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 693
The conductivity of soil determines how well or how poorly the earth conducts
electrical current (Table 30.1). Moist soil over a brackish water dome conducts best
(coastal southern swamps make better AM broadcast radio station locations), and the
sands of the western deserts make the poorest conductors.
Previous editions of this book described techniques for reducing the electrical resistance
of soil through treatment with chemicals. Currently, the author does not recommend
the approach. In addition to any possible environmental concerns, any benefit
from salting will only be temporary and the cost and time required to repetitively salt a
useful area surrounding the base of the antenna prohibitive. Salting only the area near
ground rods is futile for improving antenna efficiency because the ground currents relevant
to antenna efficiency travel laterally, near the surface of the ground—from roughly
as far away from the base of the vertical as the vertical is tall—back to the antenna feedpoint.
In contrast, focusing on deep ground rod conductivity is arguably of interest
when attempting to maximize the dissipation of a lightning surge into the ground.
As discussed in Chap. 5, the soil directly underneath a horizontally polarized antenna
is important primarily for establishing the effective height of the antenna (and,
hence, its feedpoint impedance at its erected height above the ground surface). The soil
within, say, a radius of l/4 to l/2 is important primarily for carrying radial return currents
back to vertical monopoles. But with the possible exception of saltwater, any surface
normally found under a vertical monopole is not a good enough conductor to
seriously consider it as a substitute for copper radials.
Table 30.1 lists the electrical characteristics for a range of commonly encountered
soils, along with those of saltwater and freshwater. Note that the highest-Âconductivity
soil (“pastoral hills”) in the list is still approximately 200 times lossier than saltwater—
and saltwater is nowhere near as good as copper! Bottom line: For superior vertical
monopole efficiency, lay at least two dozen radials out under your verticals and quit
worrying about your specific soil type!
Conductivity
Type of Soil
Dielectric
Constant
(siemens /
meter)
Relative Quality
Saltwater 81 5 Best
Freshwater 80 0.001 Poor
Pastoral hills 14–20 0.03–0.01 Very good
Marshy, wooded 12 0.0075 Average/poor
Sandy 10 0.002 Poor
Cities 3–5 0.001 Very poor
Table 30.1 Sample Soil Conductivity Values
Much farther from the base of your antenna, soil conductivity is important because
it affects the first ground reflection of your transmitted signal and the last ground reflection
of received signals. But for long-Âdistance (DX) communication, optimum takeoff
and arrival angles are usually in the 1-Â to 5-Âdegree range, so we’re talking about
bouncing the RF off soil at distances up to a mile away! Odds are high you don’t even