SPARK Fox Hunt Page

FOX HUNT

At our April 1997 meeting, guest speaker Rich Harrington, KN6FW, gave us a very informative talk on transmitter hunt. With Rich's permission, the following is his write-up on that subject. He also provided the schematic of the 2-Antenna Switching Direction Finder.

Rich holds transmitter hunts twice a month in different locations from Livermore to Santa Clara. If you're interested, Rich welcomes you to give him a call at 510/462-1467.

Direction finding in amateur radio is like most other facets of the hobby, you need to learn all you can about it and then practice. The great part about transmitter hunting is that the practice is fun, like going on a treasure hunt or playing hide and seek. Direction finding also has its practical aspects: one is finding jammers or other pests, and another is helping in search and rescue.

Anyone who has ever aimed a TV antenna or swung a beam has done radio direction finding. Most of the things you had to contend with for your TV antenna are present in VHF and UHF transmitter hunting. The three things that affect radio signals the most are topography, obstructions and reflections. UHF and VHF signals don't like going through hills. They don't even like trees and other plants much. Buildings (particularly those made of steel and concrete) obstruct and reflect signals. When transmitter hunting, always consider your surroundings. If a signal gets weaker as you approach a hill, the transmitter is most likely on the far side or behind the hill. If the direction of a signal suddenly points to a large building, it is most likely a reflection.

Next I will discuss a series of systems and techniques useful for T hunting.

A simple technique for radio direction finding is "body fade." Just like trees the human body attenuates radio signals. If a signal goes through you to get to your hand held radio it will be weaker than if it does not go through your body first.

Using a beam usually gives you even better accuracy than body fade. However, mounting a beam to your car can be a challenge, both from wind load and from the mechanics of rotation it. When using a beam you need an attenuator. Most S meters on FM radios saturate at about 20 to 100 microvolts. As you get closer to the hidden transmitter, you have to reduce the signal to your HT to get a change on the S meter.

The Adcock or interferometer is a very accurate direction finding antenna system. An interferometer can get a bearing to about one degree. It is made by placing two antennas out of phase so that when both antennas are equal distance from the transmitter, the signal nulls. They can be made with 2 simple dipoles or with 2 20 element beams.

A time of arrival direction finder works similar to the Adcock antenna system. It switches between the two antennas at an audio rate, typically between 300 to 800 Hz. The switching causes an effective phase modulation in your FM radio. If the antenna are equal distance from the transmitter, there will be no phase change when the antenna are switched and no tone will be heard. This system also has a sharp null.

The Doppler system gives you a direct read-out of the direction to the transmitter. It works by switching 4 antennas (3 or more can be used). As the antennas are switched, the FM radio detects the phase change from the antennas and generates a tone in the radio at the switching rate. The phase of the generated tone is compared to the signal switching the antennas to give you the direction to the transmitter.

Besides radio equipment there some other things a well equipped T hunter brings along. You will need a good continuous map of the area. A book of maps like the Thomas guide will not work. Covering your map with plastic or clear shelf paper will allow you to mark bearing on it as you hunt. Most T hunts start from high locations, bearings from high location are usually more accurate than ones from low locations. Mark your initial bearing on the map and follow it! As you get closer to the transmitter mark the bearings you get along the way. Do not follow just one signal that does not match all the other bearings you have obtained as you progress.

If you have a HT you are not using for the hunt while you are moving put it on the dash board. When you hear the transmitter in the HT you are very close.

Now that you are close to the transmitter the hunt gets more interesting, although some hidden transmitters are easy to find, because they are on a ham's car. Some hidden transmitters are actually "hidden." When you are very close to a hidden transmitter your HT will even pickup a strong signal without an antenna connected. To reduce the signal into your HT you can wrap it in aluminum foil or slip it into a metal tube. You also could use a field strength meter. Be careful to find the correct signal, because you might find the local FM station. To eliminate this problem you could use a frequency counter. When using a counter you may need an attenuator to reduce the signal to a just detectable level.

Standard T hunts are scored by the shortest distance traveled from the start to the hidden transmitter. This does not allow the use of triangulation because you are always going directly toward the transmitter.

Team hunts are a situation where triangulation can be very effective. With these hunts you can place teams on two or three high locations. This can pinpoint a transmitter to within about 2,000 feet.

2-Antenna Switching Direction Finder, Rich Harrington, KN6FW, 7/24/93