To obtain obtain some idea of the numerical values involved with
mobile, short, antennas on 160 meter and other low frequency bands,
download programs -

HELICAL3
VERTLOAD
LOADCOIL

from website below. Takes only a few seconds to download. Not zipped
up. Run immediately.

With a very short antenna, to obtain a useful efficiency, a physically
large loading coil is essential. The only space available is up the
antenna. So the best thing to do is extend the coil up the antenna
from near the base, in the form of a long slender helical winding on a
pvc plastic pipe.

The whole thing behaves as a short 1/4-wave resonant vertical. There
is a short rod at the top which is pruned to resonate the antenna in
the required band. It is inevitably a single frequency job. The
bandwidth on 160m is only a few kHz.

With a 100 watt transmitter, when both vehicles are in the low-noise
countryside, it is possible to work 100 miles or more, on groundwave,
in daylight. Much further via skywave in darkness. G3YXM has worked
transatlantic from a car in Scotland to a base station in Canada on
occasions.

Maximum overall antenna heights are about 9 feet above roof of car
with the coil about 2" in diameter, the coil extending to a height of
6 or 7 feet.

The most tedious procedure is pruning all antenas in a group of
mobiles to the same frequency on the 160 meter band. But it has been
done. Mobile to a base station is easy. For many years the standard
UK frequency for mobiles was 1930 kHz.

Matching from antenna base to a 50-ohm transmiier can be accomplished
by a single or bank of mica capacitors of several hundred pF.

For quite a number of years, mobile working on 160m was quite popular
in the UK using helically-wound antennas.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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