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TECH NOTES—GROUND RADIAL SYSTEMS
conditioning systems or structural steel frames of apartment buildings. Some amateurs have
reported good results with vertical antennas extended horizontally or semi-vertically from metal
terraces which serve as the ground connection. Alternatively, a quarterwave vertical may be
window mounted if a short ground lead to a cold water pipe or radiator can be used. If a long
lead must be used, tuned radials may be required for resonance on one or more bands. Great
care should be exercised in such installations to avoid power lines and to keep the antenna
from falling onto persons or property.
MOBILE HOME AND RV INSTALLATION
The principles of vertical antenna installations for use on mobile homes or RV's are the same as
for other installations, and they all boil down to two main considerations. The first is that of
erecting the vertical in the clearest possible spot, away from obstacles (including the MH or
RV) that can interfere with radiation from the antenna. The second is that of installing the beat
possible ground system beneath the antenna in order to minimize losses from r.f. currents
flowing in the earth below the antenna. Fortunately, the metal bodies of both MHz and RVs
can be used as highly conducting ground planes for vertical antennas in exactly the same way
that automobile bodies, etc., provide the ground system for shorter vertical antennas for mobile
operation. The metal body of an automobile, MH or RV may be viewed as one plate of a
capacitor. Since the surface area of even a small automobile is quite large and in close
proximity to the earth, its body is tightly coupled to the earth below and may be considered
simply as an extension of the earth itself—a kind of hill as far as radio frequencies are
concerned, but one having higher conductivity than the earth itself. RVs and especially MH
having much greater surface area, will therefore provide a more extensive and effective ground
system than a large number of radial wires occupying the same space as the MH or RV.
As in mobile installations, a vertical antenna may be mounted almost anywhere on the body of
the vehicle or MH and made to operate with reasonably low VSWR, but it is generally
considered that the best possible location for a mobile antenna is in the middle of the roof of
the vehicle, i.e., at the center of the vehicle's ground plane and at a point where the antenna
will not be in the "shadow" of any part of the vehicle. It is not usually convenient, or even
practical to install a relatively tall vertical on the roof of an RV or MH for any number of
reasons, so the next best procedure would be to install a vertical antenna with its base at the
same level as the roof, preferably near the middle of one of the longer sides. The exact way in
which this may be done is a matter of convenience, but a short mast extending from ground
level to the roof of the MH and RV and placed alongside the building or RV would provide a
stable and sturdy support with a minimum of mounting brackets and other modifications to the
RV or MH. For portable operation such a mast could simply be lashed alongside the RV with
the base in a shallow hole in the ground for additional support, and there would be no harm in
extending the mast a few inches above the roof level to permit attachment of ropes which
could be used to hold the mast firmly against the side of the vehicle and to prevent side sway.
This system has been used successfully with various types of RVs, travel trailers and even
passenger automobiles during portable operation. For "L" shaped mobile homes a vertical
antenna should be placed in the corner of the "L" so that the metal roof will provide
groundplane coverage over 270 degrees.
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