K7GCO wrote:
<snip>
> RF can bounce off metal
> objects but unless it bounces back at the "right everything in the right
> plane", the beams pattern and SWR are virtually unaffected. When you run
> these tests with say a 2M beam and scaled guy wires and a tower or in Eznec
> you will see there has been too much concern for guy wires affect except for
> what is right under the beam for a 1/2 wave. The multiple-insulator
> installation all the way to the ground is mostly just another "TT Band Aid"
> for a problem not properly addressed like poor feed systems used in beams.
> Fix the source of the problem first. Unless Phillistran is used for a 1/2
> wave from the tower I'd suggest an insulator at the tower, one at 5', one at
> 15' and one at 30' in metal guys.
<snip>
In some cases, K7GCO is right on the mark. However depending on the
installation, there are valid reasons (not "band aids") to use non-
conductive guy wires (or EHS broken up with insulators) for distances
beyond 1/2 wavelength from the tower.
Case 1: Sidemounted antennas
VSWR and patterns of sidemounted yagis below the top set of guys can be
altered even with the first 1/2 wavelength being nonconductive.
Case 2: Multi-tower installations
A neighbor had a 18 MHz yagi on a 56 foot freestanding tower that would
change VSWR when rotated with |Zin| varying up to 25%! The maximum change
occured when pointing at his big tower located about 100 feet away. After
replacing the big tower's EHS guys with Phillystran, the beam's VSWR no
longer varied as it was rotated. Guy wires located about 2 wavelengths
away were causing significant interaction. I believe there is an upcoming
article (perhaps in QEX) on the specifics....
Personally, I opted to use fiberglass rod guys (down to ~10 feet above
ground level) since I will be sidemounting several antennas. By removing
conductors that are not part of the antenna, any potential guy interaction
and reradiation problems are completely avoided.
--
Steve Miller N8SM millersg@dma.org http://www.dma.org/~millersg
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