The optimum pattern of radials for lightning dissipation from a tower or
vertical antenna is a pattern of 6, equally spaced around the center.
More won't hurt, but don't help a whole lot for dissipating lightning,
because 6 will utilize the available capacity of the soil. This assumes
that each radial terminates in a ground rod to a depth at least twice
the length of the radial. Each of these radials may be extended with
one or two more, also terminated with a ground rod as described.
Extending each to two more radials creates a pattern in which all
grounds are spaced optimally, that is, to a depth equal to half the
distance between them. This pattern may be extended, but the
effectiveness of the additional radials is less than that of the first
two rings because of the inductance of the greater length. That
effectiveness doesn't become zero, it is simply less.
Think of the earth as a lossy capacitor. The earth surrounding each
ground rod will quickly charge to high voltage as it absorbs the current
of a lightning stroke. Following that, it will discharge through its
conductance to the surrounding earth. That is the reason for spacing
the ground rods apart. It permits each ground rod to be modeled as
another capacitor, in parallel with its companions. The total capacity
of the group is the capacity of one times the number of rods, if they
are spaced this way. The impedance of the pattern is the impedance of
one (capacitor and associated resistance and inductance of the radial)
divided by the number of rods. Two or more ground rods close together
are very slightly more effective than one, for lightning dissipation.
The current will not necessarily be equal in all these radials. It will
divide proportionally to the admittance (reciprocal of impedance) of the
radials, which will vary because of varying soil characteristics,
spacing, etc.
The current capacity of these radials is much greater than the steady
state capacity of the same wires encased in insulation due to the short
time duration of a lightning strike. The peak current of lightning
strikes is, according to Uman, 150,000 A. Half exhibit a peak of 30,000
A. The peak is usually on the first stroke. Subsequent strokes are
typically less than half the peak current of the first stroke.
Six number 12 copper wires will carry the current, but number 6 is
recommended for mechanical endurance. The lightning dissipating radials
should be bare and buried, to take advantage of contact with the soil.
That advantage is, in typical installations, small, but it's free.
73 de WOØW
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