Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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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 691 buss or ground plane through a short length of braid or #12 wire, secured at the buss with hose clamps or sheet metal screws. Small RF accessories (e.g., a low-Âpass filter) that have no user controls on them are mounted directly to the copper sheet. These connections are an important part of all the other grounding systems discussed, and they must therefore be low impedance all the way from dc to many megahertz. Further, the ground plane or ground buss so formed must be connected to the outdoor ground system(s) with as short a run as possible, so that the least possible amounts of both resistance and inductance are introduced into the total ground path. In one installation, the author was able to drop the copper sheet down from the table to connect directly to the ground system outside the building. The run was less than 40 in. Unfortunately, some amateur radio operators and CBers use the building electrical ground wiring for the RF antenna ground of their station. Neglecting to install an outdoor ground that will properly do the job, they opt instead for a single connection to the grounded “third wire” in a nearby electrical outlet. In addition to being potentially dangerous, this is a very poor RF ground. It is too long for even the lower HF bands, it reradiates RF around the house in large quantity, and there’s just not enough of it to serve its intended purpose. Transmitters that depend on the household electrical wiring as the radio ground tend to cause radio and TV broadcast interference, as well as interference to other consumer electronics devices in their own home and in nearby buildings. Tuned Ground Wire If you use a power tool or a household appliance on the second or third floor of your home, or at the far end of a long three-Âconductor extension cord, you still have an effective power safety ground at the point of use, despite the relatively long run from your circuit breaker box in the basement. That’s because the wavelength of 60-ÂHz energy is measured in miles, and the extension cord represents a tiny fraction of a wavelength. However, from a lightning or radio standpoint, second-Â and third-Âstory grounds are not grounds at all! The reason is because at radio frequencies your equipment chassis ground may be a substantial fraction of a wavelength distant from true earth ground. (Remember that typical lightning strikes include large amounts of energy at 1 to 3 MHz and above. At those frequencies, anything longer than a few feet is likely to see a significant induced voltage across its length.) If you have ever been “bitten” or burned by RF energy when you happened to touch the chassis or a metal knob on your radio equipment while transmitting, you have felt firsthand the effect of having an inadequate RF ground in your radio room. Often the user thinks he or she has a ground simply because a wire has been run from the equipment chassis to a ground rod two floors below. That “ground” wire may be 25 or 30 ft in length—long enough to be a quarter-Âwave vertical on 20 m! An alternative that some operators use is the ground wire tuner. (MFJ Electronics makes such a unit.) These accessories insert an inductor or capacitor (or even a full LC network) in series with the ground line. The user adjusts (or “tunes”) the device for maximum ground current while transmitting on the desired operating frequency. Although these tuners can bring your radio equipment common chassis connections to a virtual RF ground for purposes of providing proper antenna tuning, a low-Âimpedance RF path to a single-Âpoint ground located outside the building, at earth level, is still necessary for lightning grounding.

692 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 Grounds for Antenna Efficiency The success or failure of a radio antenna system often hangs on whether it has a good RF ground. Poor grounds cause many antennas to operate at less than best efficiency. In fact, it is possible to waste as much as 50 to 90 percent of your RF power heating the lossy ground under the antenna, instead of sending that RF into the air. This is especially true in the case of ground-Âmounted monopoles, such as a typical quarter-Âwave vertical, because the “missing” half of the antenna is the circle of ground lying within roughly l/2 of the base of the vertical; RF energy delivered to the feedpoint of the vertical monopole and radiated into space must be balanced by return currents delivered to the transmission line via this ground path. Total effective ground resistance of typical soils in the circle beneath a vertical monopole can vary from a low value of, say, 5 W, up to more than 100 W, and RF power is dissipated in the ground resistance according to a simple resistive divider equation. The total resistance, R LOAD seen by the transmission line is RLOAD = RGND + R (30.1) RAD where R GND = effective loss resistance of ground surrounding antenna R RAD = radiation resistance of antenna The total power, P OUT , delivered by the transmission line is P = P + P OUT GND RAD 2 = I R + R LOAD ( ) GND RAD (30.2) The power radiated by the antenna is P RAD = I 2 LOADR RAD ; expressed in terms of the total power delivered by the transmission line, it is P RAD P 2 ILOADRRAD = 2 I R + R RAD LOAD = R ( ) GND RRAD + R GND RAD RAD (30.3) (30.4) If, for instance, the effective ground resistance is 30 W, and the antenna is a full l/4 vertical monopole having a radiation resistance of 30 W, also, then half the applied power, or 3 dB, is wasted in ground losses. Any increase in ground losses (usually as a result of an insufficient radial field) or decrease in radiation resistance (a shorter vertical) can conspire to drop the power radiated even further—perhaps to as low as 5 or 10 percent of the power available from the transmission line! Soil Conductivity Factors that affect ground resistance include the conductivity of the ground, its composition, and its water content. The effective RF ground depth is rarely right on the surface and—depending on local water table level—might be a few meters or so below the surface.

692 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 />

Grounds for <strong>Antenna</strong> Efficiency<br />

The success or failure of a radio antenna system often hangs on whether it has a good<br />

RF ground. Poor grounds cause many antennas to operate at less than best efficiency. In<br />

fact, it is possible to waste as much as 50 to 90 percent of your RF power heating the<br />

lossy ground under the antenna, instead of sending that RF into the air. This is especially<br />

true in the case of ground-Âmounted monopoles, such as a typical quarter-Âwave<br />

vertical, because the “missing” half of the antenna is the circle of ground lying within<br />

roughly l/2 of the base of the vertical; RF energy delivered to the feedpoint of the vertical<br />

monopole and radiated into space must be balanced by return currents delivered to<br />

the transmission line via this ground path. Total effective ground resistance of typical<br />

soils in the circle beneath a vertical monopole can vary from a low value of, say, 5 W, up<br />

to more than 100 W, and RF power is dissipated in the ground resistance according to a<br />

simple resistive divider equation. The total resistance, R LOAD seen by the transmission<br />

line is<br />

RLOAD = RGND + R<br />

(30.1)<br />

RAD<br />

where R GND = effective loss resistance of ground surrounding antenna<br />

R RAD = radiation resistance of antenna<br />

The total power, P OUT , delivered by the transmission line is<br />

P = P + P<br />

OUT GND RAD<br />

2<br />

= I R + R<br />

LOAD<br />

( )<br />

GND<br />

RAD<br />

(30.2)<br />

The power radiated by the antenna is P RAD = I 2 LOADR RAD ; expressed in terms of the total<br />

power delivered by the transmission line, it is<br />

P<br />

RAD<br />

P<br />

2<br />

ILOADRRAD<br />

=<br />

2<br />

I R + R<br />

RAD<br />

LOAD<br />

=<br />

R<br />

( )<br />

GND<br />

RRAD<br />

+ R<br />

GND<br />

RAD<br />

RAD<br />

(30.3)<br />

(30.4)<br />

If, for instance, the effective ground resistance is 30 W, and the antenna is a full l/4<br />

vertical monopole having a radiation resistance of 30 W, also, then half the applied<br />

power, or 3 dB, is wasted in ground losses. Any increase in ground losses (usually as a<br />

result of an insufficient radial field) or decrease in radiation resistance (a shorter vertical)<br />

can conspire to drop the power radiated even further—perhaps to as low as 5 or 10<br />

percent of the power available from the transmission line!<br />

Soil Conductivity<br />

Factors that affect ground resistance include the conductivity of the ground, its composition,<br />

and its water content. The effective RF ground depth is rarely right on the surface<br />

and—depending on local water table level—might be a few meters or so below the<br />

surface.

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