> That 120 radial thing is from Brown, Lewis and Epstein, and was
> originally formulated as a FCC shortcut to avoid having to do a proof of
> performance on a non-directional AM transmitter
Another reason is to maintain radiation efficiency as the ground system
degrades over time due to soil acidity. So, while it appears diminishing
returns sets in around 60 radials, it's a lot more convenient to a broadcast
station to double the amount required during an initial installation and
later years during extensive re-work, rather than wonder if signal report
complaints are due to a failing ground system.
I recently had dinner with my old boss, who is now Director of Engineering
at WLS-AM in Chicago. He indicated that they have a system of 240 radials
consisting of 120 added radials overlaid on an existing set of 120 older
radials. The new and old radial systems only come in contact only at the
base of the tower. Inefficiencies can develop when radials wires come in
contact at a distance. The mechanism for this is detailed in Radio Antenna
Engineering, by Laport.
Paul, W9AC
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