Radio direction finding (RDF) is the sport of finding a hidden transmitter with a receiver and an antenna — and the same skill that locates a stuck repeater input, a malicious jammer or a noisy power line. Clubs run it as a "foxhunt" or "T-hunt", usually on 2 meters, sometimes as a mobile chase across a county and sometimes as a timed course on foot.
The gear
- A handheld with an S-meter or signal-strength bars, and — critically — the ability to hear a
- A directional antenna. The classic is a tape-measure 3-element yagi for 2 m: a length of PVC,
- An offset attenuator or step attenuator. Close to the fox, everything reads full scale and your
- A body fade technique costs nothing: hold the handheld against your chest and turn slowly; your
- A compass, a map and something to write on. Two or three bearings taken far apart and drawn on a
very strong signal without pinning the meter.
hose clamps and a steel tape measure, built in an evening for a few dollars, and it folds to fit in a car.
antenna's pattern disappears. An attenuator restores the difference between "that way" and "the other way" and is what separates a fast hunt from a wandering one.
own body blocks the signal and the null points away from the transmitter.
map will put you within a small area before you start walking.
How a hunt works
The fox transmits on a known frequency, either continuously or on a schedule (30 seconds every few minutes is common, and much harder). Hunters take bearings, triangulate, drive or walk closer, and attenuate as the signal grows. Scoring is usually by time, sometimes by mileage.
Things that trip people up:
- Reflections. Buildings, hills and water give bearings that are confidently wrong. A bearing taken
- Multipath near the fox. The last hundred yards often reads as several directions at once; walk a
- Antenna polarization. A horizontal antenna looking for a vertical fox loses 20 dB. Match them.
from a hilltop is worth three taken in a valley.
circle and trust the pattern, not one reading.
ARDF — the international form
Amateur Radio Direction Finding is the competitive, on-foot version, run on 2 m and 80 m with five transmitters on a timed cycle over an orienteering-style course. It has national and world championships and a small, welcoming US community.
The serious use
The same equipment finds:
- Interference and jamming, which the FCC expects the community to document before it will act.
- Noise sources — arcing insulators, failing switching supplies, grow lights, electric fences.
- Emergency locator and lost-hiker searches, where amateur teams have assisted agencies.
Recorded audio, dates, times and independent bearings from several operators make a case; see interference and enforcement.
Bearings on a noise are taken exactly the same way, usually with an AM receiver, and utilities take a documented location far more seriously than a complaint. See RFI and noise.
A foxhunt is also one of the best club activities for families and newcomers: it needs no license to participate, it happens outdoors, and a Technician with a $30 handheld can win.