Practical_Antenna_Handbook_0071639586

686 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
seem, when it comes to lightning surges we want lossy soil but when it comes to radio
signals we want lossless soil!
When soil conditions do not allow 8-Âft ground rods to be fully pounded in, shorter
rods can be used, with some lessening of performance unless the number of radials and
rods is increased. As a last resort in very rocky terrain, ground rods can be laid horizontally
and buried just under the sod.
Spectral analyses of lightning surges show the bulk of the RF energy to be in the 1-Â
to 3-ÂMHz range. Thus, a key requirement of a lightning ground is that it exhibit a very
low impedance to lightning—not just at dc and power line frequencies but up through
3 MHz or more. As a practical construction matter, the radial wires and straps being
used to dissipate the energy should have no sharp bends or other sources of excessive
inductance. In Chap. 3 we noted that a current flowing in a length of wire creates a
magnetic field around the wire and that a large enough current will result in a field so
strong as to be able to actually move that wire or neighboring wires. So it should not be
surprising that large surge currents in conductors from nearby lightning strokes can
create mechanical stresses at sharp bends.
The other important thing to remember is Ohm’s law: V = IR. It does not take very
much resistance in a wire or a junction to create extremely high voltage drops (and
power dissipation) in the presence of a lightning surge. The heat generated when lightning
surges through ground wires and their connections is well above the amount
needed to melt any solder you might have available. Consequently, lightning radials
should never be soldered; the connections throughout a lightning ground system
should be brazed or mechanically clamped.
Naturally, our primary concern in grounding for lightning is protection of living beings,
followed by protection of our property and possessions. Thus, one reason we attempt
to dump the lightning surge current into a properly constructed ground system is
to minimize damage to electronics equipment inside the home or business. Many purists
feel the only way to be 100 percent sure there is no damage is to totally disconnect all
electronic equipment prior to the arrival of a storm or, better yet, immediately following
each period of use. This means disconnecting everything—power cords, headphones,
telephone lines, all audio and control cables, grounding wires, etc. Unfortunately, there
have been reports of electronics equipment being damaged during a storm despite being
totally disconnected from external wiring, so even this approach is not foolproof.
Today the state of the art in electronic equipment protection for home and business
relies on the concept of a single-Âpoint ground (SPG). In a true SPG arrangement, all signal
and control cables coming into or leaving the radio room (or entire building, for that
matter) are passed through a metal panel or enclosure just outside the point of entry
(Fig. 30.2). The panel should be ground-Âmounted or physically as close to earth as can
reasonably be attained in a given installation, and solidly attached to its own lightning
radial system. It is imperative that the telephone company ground and power utility
ground be close to the SPG panel and to each other (say, within 1 or 2 ft) and electrically
bonded together and to the SPG ground. Any other utilities or quasi-Âutilities, such as
cable or satellite TV system cabling, that enter the building should also pick up their
earth ground at the same point. If the building has a protective lightning rod system, it
should also be connected here.
The fundamental premise of a single-Âpoint ground is that a “rising tide lifts all
ships”. In most instances of surges from nearby lightning strikes causing damage to
electronic equipment, the culprit has been identified as the difference in the induced