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
694 P a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s own the land, much less have the fat wallet needed to salt acres and acres of ground. And for very long DX paths, there are additional ground reflections to consider. Ultimately, it’s probably cheaper and a lot less backbreaking labor over a lifetime to buy oceanfront property. Grounding with Radials The effectiveness of an RF ground system is enhanced substantially by the use of radials either above ground or buried just below the surface. In Chap. 10 we saw that a vertical monopole antenna is relatively ineffective unless provided with a good RF ground system, and for most installations that requirement is best met through a system of ground radials. An effective system of radials requires a large number of radials. But how many is enough? Experimental results obtained in 1928 subsequently resulted in new regulations in the United States requiring broadcast stations employing vertical monopoles in the AM band (540 to 1600 kHz at the time) to use a minimum of 120 half-Âwavelength radials, but 120 was deliberately chosen to be at least twice the number at which the experimenters had found that practical improvement ceased to be meaningful. Recent modeling work has confirmed that nowhere near that number is necessary for nonbroadcast services. Installing more than 30 or 40 l/4 radials is not only expensive and time-Âconsuming, but totally unnecessary, as well. The author has had a superior signal on 160 m for years using anywhere from 12 to 25 radials of various lengths. Unlike elevated radials, which should be as long as the electrical height of the vertical, radials on the ground can be just about any length that is convenient, up to perhaps a half-Âwavelength. A radial in close proximity to earth has lost any pretense of being resonant at a specific length; better to think of it instead as one terminal of a long, skinny capacitor. From modeling and experiments a rough rule of thumb has evolved: Aim for the tips of adjacent radials on the surface of the ground to be about 0.05l apart. At smaller spacings, no significant reduction in ground losses is noted, and when spacings start to exceed that figure, losses start to grow as an increasing percentage of the vertical’s return currents find their way into the earth instead of the copper wires. A little thought will lead you to the conclusion that for fixed tip-Âto-Âtip spacing specified as a fraction of the wavelength in question, the shorter your radials are, the fewer you need!* This seems paradoxical at first, but what’s missing is the fact that your antenna’s ground return efficiency is not constant during this comparison; truly, shorter radials are not as good as longer ones, no matter how many of them you install. Stated another way, it is a waste of time and good copper to install “additional” short radials in an attempt to compensate for their shortness. The ideal layout for a system of radials in a vertical antenna system is depicted in a view from above in Fig. 30.5. Here, the radials are laid out in a uniform pattern around the antenna element. This coverage provides both the lowest resistance and the best radiation pattern for the antenna. Tie all radials together at a common point at the base of the vertical, and connect that junction of wires to one side of the feedline—usually the shield braid when coaxial cable is used—and to the ground terminal on any remote *To see this, draw a circle with radius R (the length of a radial). The circumference of the circle is 2pR, or approximately 6.3R. If R = l/4, then C = 1.6l. Divide the circumference into little segments, each 0.05l long. Then N, the number of radials, is 1.6l ÷ 0.05l = 32. Now shorten the radials to l/8 and redo the calculation. The maximum useful number of radials is now 16.
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694 P a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s<br />
own the land, much less have the fat wallet needed to salt acres and acres of ground.<br />
And for very long DX paths, there are additional ground reflections to consider. Ultimately,<br />
it’s probably cheaper and a lot less backbreaking labor over a lifetime to buy<br />
oceanfront property.<br />
Grounding with Radials<br />
The effectiveness of an RF ground system is enhanced substantially by the use of radials<br />
either above ground or buried just below the surface. In Chap. 10 we saw that a vertical<br />
monopole antenna is relatively ineffective unless provided with a good RF ground system,<br />
and for most installations that requirement is best met through a system of ground<br />
radials.<br />
An effective system of radials requires a large number of radials. But how many is<br />
enough? Experimental results obtained in 1928 subsequently resulted in new regulations<br />
in the United States requiring broadcast stations employing vertical monopoles in<br />
the AM band (540 to 1600 kHz at the time) to use a minimum of 120 half-Âwavelength<br />
radials, but 120 was deliberately chosen to be at least twice the number at which the<br />
experimenters had found that practical improvement ceased to be meaningful. Recent<br />
modeling work has confirmed that nowhere near that number is necessary for nonbroadcast<br />
services. Installing more than 30 or 40 l/4 radials is not only expensive and<br />
time-Âconsuming, but totally unnecessary, as well. The author has had a superior signal<br />
on 160 m for years using anywhere from 12 to 25 radials of various lengths.<br />
Unlike elevated radials, which should be as long as the electrical height of the vertical,<br />
radials on the ground can be just about any length that is convenient, up to perhaps<br />
a half-Âwavelength. A radial in close proximity to earth has lost any pretense of being<br />
resonant at a specific length; better to think of it instead as one terminal of a long,<br />
skinny capacitor. From modeling and experiments a rough rule of thumb has evolved:<br />
Aim for the tips of adjacent radials on the surface of the ground to be about 0.05l apart.<br />
At smaller spacings, no significant reduction in ground losses is noted, and when spacings<br />
start to exceed that figure, losses start to grow as an increasing percentage of the<br />
vertical’s return currents find their way into the earth instead of the copper wires.<br />
A little thought will lead you to the conclusion that for fixed tip-Âto-Âtip spacing specified<br />
as a fraction of the wavelength in question, the shorter your radials are, the fewer<br />
you need!* This seems paradoxical at first, but what’s missing is the fact that your antenna’s<br />
ground return efficiency is not constant during this comparison; truly, shorter<br />
radials are not as good as longer ones, no matter how many of them you install. Stated<br />
another way, it is a waste of time and good copper to install “additional” short radials<br />
in an attempt to compensate for their shortness.<br />
The ideal layout for a system of radials in a vertical antenna system is depicted in a<br />
view from above in Fig. 30.5. Here, the radials are laid out in a uniform pattern around<br />
the antenna element. This coverage provides both the lowest resistance and the best<br />
radiation pattern for the antenna. Tie all radials together at a common point at the base<br />
of the vertical, and connect that junction of wires to one side of the feedline—usually<br />
the shield braid when coaxial cable is used—and to the ground terminal on any remote<br />
*To see this, draw a circle with radius R (the length of a radial). The circumference of the circle is 2pR, or<br />
approximately 6.3R. If R = l/4, then C = 1.6l. Divide the circumference into little segments, each 0.05l<br />
long. Then N, the number of radials, is 1.6l ÷ 0.05l = 32. Now shorten the radials to l/8 and redo the<br />
calculation. The maximum useful number of radials is now 16.