I ran across this, which was originally posted on the Broadcasting list and may
be of interest to Topbanders. Something I might add as a justification for
exceeding, if possible, the point of diminishing returns in a radial field (not
necessarily going all the way to 120) is that in many installations, some of
radials will eventually get damaged or the wire may deteriorate in certain
soils, and the redundancy may assure the ground system a longer lifetime.
Better to have it and not need it, than to need it and not have it.
Don k4kyv
> On Wed Jan 18 17:48:53 CST 2017 Mike Vanhooser novaelec at sbcglobal.net
> wrote:
> As someone who has a great deal of experience with elevated counterpoise, I
> can assure you that 4 will work just fine. I typically use 6, which is what
> the Commission prefers to see, but the performance between the two is almost
> unnoticeable. Clarence Beverage has even more experience than I do, and he
> is the one who walked it through the approval process. You can get his NAB
> paper on the subject in the documents section of his website,
> www.commtechrf.com, filename NAB1995.
The reason for 120 radials is because that is what they put in during the
original development. A prime case of "That's how we always did it" making
into a regulation. At about 60 radials you begin to hit the point of
diminishing return, and anything over 90 is virtually undetectable. The main
reason for so many is to overcome the earth losses. Once you get about .02
wavelength above ground, you have overcome the majority of that. I can show
you the ND proof from WDTW, which had 6 radials, and it is almost a perfect
circle, except for a rise to the NE where there was reradiation from the tall
streetlight poles on I-94, about 100° away.
In your situation..., were the radials elevated or on the ground? That makes a
big difference. I understand the phenomena with the VHF/UHF verticals, but MW
behaves a little differently. Another consultant friend did an experiment many
years ago to find out the directionality, if any, of a partial radial field.
He was installing a new ground system, so he plowed in 60 radials in a 180°
arc. Here was an AM tower with half the radial field missing, so what would it
do? He ran a ND proof on the antenna, and was amazed at what he saw. The
pattern was completely round, with no discernible attenuation over the missing
radials. He plowed in the remaining 60 and saw no change, other than a slight
increase in radiation efficiency by going from 60 to 120 radials. So an
adequate counterpoise at MW frequencies produces a true omnidirectional
radiation pattern, whether in the ground or in the air. It just takes a whole
lot less copper
> when you go airborne.
http://lists.radiolists.net/pipermail/broadcast/2017-January/172560.html
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