> I have an AV 640 on a brand X Aluminum tower mounted on the
> West end of my shop. It works very well on 40 and quite
> good on the rest of the bands except 20. The match is good,
> but it appears to be a good dummy load on 20. I've never
> taken the time to trouble shoot it to find out the problem.
> It has a good looking SWR curve that is very close to what
> the manual predicts, but it neither hears, or gets out well
> on 20.
Are the "radials" (counterpoise) connected to your tower? You
will find that one MUST isolate the radials/counterpoise from
the tower and use a good balun to decouple the feedline from
the radials on ANY elevated vertical.
Without decoupling, the feedline and/or tower becomes a random
radial (vertical radiator) and, depending on frequency, length,
ground connection, as well as a host of other considerations
can really screw up the pattern of an elevated vertical antenna.
A classic problem would be an undecoupled feedline/tower that
would elevate the take off angle to something on the order of
45 degrees which would be too high for reasonable performance!
The takeoff angle for elevated verticals are also sensitive
to height above ground (information in the ARRL Antenna Book
and/or modeling programs) ... pick your height carefully to
minimize the problems on specific bands.
... Joe, K4IK
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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