Fox Hunting 101 - Part 1

[VA3KOT John]

Did you know you can listen to Owen Sound's local AM radio broadcaster CFOS on a frequency of 1.680MHz?

That's just below the amateur 160m band. CFOS broadcasts on 560KHz but, if you go up to the transmitter site on Story Book Road you can also listen to the station on the third harmonic of its fundamental frequency. Three times 560KHz equals 1680KHz.

The same works for ham radio. If a signal is sent out on the VHF Simplex calling frequency of 146.520MHz, it can also be received on a UHF frequency of 439.56MHz. But, you have to be very close to the transmitter to hear it. Radio transmitters employ a LPF (Low Pass Filter) in their output circuitry to reduce interference on harmonics of the fundamental frequency. The LPF significantly - but not completely - removes harmonics.

We can use this technique as an aid to locating hidden radio transmitters, also known as "foxes". A popular ham radio activity known as "fox-hunting" employs this technique, and many others we can explore in subsequent posts and discussions. Fox-hunting is a useful skill, not just for fun but can also be used for locating malicious ham band activity such as jammers (unfortunately that is quite common in the Greater Toronto Area).

The general set of skills used in fox-hunting is called RDF (Radio Direction Finding). RDF is used by the Canadian Coast Guard to locate vessels sending out distress signals on the Great Lakes.

Why don't we get a group of GBARC members together to do some local fox-hunting. Experienced fox-hunters can help train newcomers in the techniques used. A typical event lasts a couple of hours and ends when a hunter successfully tracks down a hidden low power transmitter. Please reply to this post if you would like to know more, or get involved.

In the meantime, start saving your empty baked bean cans. I'll explain why in Fox Hunting 101 - Part 2.