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Above Ground (Elevated) Installations
The problem of ground loss resistance may be avoided to some extent by mounting a vertical
antenna some distance above the earth over an artificial ground plane consisting of resonant
(usually 1/4-wave) radial wires. Four resonant radials are considered to provide a very low-loss
ground plane system for vertical antennas at base heights of 1/2-wave or more. This arrangement
contrasts favorably with the more than 100 radials for zero ohms loss resistance at ground level, and
since 1/2-wave is only about thirty-five feet at 20 meters, very worthwhile improvement in vertical
antenna performance can be realized, at least on the higher bands, with moderate pole or tower
heights. At base heights below 1/2-wave more than four radials will be required to provide a ground
plane of significantly greater conductivity than the lossy earth immediately below the antenna: even
so, a slightly elevated vertical with relatively few radials may be more effective than a ground-level
vertical operating over a larger number of radials if only because the former is apt to be more in the
clear. Resonant radial lengths for any band may be calculated from the formula:
Elevated antennas may interact with the coaxial cable and cause RF feedback into the shack. It is
recommended that a Feedline Current Choke be installed at the base of the antenna near the
feedpoint connection.
Figure 1
shows the basic ground plane system for elevated verticals. Radials may
slope downward as much as 45 degrees without any significant effect on operation or
performance. Radials for different bands should be separated as much as possible and
the far end of each radial insulated from supporting wires.
Figure 2
shows a ground
plane system that uses four resonant radials for 40 meters, another set of four for 20
meters, and a third set for 10 meters. A separate set for 15 meters is not ordinarily
required because the 40 meter radials operate as resonant 3/4-wave radials on that band.
Figure 1
At the lower heights the separate wires of this system may provide enough
capacitance to ground to permit low SWR operation on 80/75 meters as well,
but it is probable that at least one resonant radial will be required for low SWR
on that band. It’s important to note that cutting each conductor to a specific
frequency will not work unless you separate it, angling each conductor away for
most of its length because the longer ones will detune the shorter ones.
Figure 2
Above Ground Radial Systems
The 12-radial system of
Figure 2
is a very good one, but it requires at least 12 tie-off points.
Butternut
®
has developed a multiband radial made of 300 ohm twinlead that resonates
simultaneously on 40, 20, 15 and 10 meters. Four such radials offer essentially the same ground
plane performance as the system of
Figure 2
but require only 4 supports. These multiband radials
plus additional wire for an 80 meter radial are available separately (
BUT-STR-II
) or as part of the
Butternut
®
roof mounting kit (
BUT-RMK-II
).
There are times when physical restrictions will dictate the use of
fewer than four radials, and at least one manufacturer recommends 2
radials per band, the radials for each band running 180 degrees away
from each other. A simpler (and no doubt less effective) system is
shown in
Figure 3
. Since only one resonant radial is used per band
the antenna will radiate both vertically and horizontally polarized
Figure 3
