For most of the past century the intractability of the equations was the
excuse for just laying down "textbook" overkill radial systems. If you
can't solve the "real world problem", then just change the real world to
match the problem you can solve!!! (Google "spherical cow").
The FCC says:
"(4) At the present development of the art, it is considered that where a
vertical radiator is employed with its base on the ground, the ground system
should consist of buried radial wires at least one-fourth wave length long.
There should be as many of these radials evenly spaced as practicable and in
no event less than 90. (120 radials of 0.35 to 0.4 of a wave length in
length and spaced 3° is considered an excellent ground system and in case of
high base voltage, a base screen of suitable dimensions should be
employed.)"
The FCC exempts stations from proving efficiency through tedious (and
expensive) full field-strength measurements when the station has at least
*90* 1/4 wave radials.
I assume this minimum of 90 radials is what they think is a safe amount over
the 50-60 radials that have generally shown to be up near 100% efficiency.
Most of the FS measurements I have seen wobble around a few dB, so the
engineer has to use a mean value of what he measures. This gives some
latitude to "fudge things" a few dB by how measurements are made or
interpreted.
None of this means much for Ham radio. We would be very lucky to notice six
or eight dB (or more) shortfall without an A-B comparison. We tend to
believe "the signal was strong so it must be 100%", when we are actually
unlikely to know or notice a six dB change without A-B comparisons.
73 Tom
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