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Old June 4th 04, 02:41 AM
K7JEB
 
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I was hoping someone else would jump in and answer, but
here goes anyway...

Eric Schumacher, WB6KCN, asked:

Can anyone give me a brief explanation of the theory
of operation of a commercial FM broadcast antenna.
Specifically the Jampro JMPC. Picture at:


http://www.jampro.com/fma/jampjmpc.htm

It looks to me like a couple of 1/2 wave dipoles
each folded into V and fed in phase using two gamma
matches. If this is true I am puzzled by the tap
point chosen. It seems further along the dipole
than I am used to seeing, even considering that the
feed for the dipole is likely 100 ohm. The feed
must be very inductive.


When the ends of a dipole are folded back, the radiation
resistance decreases and matching schemes have to become
more extreme. The Jampro antenna does not look that
exceptional to me. You're probably right about a certain
amount of reactance accompanying the feedpoint resistance.
This could be tuned out by a matching network hidden in
the feedline/support pipe.

Polarization is stated as RH circular. How is this controlled?


To my way of thinking, the circular polarization is due to
the horizontal spacing between the horizontal and vertical
dipoles and the phasing of their excitations. If that is
done in a certain way, the far-field radiations from the
vertical and horizontal dipoles will be in time-quadrature
at all azimuthal bearings, the necessary condition for circular
polarization.

If I look at this antenna in a certain way, it looks like
a highly truncated, half-wave section of a bifilar helical
antenna radiating in the radial mode with the two dipoles
simulating the currents flowing in a small segment of such
a structure.

My guess is that getting this antenna to produce good
circular polarization and a good match and uniform
azimuth coverage were the end results of a hell of a lot
of tweaking on the antenna range.

Jim Bromley, K7JEB
Glendale, AZ