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energy, and the pattern will not be completely omnidirectional. For true ground plane action and
predominantly vertical polarization no fewer than three equally-spaced radials should be used.
Figure 4
illustrates the construction of a multi-band radial which is resonant on 40, 20, 15 and 10
meters. Good quality 300 ohm TV ribbon lead should be used (velocity factor is critical), and the
twinlead conductors should employ
at least one strand of steel wire to
support the weight of the radial.
Figure 4
Four such radials will be the practical equivalent of the system shown in
Figure 2
for operation on
40 through 10 meters. Regardless of the number of radials used in either elevated or ground level
systems, all radials should be attached to the ground connection at the antenna feedpoint by the
shortest possible leads. An elaborate radial system at ground level, for example, cannot be used with
a vertical antenna on a rooftop or on a tall tower, for the length of the ground lead would effectively
become part of the antenna, thus detuning the system on most or all bands.
Metal Towers and Masts
If a metal mast or tower is used to support a vertical antenna all radials should be connected to the
mast or tower at the ground connection of the antenna feedline. This is because one of the functions
of a resonant radial is to detune a supporting metal structure for antenna currents that might
otherwise flow on the structure and thus turn the vertical antenna system into a vertical long wire
with unwanted high-angle radiation.
Other Mounting Schemes
In cases where a resonant vertical antenna may neither be ground mounted nor used with an
elevated ground plane, operation may still be possible if connection can be made to a large mass of
metal that is directly connected or capacitively coupled to the ground, e.g., central air 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 1/4-wave 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 Recreational Vehicle 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 best possible ground
system beneath the antenna in order to minimize losses from RF currents flowing in the earth below
the antenna. Fortunately, the metal bodies of both Motor Homes and Recreational Vehicles 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, Motor Home or Recreational Vehicle 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.
