See my article on Reverse Fed Elevated GP Antennas
in JUNE 1994 QST or any recent ARRL Handbook.
Basically, the Top Load PLUS tower / mast form the
'other half' of a resonant half wave.
Originally, I had 3 slopers fed at 60 ft on an 80 ft tower
with a TH6 on top. The TH6 looked like approximately
40 ft of equivalent top loading plus 20 ft of tower = 60 ft
which is pretty close to the other 1/4 WL needed on 80M.
Someone suggested tieing the 3 slopers together instead
of having 3 feedlines and switching. VOILA, a resonant
elevated GP antenna with sloping radials. This antenna
seemed to have a VERY low angle of radiation which
was not always the best match to propagation on 80M.
When I installed nearly horizontal elevated radials at 15 ft,
25 ft below a TH7 on a 40 ft tower I achieved a wider vertical
pattern. The feed impedance was 17 ohms resistive at
3.6 MHz which I matched using a coaxial transformer to
75 ohms and fed with CATV hardline. This antenna has
proven to be quite effective.
Since the feedpoint was at 60 ft and the base of the tower
was grounded, I'm wondering if the lower portion of the
tower wasn't effectively isolated on the taller tower, i.e.,
at 1/4 WL above ground, the impedance of the tower
should be quite high and therefore little or no current
would flow in the lower portion whereas the upper portion
(a resonant 1/4 WL) would present a low impedance
at the feedpoint taking all (or most) of the current.
de Tom N4KG
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