Practical_Antenna_Handbook_0071639586

390 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
rigs, and the crowded conditions on the 80-, 75-, and 40-m bands, it becomes a matter of
more than academic interest how they might increase the signal strength from their
portable (or mobile) emergency station. Anything they can do, easily and cheaply, to
improve their signal is like having money in the bank. Fortunately, there are several
tricks of the trade that will help.
In cases of emergencies on most highways, we are likely to be in range of some repeater,
so we would use a VHF band (probably 2 m) to contact police or other emergency
services—perhaps through manual assistance from another ham on the repeater,
perhaps through a repeater autopatch. In fact, with the wide availability of repeaters
around the country it behooves any amateur backpacking or four-wheeling into remote
areas to be familiar with nearby repeater locations and frequencies.
As we saw in the preceding chapter, the HF mobile configuration is inefficient by its
nature, and little can be done to improve matters . . . at least while remaining capable of
being operated “in motion”. But if we recognize that the operator needing emergency
or urgent communications is most likely not driving down the road somewhere but is
stopped in a remote location and needs to contact someone outside his local area, we
can improve the performance of the “mobile” station by employing techniques not
available to the mobile operator who is “in motion”.
Most amateurs with experience in mobile HF operation will attest to the need to
enhance their signal if they are to be effective in providing emergency communications.
Setting aside for the moment the issue of output power levels, mobile radiation efficiencies
are simply too poor to consistently compete with the larger and better installations
found at most home and fixed (or base) stations. There are three primary reasons for
this:
• The radiating portion of virtually every mobile HF antenna is substantially less
than l/4 in length, making its radiation resistance extremely low and difficult
to match with low losses.
• Below 2 m, most vehicle bodies make very poor RF grounds, thus further compounding
the problem by dissipating a greater portion of the transmitter output
power in the resistance of the lossy ground.
• In most emergency situations, the preferred coverage distance is considered
“close in” for HF propagation modes, even though it may be beyond the reach
of the nearest 2-m repeaters. The use of a conventional mobile antenna mounted
on the vehicle (in situ) results in a vertically polarized signal with maximum
amplitude at relatively low wave angles, while ionospheric propagation modes
for the distances involved overwhelmingly favor an antenna that puts most of
its energy into the higher wave angles. This is a perfect match for a low, horizontally
polarized radiator.
Let’s discuss each of these limitations in more detail, with the intention of identifying
simple workarounds for the temporary situations that most emergency and portable
operations imply. Keep in mind that throughout the discussion we are assuming
that the vehicle and the mobile gear it is carrying are in a fixed location for the duration
of the emergency or portable activity.