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TECH NOTES—GROUND RADIAL SYSTEMS
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 resistance is the textbook figure of 35 ohms. The
feedpoint impedance would then be 15+0+35 = 50 ohms, and the antenna would be
perfectly matched to a 50 ohm coaxial line. Since the radiation resistance is an index of the
amount of applied power that is consumed as useful radiation rather than simply lost as heat in
the earth or in the conductor, the radiation resistance must be kept as high as possible in
relation to the total feedpoint impedance for maximum efficiency. Efficiency, expressed as a
percentage, may be found by dividing the radiation resistance by the total feedpoint impedance
of a resonant antenna, so under the conditions assumed above our vertical antenna would
show an efficiency of 35/50 = 70%. As a vertical antenna is made progressively shorter than
one-quarter wavelength the radiation resistance drops rapidly and conductor losses from the
required loading inductors increase. A one-eighth wave inductively loaded vertical would have
a radiation resistance of something like 15 ohms and coil losses (or trap losses for multiband
antennas) would be in the range of 5 ohms. Assuming the same value of ground loss
resistance (15 ohms), the feedpoint impedance would become 15 + 5 + 15 = 35 ohms and
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