Introduction to Radio Direction Finding
 
RDF or radio direction finding, fox-hunting, hidden transmitter hunting or just T-hunting, bunny hunting, and jammer hunting are different terms for the same thing, sort of. Sometime you may be asked to help in a "turkey hunt" or in a "maverick hunt." How about a "kurchunker hunt" to try to find who's been messing with the local repeater? Whatever it's called, two meanings usually cover the purpose of the activity, transmitter tracking, or just tracking, and transmitter locating and retrieval, or just locating. Usually the activity is to do a combination of the two, tracking and locating. If you are tracking tagged animals in the woods, looking for a lost model rocket somewhere on a sod farm, or collecting data from a small balloon you released half an hour ago, you'll still use many of the same techniques you learned while looking for that transmitter hidden under a water faucet as part of a Field Day contest.

There are numerous practical applications for RDF. It's been an important technology to the military as far back as World War I. Today we use it to find lost children and animals and lost hikers; and now cross-country skiers wear transmitters in case of a bad experience with an avalanche. Over the years many lives have been saved by the quick location of downed aircraft in remote areas. Many use RDF to keep track of and study land animals, flying animals, and creatures that live in water.

 
 
Tracking animals in the wild is a common use of RDF. Attaching a small transmitter to a box turtle allows the scientist to study the movements of the animal in its habitat, or to again find the animal in order to replace the battery in the transmitter, or to remove the transmitter once the studies are completed. Hunters often equip their hunting dogs with transmitter collars in order to find them again. Falconers attach small transmitters to their birds of prey to keep track of them. Oceanographers and other scientists use transmitters on whales, dolphins, sharks, seals, and so on.

Animal tracking transmitters are available from a few sources. They are usually of high quality, and work very well. Specialized receivers and directional antennas are also available from the same sources. However, the person or organization using the transmitters must obtain a permit from the FCC in order to legally use them. The transmitting frequencies are around 218 MHz., and there are three separate bands. One band is for tracking animals on the ground. One is for tracking animals that fly, and one is for tracking animals in or on water.

 

RDF could also stand for "radio direction and finding." Most people agree that the basic idea of RDF is to find the direction of a transmitter, and then to find the transmitter. Sometimes you may only need to track a moving transmitter, such as a weather balloon. Transmitter tracking often requires RDF techniques in order to keep the directional antenna aimed at a weak-signal source.

Many hams get into transmitter hunting as a sport, and some of them get really good at it. Usually it's more art than science. ARDF (Amateur Radio direction finding) and DFing (more terms) are quite popular at regular events such as Field Day, and JOTA (Jamboree on the Air) a yearly event for the Boy Scouts. Many ham groups and clubs have formed for the single activity of DFing.

 
The three basic items that make up an RDF system are a receiver, a directional antenna of some form, and a transmitter or transmitting source. If you are one of a group hunting the same transmitter, you need just a receiver and a directional antenna.
 
You'll also need a few accessories such as a cable to connect the antenna to the receiver. Depending on the receiver, you may need an adapter connector. Eventually you may need or wish to add an attenuator.
 
 
www.adepinstruments.com       12/24/15

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