Howard asked about mounting an 18avq on top of a 50' tower...
Tim replied...
"If you slope the radials, that will lower the impedance a bit, for a full
size 1/4 wave ground plane it will match 50 ohms...either way it shouldnt
change more than about 20 ohms and should still present a decent match to 50
ohm coax. However another point, moving the vertical that high may help
with receive signals, but a lot of your transmitted signals will go straight
up or nearly so, fine if you want short DX."
-0-
Tim's comments regarding the match are consistent with my experience.
I've had elevated GP's, both multi and single band, up for years, with
generally good results. You'll need resonant radials on each band,
so things might get a bit spider-web like. (and having only TWO resonant
radials per band isn't very effective. I use a symmetrical pattern of 4, at
a
minimum.)
I differ on the topic of radiation pattern. Not sure about the source of
Tim's caution, but if we're talking about 2 resonant radials...it's just
a bent dipole, and you WILL get high angle stuff out.
My experience on 40 and 20, in particular, is that as the antenna was raised
above .25 wavelength, stateside signals dropped off, and long distance stuff
came up appreciably. My 40m GP was very strong into Hong Kong, on a
consistent basis.
It was 2 S units down from dipoles/vees at 35 to 70' on stateside paths,
and
topped the heap from EU to Asia.
After one season, I removed the traps from the 18AVQ, and made it a 33'
vertical
for 40 only. That picked up another few dB. Incidentally, I used a 50'
pushup
mast, held vertical by a maple tree. There was no connection to the mast,
except
whatever static drain system the vertical has, in its base. The tower was
not part
of the antenna.
A question for the modellers might be...what would the effect of feeding the
tower
be on radiation pattern? I'd expect it would be different for each band.
N2EA
jimjarvis@ieee.org
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