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Radio direction finding — edited Aug 31, 2026 by Unknown editor
This is the first recorded version, so every line is shown as added. Note: Imported from content/wiki
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Radio direction finding (RDF) is the sport of finding a hidden transmitter with a receiver and an
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antenna — and the same skill that locates a stuck repeater input, a malicious jammer or a noisy power
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line. Clubs run it as a "foxhunt" or "T-hunt", usually on 2 meters, sometimes as a mobile chase across
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a county and sometimes as a timed course on foot.
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## The gear
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- **A handheld** with an S-meter or signal-strength bars, and — critically — the ability to hear a
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very strong signal without pinning the meter.
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- **A directional antenna.** The classic is a **tape-measure 3-element yagi** for 2 m: a length of PVC,
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hose clamps and a steel tape measure, built in an evening for a few dollars, and it folds to fit in
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a car.
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- **An offset attenuator** or step attenuator. Close to the fox, everything reads full scale and your
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antenna's pattern disappears. An attenuator restores the difference between "that way" and "the other
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way" and is what separates a fast hunt from a wandering one.
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- **A body fade** technique costs nothing: hold the handheld against your chest and turn slowly; your
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own body blocks the signal and the null points away from the transmitter.
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- **A compass, a map and something to write on.** Two or three bearings taken far apart and drawn on a
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map will put you within a small area before you start walking.
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## How a hunt works
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The fox transmits on a known frequency, either continuously or on a schedule (30 seconds every few
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minutes is common, and much harder). Hunters take bearings, triangulate, drive or walk closer, and
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attenuate as the signal grows. Scoring is usually by time, sometimes by mileage.
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Things that trip people up:
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- **Reflections.** Buildings, hills and water give bearings that are confidently wrong. A bearing taken
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from a hilltop is worth three taken in a valley.
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- **Multipath near the fox.** The last hundred yards often reads as several directions at once; walk a
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circle and trust the pattern, not one reading.
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- **Antenna polarization.** A horizontal antenna looking for a vertical fox loses 20 dB. Match them.
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## ARDF — the international form
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Amateur Radio Direction Finding is the competitive, on-foot version, run on 2 m and 80 m with five
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transmitters on a timed cycle over an orienteering-style course. It has national and world
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championships and a small, welcoming US community.
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## The serious use
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The same equipment finds:
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- **Interference and jamming**, which the FCC expects the community to document before it will act.
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Recorded audio, dates, times and independent bearings from several operators make a case; see
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[interference and enforcement](/wiki/rules/interference-and-enforcement).
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- **Noise sources** — arcing insulators, failing switching supplies, grow lights, electric fences.
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Bearings on a noise are taken exactly the same way, usually with an AM receiver, and utilities take
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a documented location far more seriously than a complaint. See
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[RFI and noise](/wiki/station/rfi-and-noise).
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- **Emergency locator and lost-hiker searches**, where amateur teams have assisted agencies.
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A foxhunt is also one of the best club activities for families and newcomers: it needs no license to
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participate, it happens outdoors, and a Technician with a $30 handheld can win.