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TECH NOTES—GROUND RADIAL SYSTEMS
In almost every case the efficiency of a vertical antenna will be
greater if radial wires are used to improve ground conductivity as
in figure 2. It’s important to note that there’s no point in cutting
radials to any particular length when ground mounting because
the earth will detune them anyway. All you want to do is make
the surface of the earth around the antenna more conductive than
it is ordinarily.
If you can’t copper-plate the backyard, the best approach is to
run out as many radials as possible, each as long as possible
around the antenna in all directions. Radials may be left on top of the ground
however they should be buried for the sake of pedestrians and lawnmowers.
How long should radials be? A good rule is no shorter than the antenna is tall
because 50% of your losses will occur in the first 1/4
8
out from the antenna. If
you have more than a dozen radials, they must be longer to get the most out of
them which is why the FCC specifies 113 wires each .4
8
for AM broadcast
stations—the equivalent of a zero-loss ground plane. Obviously, for most ham
work this would be overkill.
In some cases wire mesh (i.e. chicken wire) may be used as a substitute for radial
wires and/or a ground connection, the mesh or screen acting as one plate of a
capacitor to provide coupling to the earth beneath the antenna.
It should be noted that a ground rod is useful only as a d.c. ground or as a tie point
for radials. It does little or nothing to reduce ground losses at r.f. regardless of
how far it goes into the ground.
Bare wire, insulated, any gauge, it doesn’t matter. The current coming back along
any one wire won’t amount to that much.
EFFICIENCY
The importance of reducing losses in the ground system can be seen from an
examination of a vertical antenna's feedpoint impedance which at resonance
consists of three components: antenna radiation resistance; conductor loss
resistance; and earth loss resistance. An unloaded quarter-wave vertical antenna
has a radiation resistance of about 35 ohms with negligible ohmic or conductor
loss, but ground loss resistance may be very great if no measures are taken to
reduce it, and in some cases ground loss R may even exceed the antenna radiation
resistance. These three components may be added together to arrive at the
feedpoint
impedance
of a
resonant
(no reactance) antenna. For the sake of
illustration, assume that the ground loss beneath a quarter wavelength vertical
antenna is 15 ohms, that conductor loss resistance is zero, and that the radiation
Summary of Contents for HF9V
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