolved into the improved revision 3 PicoFox, incorporating lessons learned from early production units and feedback from users.
What Makes the PicoFox Interesting?
Despite its small size, the PicoFox offers a useful collection of features:- Adjustable transmitting power: Approximately 0.1 to 65 milliwatts, allowing organizers to vary the difficulty and range of a hunt.
- Rechargeable battery: A built-in 1,200 mAh lithium-polymer battery.
- USB-C connectivity: Used for charging and programming.
- Custom audio transmissions: Plays prerecorded WAV files, allowing each hunt to have its own distinctive sound.
- Programmable Morse code identification: Automatically identifies the transmitting station.
- Simple configuration: Settings can be changed by editing a text file through a computer's USB connection.
- Open-source design: Hardware and firmware information is available for amateur radio experimentation and development.
The PicoFox's compact construction makes it easy to conceal, while its adjustable output power allows it to serve in everything from short-range searches to more challenging hunts.
MSRDF Puts the PicoFox to Work
The Mississippi Radio Direction Finding Association has been testing the PicoFox during our 3rd Saturday T-Hunts, and our members have particularly enjoyed its customizable audio recordings.
Instead of listening for the usual electronic tones, hunters might hear a recorded voice, an unusual sound effect, or a message prepared specifically for that day's event.
We can give each hunt its own personality simply by loading a different recording into the transmitter. The possibilities are especially appealing for our themed and special-event hunts.
The recordings are relatively easy to prepare using audio-editing software such as Audacity. Once converted to the required WAV format, they can be copied to the PicoFox through its USB connection.
Our experiences have been very positive, and we plan to incorporate PicoFox transmitters into more MSRDF events in the future.
Additionally, with a bit of programming advice from AI6YM, we plan to make the PicoFox the standard transmitter required in all PRSD Events. Precision Radio Signal Detection (PRSD) is a brand new type of RDF event created by our President, K1REZ to fill a void in radio direction finding events. We plan to hold the world premier of this new type of event in January 2027.
An Open-Source Project Worth Following
One of the PicoFox's most appealing characteristics is that AI6YM has made the hardware design and firmware source code available to the amateur radio community.
Experimenters can study the circuitry, explore the software, and potentially develop their own modifications, subject to the project's licensing terms.
Justin has also documented the transmitter's development with an unusually candid account of the engineering challenges encountered along the way.
His two articles, The PicoFox Saga and The PicoFox Saga: Chapter 2 — Revenge of the Spurs, are well worth reading for anyone interested in the technical side of amateur radio equipment.
Our Impression
The PicoFox combines practical radio direction finding capabilities with the flexibility to make transmitter hunts more entertaining.
Its small size, adjustable power, straightforward programming, and custom audio make it a welcome addition to our equipment collection.
We're looking forward to finding even more creative ways to put it to work!
Learn More
The PicoFox Saga — Original Development Story
https://ai6ym.radio/the-picofox-saga/
The PicoFox Saga: Chapter 2 — Revenge of the Spurs
https://ai6ym.radio/the-picofox-saga-chapter-2-revenge-of-the-spurs/
PicoFox — Official Product Information and Specifications