Fox hunting and ARDF

What it is

A fox hunt is a game of hide and seek on the radio. Someone hides a small transmitter, the "fox", and everyone else finds it with direction-finding gear. The organised, competitive form is ARDF (Amateur Radio Direction Finding), also called radiosport, a footrace between several hidden transmitters with a map and a compass. Both teach Radio direction finding basics in a fun way, and both are a good first step toward tracking down real interference.

Two ways to play

	Casual fox hunt	ARDF competition
Foxes	Usually one	Typically several, each with its own identifier
Where	Town or countryside, often by car	On foot, in woods or open country
Gear	Handheld or mobile Yagi, loop or Doppler set	Compact handheld receiver and directional antenna
Scoring	First to find it	Find the required foxes, fastest wins
Often organised by	A local club	Amateur radio societies, national and international

Casual hunts are friendly: a club hides a transmitter, and a few cars or groups of walkers set off. Competitions are strict about the course, time limits and rules, so check the current rules of the event you enter.

How a competition runs

In the classic format, only one fox transmits at a time. Each takes a turn of about a minute, then the cycle repeats. So you can only take a bearing on a given fox during its minute, and you plan your route around where you think the others are. Between bearings you move, using the map to decide where to try next. Competitors visit the foxes in any order, and the clock keeps running.

The final approach

Far from the fox you take bearings and walk toward the crossing point. Close to it the signal gets so strong that the receiver is overloaded and reads full scale from every direction, so you can no longer tell which way is best. The cure is to reduce sensitivity as you approach: an attenuator in the antenna line, or the receiver's own gain control.

Free-space signal level rises about 6 dB each time the distance halves. A few turns of the attenuator, 10 dB or 20 dB at a time, keep the meter in the middle of its range, where it moves as you swing the antenna. That is how you finish: switch in more attenuation, find the direction, walk, repeat, until you are standing next to the fox with the receiver almost deaf.

Tips

• Practise on a friend's transmitter before a real event.
• A fox is an amateur station like any other. A licensed operator is responsible for it, and it must identify as the rules require; see Part 97 in plain words.
• Terrain matters. On a hunt, a signal that seems to come from the wrong way is often a reflection off a hillside or building.
• Stay safe: watch for traffic, uneven ground, wildlife and weather, and respect private land.

Related

• Radio direction finding basicsBearings, triangulation and null techniques.
• Direction-finding antennasLoops, Adcock arrays and Yagis for finding a transmitter.
• The Yagi-Uda beamDriven element, reflector and directors.
• Handheld radio antennasRubber ducks, whips and getting more from an HT.
• Clubs, Elmers and communityHow to find mentors, clubs, nets, hamfests and learning groups.
• The radio spectrumLF to microwaves: where the bands sit and what each is good for.