I have made numerous attempts to "detune" my vertical over the past
6 years. I admit at this point I have no idea what I did wrong or
what to try next. I have what I believe to be evidence the vertical
is interacting significantly with the several Beverages, which of
necessity are close to it. I attempted to follow the advice in "Low
Band DXing" and on W8JI's outstanding web site but I didn't get very
far.
The vertical is 100 feet of Rohn 25 tower with a ~30 foot long 7
element 6 meter beam sitting at 103 feet. There are approximately
100 on-ground radials ranging from 60 to 200 feet in length. It is
gamma matched by a 4.5 inch triangle of #6 wires spaced
approximately 30 inches off one side of the tower. The short is at
about 32 feet (from memory, may be off a couple of feet in either
direction), BUT the 3 wire gamma "rod" continues to approximately
the 60 foot level. Is this a problem? Should I get rid of the
"excess" length of the gamma match?
Previous attempts to detune the vertical ended in frustration. First
I attempted to use the gamma match as the loop since that would be
very convenient. Using an MFJ-259B I was able to get the loop down
to about 2 ohms. This occurred with about ~900 pf capacitance vs the
few thousand pf I was expecting. It did not seem to have any
noticeable affect on what I presume to be vertical / Beverage
interaction. I tried the same thing with a similar sized loop placed
half way up the vertical on the side opposite the gamma match, with
similar results.
Perhaps I am wrong about the nature of the problem? Local noises
(eg. plasma TV) don't change by the expected amount (F/B of
Beverages) when I switch among Beverages. Often, while listening to
such a noise on one Beverage, switching from the vertical to a
different transmit antenna (thus leaving the ~250 feet of coax
feeding the vertical open at the shack end), there will be a
dramatic change in noise on the Beverage, depending on the
particular noise and Beverage selected. I take this as evidence the
vertical is re-radiating noise and that detuning it should help. Is
that a valid assumption?
Is there any other method I can use to detune the vertical? How
about listening to a signal coming from the back of a Beverage and
tuning the vertical decoupling section for minimum signal on the
Beverage? Would that be valid? Any other ideas?
Noise is getting out of hand around here. Every time I listen there
seems to be a new one. Of course I will continue to track them down
and attempt to mitigate at the source. But the need to get my
station receive performance as good as it can be has never been more
evident.
73,
Paul N1BUG
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Topband reflector - topband@contesting.com
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