Fox hunting
| Fox hunting | |
|---|---|
| Also called | Hidden transmitter hunting, T-hunting, radio direction finding (RDF); ARDF for the international sport |
| Object | Locate a hidden transmitter, the "fox", using direction-finding equipment |
| Common bands | 2 meters; 2 meters and 80 meters for ARDF |
| Formats | Mobile hunts by car; on-foot ARDF with map and compass |
| Equipment | |
| 2-meter antennas | Handheld Yagi, tape-measure Yagi |
| 80-meter antennas | Ferrite-rod loop with a sense antenna |
| Close-in tools | Offset attenuator, body fade |
| International rules | IARU ARDF rules |
Fox hunting is the amateur radio activity of finding a hidden transmitter, called the fox, using direction-finding equipment. Hunters follow the fox's signal and try to reach the transmitter as quickly as possible, by car, on foot or both. It is also known as hidden transmitter hunting, T-hunting or radio direction finding (RDF), and its international on-foot sporting form is called Amateur Radio Direction Finding (ARDF). Beyond being a game, it teaches exactly the skills used to track down jammers, stuck transmitters and sources of interference.
How a hunt works
A directional antenna picks up signals best from one direction. By turning it and noting where the signal peaks, or with some antennas where it drops into a sharp null, a hunter gets a bearing, a compass direction toward the fox. Bearings from two or more places, drawn on a map, cross roughly where the fox is, a method called triangulation. Hunters then move in, taking new bearings as the signal grows stronger.
The main trap is reflection. VHF signals bounce off hills, buildings and large vehicles, so a bearing can point at a reflection rather than the fox. Bearings from high, open places are more reliable, and one that doesn't agree with the others is worth taking again.
Mobile hunts
In the US, many fox hunts are mobile T-hunts run by local clubs, usually on the 2-meter band. The fox, an automatic low-power transmitter or a ham with a radio, is hidden somewhere in an agreed area and transmits on a set simplex frequency, continuously or at intervals. Like any amateur station, it must identify with its call sign. Teams start from the same place, take bearings and set off. Depending on local rules, the winner is the first to find the fox or the team that covers the fewest miles.
Mobile hunters use a directional antenna, often a small Yagi held out a window or mounted on a mast that can be turned from inside the car, or a Doppler direction finder, which switches rapidly among several antennas on the roof and shows the bearing on a display. A team of two, one driving and one working the radio, is both safer and faster. Near the end, most hunters park and finish on foot.
ARDF
ARDF is an organized on-foot sport, run internationally under rules set by the International Amateur Radio Union (IARU), with national and world championships. It combines direction finding with orienteering: competitors run or walk through woods and fields with a map, a compass and a lightweight receiver, looking for several foxes hidden in the terrain. Events are held on 2 meters and on the 80-meter band.
In the classic format, five foxes share one frequency and take turns, each transmitting for one minute in a repeating cycle and sending a different short Morse code identifier so competitors know which one is on the air. A separate beacon guides them to the finish. Competitors, grouped by age and gender, must find the foxes their category requires, in any order, within a time limit. Shorter formats such as the sprint and foxoring are also part of the sport.
Equipment
- Handheld Yagi. A small two- or three-element Yagi for 2 meters, held in front of the hunter with a receiver attached, gives a clear peak toward the fox and a weaker response off the back.
- Tape-measure Yagi. A cheap, popular home-built version has elements cut from a steel tape measure, mounted on a PVC pipe boom. The flexible elements fold out of the way of branches and spring back.
- Offset attenuator. Close to the fox, its signal overloads the receiver and leaks in through the radio's case, so the antenna stops giving useful bearings. An offset attenuator mixes the signal from the antenna with a local oscillator, producing a copy a few megahertz away. With the receiver tuned to that copy, signal leaking through the case goes unheard, and the copy's strength can be turned down with a knob all the way to the fox.
- Body fade. Very close in, a hunter can hold a handheld radio against the chest and turn slowly in a circle. The body blocks the signal, so it is weakest when the hunter's back faces the fox.
- 80-meter receivers use a small ferrite-rod loop antenna, which has two sharp nulls in opposite directions, plus a short whip called a sense antenna that shows which of the two directions is the right one.
Finding interference
The same methods solve real problems. Hams use direction-finding gear to locate a stuck microphone holding a repeater on the air, deliberate jammers and noisy electrical equipment, and then share their findings with repeater owners, utilities or the FCC. Search-and-rescue teams and the Civil Air Patrol use similar techniques to find emergency beacons from downed aircraft. Many clubs treat fox hunts partly as training for these jobs.