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 687 Figure 30.2 Single-Âpoint ground (SPG) system brings grounds for all utilities, antenna feedlines, and control lines together at a common entry point for the entire building. surge voltages on two different cables or within two different systems. As one specific example, the author lost a fax machine, a cordless telephone base station, and a number of PCs—each with an internal fax modem card connected to the telephone line—during thunderstorms on two separate occasions back when his telephone line and ac mains entered—and were grounded—at opposite ends of his home. The author’s transceivers, with no connection to the telephone system but with their inputs connected to coaxial cables in the SPG enclosure, sustained no damage during those same storms. All radio transmission lines, all rotator control cables, all remote switch control lines must go through the SPG. At the very least, the GND lead for a set of control wires should be grounded at the SPG. The same is true for the shield braid of each coaxial cable entering or exiting the building. Open-Âwire transmission lines should, at a minimum, have spark gaps from both sides of the line to the SPG ground and ideally would have remotely operated contactors that short out both sides of the transmission line to SPG ground when that line is not in use. “Hot” leads—the center conductor of coaxial cable, the control lines to remote antenna selectors, rotators, etc.—can be handled either of two ways: • Use the normally closed contact of relays or contactors to directly ground them all when they’re not in use. • Route each and every hot lead through a lightning protection device.
688 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 If you opt for the second approach, you can purchase units manufactured by ICE, Alpha Delta, and others (see Fig. 30.3). Some of these designs employ metal-Âoxide varistors (MOVs) between each control line and ground. Others use gas discharge tubes for protecting any equipment connected to the hot center conductor of coaxial cables. One complaint by some amateurs is that these devices tend to fail “open”, making it impossible to know whether they’re still operational or not. Further, MOVs have been shown to fail gradually, becoming less and less useful with each nearby lightning surge. The least expensive devices are shunt protectors; that is, they operate by shunting the lightning-Âinduced surge to ground. Another category, generally costing more but preferred by some experts, employs series protection techniques. Coaxial signal cables and rotator or relay control cables that have been routed down the legs of metal towers should have their shields and GND leads connected directly to the tower at both the top and bottom. Of course, the bottom connection should be directly tied to a lightning ground radial field emanating from the base of the tower in all compass directions. As mentioned earlier, these connections should be mechanical or brazed, since solder will not survive a lightning surge. Cables dropping from masts and other antenna support structures, including trees, should have their shields and GND leads connected to an earth ground at both the top and the bottom of the vertical span. For maximum protection from lightning surges, the key directive is “Ground everything well and often!” Note Grounding cables at the tower is not a substitute for a good single-Âpoint ground at the point where all the exterior cabling enters the building. At RF, distance is not your friend! A good lightning ground at the tower, while highly desirable, has nothing to do with protecting the electronic equipment inside the building from lightning-Âinduced surges on power lines, telephone lines, or even the cabling between the tower and the building. Figure 30.3 SPG enclosure contains surge suppressors on all control lines and antenna feedlines.
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688 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 />
If you opt for the second approach, you can purchase units manufactured by ICE, Alpha<br />
Delta, and others (see Fig. 30.3). Some of these designs employ metal-Âoxide varistors<br />
(MOVs) between each control line and ground. Others use gas discharge tubes for protecting<br />
any equipment connected to the hot center conductor of coaxial cables. One<br />
complaint by some amateurs is that these devices tend to fail “open”, making it impossible<br />
to know whether they’re still operational or not. Further, MOVs have been shown<br />
to fail gradually, becoming less and less useful with each nearby lightning surge. The<br />
least expensive devices are shunt protectors; that is, they operate by shunting the<br />
lightning-Âinduced surge to ground. Another category, generally costing more but preferred<br />
by some experts, employs series protection techniques.<br />
Coaxial signal cables and rotator or<br />
relay control cables that have been routed<br />
down the legs of metal towers should have<br />
their shields and GND leads connected directly<br />
to the tower at both the top and bottom.<br />
Of course, the bottom connection<br />
should be directly tied to a lightning ground<br />
radial field emanating from the base of the<br />
tower in all compass directions. As mentioned<br />
earlier, these connections should be<br />
mechanical or brazed, since solder will not<br />
survive a lightning surge. Cables dropping<br />
from masts and other antenna support<br />
structures, including trees, should have<br />
their shields and GND leads connected to<br />
an earth ground at both the top and the bottom<br />
of the vertical span. For maximum protection<br />
from lightning surges, the key<br />
directive is “Ground everything well and<br />
often!”<br />
Note Grounding cables at the tower is not a<br />
substitute for a good single-Âpoint ground at the<br />
point where all the exterior cabling enters the<br />
building. At RF, distance is not your friend! A<br />
good lightning ground at the tower, while highly<br />
desirable, has nothing to do with protecting the<br />
electronic equipment inside the building from<br />
lightning-Âinduced surges on power lines,<br />
telephone lines, or even the cabling between the<br />
tower and the building.<br />
Figure 30.3 SPG enclosure contains surge<br />
suppressors on all control lines and antenna<br />
feedlines.