CERN Amateur Radio Club – CARC

Fox hunt ! Direction finding

Hunt the Signal: Radio Direction Finding, Fox Hunts, and Ham Radio Adventures

In a world saturated with signals, there’s a certain thrill in tuning in, tracking down, and pinpointing the exact location of a hidden transmitter. For ham radio enthusiasts, that’s the heart-pounding fun of fox hunting, or radio direction finding (RDF)—a real-world game of hide and seek played with antennas, receivers, and a healthy dose of brainpower. Whether you’re chasing a hidden “fox” in your local park or experimenting with advanced geolocation methods like TDoA (Time Difference of Arrival), this side of ham radio combines exploration, engineering, and a bit of detective work.

What Is Fox Hunting?

In ham radio, a fox hunt is a competition where participants use radio direction-finding techniques to locate a hidden transmitter (“the fox”) that periodically emits a signal. Think of it as geocaching for radio geeks—but instead of GPS coordinates, you’re using signal strength, direction, and timing to close in on your target.

Fox hunts can be:

  • On foot, across parks or urban areas with handheld gear
  • Mobile, with teams in vehicles covering larger areas
  • Automated, using fixed receivers and time-synchronized measurements

And it’s not just for fun. These same techniques are used in wildlife tracking, search and rescue, and even locating illegal or interfering transmitters in crowded spectrum environments.

Directional Antennas: Your RDF Best Friends

At the heart of any successful fox hunt is a good directional antenna. These antennas allow you to determine the direction of strongest signal (or, cleverly, the weakest signal using signal nulls), helping you triangulate the fox’s location.

Popular homemade options include:

  • Tape measure Yagi: lightweight, portable, and easy to build with just a few elements and a PVC boom
  • Loop antennas: compact and great for detecting signal nulls—especially handy in dense environments
  • Phase-loop systems: using two small loops and a phase shifter circuit for fine-grain direction finding

Combine one of these antennas with a handheld receiver or a dual-VFO HT like the Yaesu FT-60R, and you’re ready to hit the ground running.

Build guides:

TDoA and High-Tech Tracking

While classic fox hunting is analog and hands-on, there’s a growing tech-savvy side to the game. Enter TDoA—Time Difference of Arrival. This technique uses multiple remote receivers, each synchronized to a common time reference (like GPS), to detect when a signal reaches each site. By comparing the timing offsets, you can calculate hyperbolic curves of possible source locations and find the transmitter’s origin point with impressive accuracy.

TDoA is often used in:

  • Amateur TDoA networks (like those integrated with KiwiSDR)
  • Open-source tools like DF Aggregator and SondeHub TDoA systems
  • Spectrum monitoring projects like SatNOGS, for geolocating satellites and terrestrial transmitters

If you’re into SDR and digital signal processing, this is a rabbit hole worth diving into.

Community, Contests, and Exploration

Fox hunting is often a social sport, with local ham clubs organizing events, workshops, and even statewide mobile competitions. Some clubs hide a low-power fox transmitter weekly as a standing challenge. Others make it more intense with time limits, prizes, and obstacles.

Whether you’re solo-hunting a practice beacon in the woods or joining a multi-club urban hunt, you’ll sharpen your radio skills and build relationships in the amateur community.

Where to start:

Why It’s Addicting

Fox hunting turns ham radio into a game of real-world signal sleuthing. You’re not just talking into a mic or logging digital contacts—you’re chasing down rogue RF energy, solving puzzles in motion, and building gear that rewards precision and practice. And as your skills grow, so will your ability to decode the world of signals around you—intentional or otherwise.

So grab your soldering iron, pack your Yagi, and start scanning. There’s a fox out there waiting to be found—and the only thing between you and the signal is your curiosity.