Bruce-K1FZ wrote:
AM broadcast band antennas 5/8 & 1/2 wave tall are rarely used any more.
I noticed a reduction of the tall AM towers starting about the 1960's.
Guy Olinger replied:
Wouldn't that have corresponded to the FCC's reduction in "clear-channel"
strategies, and more local market driven commercial agendas?
Class A (50 kW, non-directional, 24/7) AM broadcast stations use monopoles
not much less than 180 degrees in electrical height. The most common height
is 195 degrees, as that produces a groundwave field that isn't much less
than from a 225-degree monopole, but the 195-degree height does not have the
high-angle lobe of the 225-deg height that can cause nighttime
fading/interference to its own groundwave at the fringe its groundwave
signal.
Class A stations cannot use a monopole much shorter than 180 degrees,
because they do not produce at least the minimum groundwave field required
by the FCC for Class A stations.
And the whole point of the 1937 Brown, Lewis and Epstein study was that
you DIDN"T need tall towers for daytime coverage.
The point of the BL&E study was to investigate the fields radiated by
monopoles of various heights when using buried radial systems with wires of
various length and number. They limited the heights of the monopoles they
measured to about 98 degrees, as that height was sufficient to illustrate
the effects of the various radial ground systems they used. In the best
cases, the fields they measured were within several percent of the maximum
field possible from a perfect monopole of that height when driven against a
perfect ground plane.
If BL&E had used radiator heights to 5/8 WL then the fields they measured
would have a new, higher reference value for a perfect monopole system of
those heights. But again, in the best cases the measured fields would
approach that new theoretical value within several percent.
Also note that even 1 kW, non-directional, 24/7, Class C AM stations may use
tall towers if they want to improve their groundwave coverage areas. One
such example is WSAM, 1400 kHz in Saginaw, MI, which uses a 180.4-degree
monopole.
R. Fry
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Topband reflector - topband@contesting.com
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