F4IOZOlivier · radio amateur · JN18FS

Fox hunt direction finding (ARDF)

A beacon is hidden in the park: turn the antenna, take the bearing, move, draw the lines and place the fox where they cross. All games

The fox is on the air somewhere

Turn the Yagi to the S-meter's peak, take the bearing, walk, take another. Then place the beacon: the closer, with few bearings and little walking, the more you score.

Badges

All-rounderFinish a game of each game.
First foxPlace a fox within 100 m.
ThriftyA fox within 100 m with only two bearings.
TriangulationWithin 50 m with three bearings at most, at the Triangulation level.
No clue neededEvery fox of a game within 100 m, with no sweep and no clue.
Not fooledWithin 80 m at the Echoes level.
ARDF masterAll three foxes within 100 m.
Woodland runnerPlay ten games.

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Amateur radio direction finding

ARDF (or “fox hunting”) is run in a forest, map and compass in hand: five beacons, MOE, MOI, MOS, MOH and MO5, each transmit for a minute in turn on 2 m (144.50–144.90 MHz) or on 80 m (3.510–3.600 MHz). They are found with a receiver and a directional antenna.

The bearing: a Yagi hears mostly in front of it. Turn it slowly, note the azimuth where the signal is strongest. Near the fox, add attenuation to keep a sharp peak.
Triangulation: two bearings taken from places far apart, at right angles if possible, cross at the fox. With three or more, look for the point nearest all the lines (least squares): a small triangle means a good measurement.
Echoes: a cliff, a building send the signal back; behind a hill, the direct path weakens and the echo can win. A bearing that crosses none of the others is suspect: take one from somewhere else.
The S-meter: S9 is −73 dBm on VHF, each S-unit 6 dB. Doubling the distance costs 6 dB in free space; the forest takes more.

The game's numbers A 0.5 W fox on 144.7 MHz, the free-space loss plus 10 dB/km of forest, 18 dB behind a hill, 6 dB per echo (image method), a 3-element Yagi with a 77° beamwidth and 20 dB front/back. The park is imaginary, drawn at random.