Practical_Antenna_Handbook_0071639586

518 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
RDF at VHF and UHF
It should be apparent from the preceding paragraphs that obtaining an easily portable
antenna with a unidirectional pattern at HF requires some mechanical and electronic
complexities. At VHF and above, such patterns are far easier to attain. Even a four-Â
element 6-m Yagi is reasonably easy to manipulate by hand, and the task gets simpler
as one goes higher in frequency. At even higher frequencies, the antenna of choice is the
parabolic reflector.
Perhaps a more difficult task at VHF and above is finding a way to get signal
strength information from the receiver being used. In today’s marketplace, very few
VHF/UHF FM mobile rigs include any visual metering, even when the AM aircraft
band is present. A rare rig may have a dc voltage proportional to received signal level
available on the rear panel, but the most likely transceivers to have S-meters (or their
modern equivalent, the LED “bar” meters) are the relatively expensive “multimode”
rigs intended primarily for base station use.
Fox Hunting
An activity popular with ham radio operators for many decades is fox hunting. A fiendishly
clever ham (the “fox”) would hide with a mobile or portable transmitter (usually
on either 10 or 75 m). The “hunters” would then RDF the fox’s brief transmissions and
try to locate the transmitter. If you were first to locate the antenna, you won—and got
to be the fox the following month!
Today, fox hunting is as likely to be held on VHF and UHF as on the lower frequencies.
Because the propagation modes can be so different around a geographic region,
both types provide many lessons.
Tracking Down Electrical Noise
A practical use for fox hunting skills is tracking down sources of electrical noise that
can mask weak signals. Most electrical noise comes from close by: every electrical appliance
that plugs into the wall (and even a few that don’t!), every wall adapter, every
computer or fax machine or wireless modem, every power line and power pole,
every high-efficiency heating or cooling system, every on-demand hot water heater,
every electric blanket, every touch lamp, and every streetlight or yard light is a suspect
until proven “innocent”. In most cases, a good technique is to start your “fox
hunting” activities on the frequencies where you first noticed the noise and then
gradually move upward in frequency as you get closer to the source. There are at least
three reasons for that:
• Noise energy in the VHF and UHF ranges is not likely to travel via as many
ambiguous propagation paths that can lead to contradictory results.
• Noise energy in the VHF and UHF ranges from most unintentional radiators is
much weaker than at the lower frequencies; if you can hear it at all at VHF and
UHF, you’re probably very close to the source.
• It’s far, far easier to hold and rotate a high-gain, sharp beam VHF or UHF
antenna by hand than it is to get the same directivity on HF!