Topband: Best angle of radiation ?

[Richard Fry]

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