Jerry Sevick, W2FMI, has an interesting comment about the "120" number in
his book, "The Short Vertical Antenna and Ground Radial." At the end of
the first chapter he notes:
"...it should be mentioned that the world standard for the number of
radials to be used with verticals in the AM broadcast band is 120. This
number was based on the classic paper published in 1937 by Brown, Lewis,
and Epstein. During the course of a business meeting with Dr. Brown, I
asked him how he and his colleagues arrived at the 120 radial
figure--because I was quite sure 100 would work as well. His answer was
interesting.
"He said that he and the others had been thinking in terms of 100 radials,
but the farmer who plowed in 100 radials had wire left over because copper
is soft and stretches easily. When he asked what to do with the extra
wire, the farmer was told to plow it in. The result was a world standard
of 120 radials."
That's an interesting story, but the story-teller must never have looked at
the papers.
BL and E used 113 radials maxium, not 120.
Brown, Lewis, and Epstein's papers are all over the web, if you search for
them.
The FCC says:
"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.)"
So you see, the FCC requires 90 radials unless you prove you can make
efficiency with fewer. They do not say 120 quarter wave radials, they
require 90 1/4 wave or longer, and say 120 radials .35 to .4 wl is
"considered excellent".
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