- Written by Brian Keahl
- Category: Emergency Communications
- Published: 24 March 2015
- Hits: 1728
This topic is based upon a presentation made by WD8LQT (John Playford) at the last West Georgia Amateur Radio Club meeting, in preparation for an upcoming “Fox Hunt”.
“Fox Hunting” is a fun hide-and-seek using radio that also provides practice in locating malfunctioning transmitters, generators of harmful interference, and even lost people & downed aircraft.
In a “fox hunt” the “fox” hides a radio transmitter for the hunters to locate. The radio usually transmits a message for a period of time (30 seconds to a minute), followed by an equal period of silence. The message is often a Morse Code ID of the unit and/or an audio message identifying the station.
Hunters can use a variety of equipment, ranging from a simple scanner to elaborate receivers that will literally point the hunter towards the signal by measuring the time difference between signal receipt between two antennas. We'll cover a few of the basic techniques used now, but before we do it's important to keep in mind that all the different options have strengths and weaknesses:
Standard hand-held transceiver or scanner with whip antenna(s). It's as simple as using your ears. By shielding the antenna with your body or a reflector like tinfoil or aluminum you can listen for changes in signal strength to determine transmitter direction. Just shield the receiver and antenna and slowly turn around until the signal decreases to its lowest point. At this moment the transmitter is directly behind you. Antennas of varying gain can be used to attenuate the signal as well. I've also found that removing the antenna, laying a piece of paper in the connector, and reattaching will often attenuate the signal.When you get particularly close, removing the antenna will attenuate the signal even more. If removing the antenna causes a complete loss of signal, try removing the antenna but holding the base of the antenna close to the connector - capacitive coupling will often deliver an attenuated signal. Just vary the distance until it is degraded enough to have some static, and then direction find as usual. This last technique worked very well for me on a recent fox hunt.
The upside is you can use equipment out of the box. The downside is the equipment isn't as directional as other options.
Standard hand-held transceiver or scanner with Yagi antenna(s). This works in much the same way as the previous method. A 2-meter Yagi antenna looks similar to an old style VHF television antenna and works much the same way. It is directional, allowing you to point it towards a signal source for greater signal reception and transmission. Likewise, as the antenna is pointed askew from the source the signal will drop off. The greater the gain of the Yagi the more rapidly it will drop off as your aim drifts away from the transmitter. However, these antennas can be prone to overload as you get closer to the transmitter, making it difficult to close the last leg of locating the transmitter. You also must be sure you aren't picking up the “fox” from the back-end of the Yagi, as they usually have gain from the back end. Arrow Antenna produces a range of portable Yagis.
Standard hand-held transceiver or scanner with Tape Measure Yagi Antenna. You can make your own Yagi antenna with a metal tape measure! It'll work pretty much the same way as any other Yagi and the measurements on the measure makes it easy to cut the segments to the right length. So consider building your own Tape Measure Yagi.
Standard hand-held transceiver or scanner with Loop antenna(s). Like Yagis, Loop antennas have directional characteristics like Yagis but is also less affected by reflected signals. These antennas tend to be receive only as they are usually not tuned. They also have negative gain as a result of not being tuned, meaning another antenna may be necessary if further from the transmitter or you may not pick up the signal at a distance. Arrow also produces a loop antenna for fox hunting.
As you get closer to the transmitter you will need to attenuate the signal so you can detect changes in the signal when close to the “fox”. There are several ways to gradually attenuate the signal on a scanner or HT.
Using Your Body
We previously discussed using your body to attenuate the signal. Your body will block signals coming from the transmitter. Holding the radio in front of you and turning around will yield the lowest signal when the transmitter is behind you. So you'll need to do a 180 degree turn to be facing the transmitter after detecting the lowest signal.
You can use a reflector, such as tin foil, a half-cut tin can, or other metal to attenuate the signal from the side of the antenna the reflector is held against. Whatever side the reflector is not obscuring will be facing the transmitter when the signal is the strongest. When the signal is weakest the reflector will be facing the transmitter, as it is blocking or attenuating the signal.
There are devices designed to go between an antenna and the radio that allows the operator to adjust the attenuation of the signal strength with an adjustment knob. You can see an attenuator and kit for purchase here. You can also build your own simple attenuator by placing a resister in series with the antenna.
A Tinfoil tube is a great tool to be used with a whip or rubber ducky antenna. A simple paper towel tube can be cut to size and wrapped in tinfoil (including one end). The tube can be slid over the antenna to varying degrees to attenuate the signal to the extent needed. The closer the tube is to the size of the antenna, the better, so if lower diameter tube can be found, use it. Of course, you could cut the tube down the length, roll it smaller, tape it, and wrap in tin foil.
A paper clip can function as a highly-attenuated antenna, better at receiving than no antenna, but with less gain than even a rubber ducky. This can be useful for the “in-between” distance from the transmitter. The paper clip, bent straight will fit into the center pole of a BNC connector on a radio nicely. Other connector types may require a little creative engineering on the paper clip.