Practical_Antenna_Handbook_0071639586

684 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
Under the right (or “wrong”, depending on your point of view) conditions, the
earth and its atmosphere in localized regions of the globe can form a power supply of
virtually infinite energy. Electrical potential differences of millions of volts build up
between objects on the earth and particles in the earth’s atmosphere directly above.
(Think in terms of an extremely large parallel plate capacitor being charged through a
series resistance by an infinite supply voltage, so that there is a time interval associated
with how quickly a thunderstorm can “recharge” a given area of a cloud.) At some
point, this voltage becomes large enough to jump across air by ionizing or breaking
down the air molecules. (A typical breakdown figure is something less than 100,000 V/
in of dry air, but the exact voltage depends on the shapes of the two electrodes, and
there is much about high-Âvoltage phenomena that is nonlinear.) When there is moisture
in the air, the voltage required to ionize the path to earth drops substantially. Any wonder,
then, that it is virtually impossible to predict where lightning will strike? Have you
ever seen or created a map of humidity versus position in your backyard, for instance?
The jagged shape of a lightning bolt is simply the result of the stored energy in our atmospheric
system taking the “path of least resistance” . . . literally.
And what could provide less resistance than a wire or a metal tower standing tall
in someone’s backyard? The very attributes of antennas and their supports that we
value are the same characteristics that make them more likely to be targets of frequent
lightning strikes. Similarly, tall trees are frequent targets because they exhibit a finite
resistance and are capable of electrical conduction, especially when the sap is running.
The old adage is a good one: “In a thunderstorm, seek shelter but don’t stand under
tall trees.” To that we can add, “Don’t stand under radio towers and utility poles, either.”
The second characteristic of lightning that we have to deal with is that a typical
stroke passes peak currents of thousands of amperes (20,000 A is often cited). The heat
generated by such a huge current flowing through even a minuscule resistance is sufficient
to blow apart concrete tower bases and vaporize not just solder connections but
entire antennas! Thus, in addition to immediate death or permanent disability from the
electromagnetic effects of being hit by lightning, there is a comparable risk from proximity
to exploding objects, extreme radiated heat, splashes of boiling water (from the ground)
or tree sap, and flash fires (from overheated wires in the walls of wood frame dwellings).
There is no known way to totally prevent a lightning strike from coming to your
neighborhood. Thus, the first purpose of a ground system for lightning protection is to
minimize the damage caused to people and property from a lightning strike that is almost
certain to find you someday. To that end, the current professional consensus is to
attempt to harmlessly dissipate in the loss resistance of the nearby earth as much of the
energy in the lightning bolt as possible. The level to which you, the station owner, can
do that is determined by the unique characteristics of your site, your tower and cabling
configuration, and your wallet.
Bear in mind that lightning does not know the difference between a tower erected
for radio communications and a length of house wiring in the attic of a three-Âstory
home. The tower may well be somewhat higher than the attic wiring, but both are ultimately
grounded and both are “juicy” targets for a lightning bolt looking for a path to
earth ground. If conditions are right (such as the air immediately above the house being
of higher humidity than the air near the top of the tower), the house may well be more
likely to be hit than the tower.