Practical_Antenna_Handbook_0071639586

C h a p t e r 3 0 : G r o u n d i n g f o r S a f e t y a n d P e r f o r m a n c e 697
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Feedline
(A)
Vertical dipole
(B)
Bottom wire
split into strands
(C)
Ground plane vertical
Figure 30.6 Transforming a vertical dipole to a l/4 ground-Âplane vertical.
thinner strips, angle them upward by 45 degrees, and space them equally around the
azimuth circle, we end up with a ground-Âplane vertical. Thus we have created an artificial
ground plane high in the air, right where the vertical monopole needs it. A little knowledge
of trigonometry will convince the user that three radials, spread 120 degrees apart,
are sufficient to provide a return current path for all compass directions.
The ground-Âplane antenna is fairly easy to implement at frequencies as low as 7
MHz. In the 1960s and 1970s a very popular antenna was the Hy-ÂGain 14-ÂAVQ (now the
AV-Â14AVQ), a trapped vertical for 40, 20, 15, and 10 m. Propped atop a 20-Â to 30-Âft mast,
with a handful of radials sloping down from the base of the antenna, the antenna acquitted
itself very nicely as a low-Âangle radiator on 40 m. In some installations the radials
did double duty as guy wires for the mast. In such an installation, each combination
radial/guy wire must have an insulator in it at a distance of l/4 along the radial from the
base feedpoint of the radiating element.
In recent years increasing numbers of hams have installed elevated radials under
80-Â and 160-Âm verticals. Unlike VHF ground-Âplane antennas or even the 14-ÂAVQ, running
controlled comparative experiments on 80 m and 160 m is a major undertaking.
What has gradually come out of the anecdotes, experiments, and modeling, however, is
that a handful of elevated radials (between four and eight) can be as effective as 30 or
more radials on the ground—but only if the vertical and the elevated radials are well
above the earth. Ideally, elevated radials should be at least l/8 above earth ground, but
good results have been reported with lower installed heights. Few amateurs have the
resources to put up 160-Âm verticals whose base is 30 to 70 ft above the earth, so elevated
radials are frequently installed as shown in Fig. 30.7.
Compared to earth ground, which is lossy (dissipative), copper radials are essentially
lossless. When elevated radials are too close to earth, radial return currents for the
vertical are shared between the radials and earth, thus allowing some substantial fraction
of the vertical’s return currents to be dissipated in the lossy earth. When the vertical
and the elevated radials are far enough away from the earth’s surface, virtually all