Our chairman, Rob / ZS1SA started off by welcoming us all, and announced some administrative notices and decisions made at the Committee Meeting held on the previous Monday. Then he introduced us to OM Colin / ZS1RS, who was there to give us a talk and demonstration on Fox-Hunting.
Colin has had extensive experience in this particular aspect of our hobby. Locating radio transmitters by way of direction-finding techniques, using directional antennas to locate the source, has many practical applications in addition to the sport aspect. For example, a burglar alarm company in Gauteng enlisted the radio hams to succesfully locate an alarm transmitter that had gone off-frequency. And, of course, the location of illegal transmitters that cause interference on amateur radio frequencies is required from time to time.
One message Colin gave us is "Learn to trust your equipment". By this he meant, once you have tested your DF antenna and are familiar with its quirks, it usually gives consistent readings. He showed us some relatively simple equipment anyone can build to use effectively in DF hunts. The sky is the limit, though, in terms of complexity and cost for those who really get involved in depth.
Simple techniques were shown, such as using your body as a shield (and therefore a direction indicator), and using a cardboard cylinder wrapped in tinfoil to "hide" the receiver in to attenuate the signal appropriately.
The aspect of Attenuation was also discussed. The important thing here is that, at the appropriate level, the directionality of a signal can be clearly indicated: Too high a signal level and the antenna shows no directionality. Too low a signal and the squelch doesn't break open. At just the right level of attenuation, the direction of the transmitter is relatively apparent as the signal breaks through the squelch when the antenna is pointed in the right direction. When closing in on the "fox", attenuation becomes critical.
The sport aspect of fox-hunting is mostly carried out on the two-metre band frequencies. RF signals are sufficiently directional at this wavelength, the antennas are manageable at this size and most hams have 2m rigs. Colin showed us a variety of antennas and other equipment used to locate the direction of transmitters.
After demonstrating these items in the clubhouse, Colin went outside to hide the "fox", and some of us went out to try and locate it. Things were made easy for the "hunters" - the transmitter was located in a relatively open area and so not difficult to find.
Truly diabolical techniques used in more competitive situations include...
Placing the "fox" next to a long fence, which becomes part of the radiating setup...
Placing vertical dipole antennas of resonant length in trees to mis-direct the hunters...
Operating the "fox" via your vehicle's own antenna, in a car park full of other vehicle antennae...
Moving the location of the fox (though this is considered to be unsporting)..
A variety of disguises (limited only by the imagination) for the fox and its operator!
This was a particularly interesting club meeting, and we hope to hold a proper fox hunt event soon. Thanks to Colin for stimulating all of our interest!
|Rob / ZS1SA opens the meeting|
|Colin / ZS1RS shows an easily-built 2m DF dipole and attenuator|
|The Active Attenuator (the silver box) is an RF diode mixer|
|Colin shows us a home-brew planar loop antenna with signal strength meter|
|The simplest attenuator (and highly effective) ! A cardboard tube with tinfoil tape.|
The handie slides into the centre of the tube to attenuate the signal
|Colin demonstrates using the body as a directional RF shield|
|A simple switched attenuator box that goes between antenna and rig|
|A more complex doppler DF indicator, used in mobile DF hunts|
|The back of the DF planar-loop antenna, with signal-strength meter.|
|A home-brew doppler-shift DF antenna. A change in audio pitch indicates direction|
|The members go outside to try to locate the fox|
|Rob / ZS1SA finds that a signal null indicates direction more accurately than a peak|
|Judicious use of the attenuator is critical as one gets closer to the fox|
|Two young guys showed great interest! The fox antenna is poking up just to the left of the kid sitting on the concrete|
|This commercial "fox" is a milliWatt CW transmitter. Its frequency and CW message can be programmed by computer|
|Further discussions inside. Colin regaled us all with some amazing fox-hunting stories!|