There’s been talk lately about Club-sponsored “Fox Hunts,” or transmitter-hunts (T-hunts).
I have no idea how many Club members have had experience with Radio-Direction-Finding (RDF), but it is a valuable skill. Besides the fun of an actual contest, T-hunting skills are helpful for finding a signal that is, say, jamming a repeater, or perhaps power line or other interference, or even Cable TV “leakage.” Some T-hunters even use their skills to work with the Civil Air Patrol or the US Coast Guard Auxiliary to help find lost or downed planes. A “Fox Hunt” helps to build RDF skills.
The required equipment is somewhat specialized, but it’s easily obtained or assembled, using the literature which is found on the subject. For instance, the ARRL website has several good articles that describe different kits, which are generally still obtainable from the authors. The articles also explain the techniques needed to master the art of finding a transmitter which may be on for only limited periods, and whose signal is hidden among multi-path reflections. A T-hunt can be done in any kind of area and terrain. For instance, it may be limited to a small park, where the “fox” has only a few milliwatts of power, or it could be “unbounded” – like some contests that go on in California – where the hunt could encompass as much as 400 miles.
Whether T-hunting by car or on foot, at the end, it will pretty much always require some hunting on foot. So the equipment you use must be adaptable for utilizing in a car or on foot-which is not always practical or even feasible. And, the equipment used in a car must be “operator-friendly” – so as to avoid creating a collision. In the final analysis, the equipment used in a car is different than that used on foot – which is why there are so many designs of the same basic equipment. If a vehicle is needed for the T-hunt, then it is usually necessary to have some portable equipment that is lightweight and easily used while running through undergrowth in the dark, for the final leg of the hunt.
RDF equipment falls into 2 categories. The first is an attenuator, so that your S-meter doesn’t peg when you get close to the transmitter – and to help give you an indication of range. The second, and more complex is the actual direction-finding equipment, which is to point you the right way. Essentially, it can be done by using a directional antenna (or a means of simulating one), taking a reading, then moving some distance and taking another reading. By “triangulating” in on the signal, you determine which direction to move. S-meters and/or LED displays, or even audio tones, are generally used to indicate direction, while S-meters or audio tones are used with an attenuator to indicate signal strength. Much of the mobile RDF equipment utilizes multiple antennas, which are rapidly switched in & out to simulate a rapidly rotating antenna. This allows utilization of the Doppler effect to determine directionality. Other means are “balance” detectors, which utilize dual antennas for right-left determination, or 4 antennas for right-left and front-back. When on foot, it’s generally enough to use 2 antennas for right-left, and use your body rotation to determine front-back. The “HandiFinder,” a handheld RDF device, is based on that principle – it produces an audio tone when one antenna receives the signal stronger than the other; loudness (in either earphone) indicates the amount of un-balance; a “null” produces no audio. Practice with this device will allow the operator to determine direction on foot very rapidly. An attenuator is helpful to determine distance from the transmitter.
So, you can see where there is lots of potential for developing skills, using various RDF devices, and for determining which ones are most useful at which times. Building kits, or devices of your own design, is also fun. Developing improvements on existing devices is always useful and fun.
Usually contest rules allow working in teams or as individuals, but small 2 or 3 man teams are most
fun and are usually necessary to handle all the equipment. Working in teams is also a good way for us to get to know each other better, and to build teamwork. Things can become very competitive, so, your team should always remember the adage, “Never trust anything said by a T-hunter or a T-hider.” Let’s get out and have some fun!
Chuck Teeters – W4MEW
Published in the August 2005 edition of The Splatter