Codan Radio Radio Repeater Systems Training Guide - Manual (Page 41)

RADIO REPEATER SYSTEM | TRAINING GUIDE
Chapter 4: Installation and Maintenance Page 33
LIGHTNING PROTECTION
There is no guaranteed method of eliminating damage due to lightning, but there are ways to minimize
the damage received from the result of a direct hit and to practically eliminate damage from all but the
most severe indirect strikes. The accepted method to suppress damage from lightning strikes is to
provide an easy path or multiple paths for these strikes in their travel to ground. The less the resistance
of the path to ground, the sooner the charge can be “bled off”, preferably before the charge can build to
the point where damage is caused.
The behavior of a current surge is infl uenced by other variables (normally undesirable ones such as
inductance, resistance and capacitance) which can, in this case, assist in reducing destruction. Bends
in coaxial cables, twisted together power cabling, and feed-through lightning arrestors are examples.
There are at least two schools of thought concerning grounding of equipment. One is that everything
should be attached to everything else and then grounded through a large diameter copper wire or
bar. Another is to provide a “cone of protection” by installing a rod or wire at the highest point of your
installation with #4 or larger copper wire running directly to ground. By minimizing resistance between
a structure and ground, we can minimize damage to equipment within a hypothetical logarithmic cone,
where the top of the cone is the highest point which is directly connected to the ground and the radius
of the base of the cone is the same as the distance from the top of the cone to ground. Inside antennas
and repeater equipment are left to fl oat within the resulting “cone of protection”. Outside antennas
would be connected to the main ground as it drops down the tower or pole. Any wire connections,
copper to copper, should be made by welding or double clamping. Non-similar metals will have to be
clamped and checked every few years for cleanliness of the joint and for tightness. Non-corrosive,
conducting compounds are available for this purpose.
When using a fi berglass shelter, the steel spider should be connected to the main ground. Ensure that
the coaxial cables are as far as possible from the ground wire. Soil resistivity increases with decreases
in temperature, therefore, ensure that your ground rod(s) is long enough to penetrate below the frost
level. Radials, equal to the height of the structure and connected to the main ground, will increase your
protection as will a surrounding ring of buried copper wire connected to these radials using ground rods
at each connecting point. Such a circle and radials provide protection from “ground bumps” (charges
conducted through the ground from nearby lightning hits). When using ground rods drilled into rock, use
a conductive grout. Use two ground rods at a tower or pole base, 180 degrees apart. Do not install the
ground rods within a concrete tower base - they have been known to blow it apart.