> Why is it when a tower has a guy point behind an FM antenna,
> the guy wires look "white" when they reach the apature of
> the antenna? Do riggers install some sort of sheathing
> around the guy wires to inhibit their affect on the antenna
> pattern or is it to simply protect them from overheating
> close to the antenna?
I gotta guess since I've only worked with self-supporting towers
for FM and with a couple of old ones where the FM antenna was
placed at the very top of the AM tower, above the highest guy
point.
My guess is that this is not sheathing; rather a non-conductive
section of the guy assembly.
First, let's assume we're talking about an FM sharing a tower with
an AM. Guyed AM installations used to use a close-spaced series
of "johnny ball" insulators near where they join to the tower. In
the last couple of decades there has been a shift to using fiberglass
rods with aluminum clevis ends, one of which mates to the tower; the
other to the eye in a preform on the guy cable.
So, if it's a shared AM/FM, that would account for about 3-feet
of what looks like white material (due to their shape, some call
these devices "dog bones". Of course the 3-foot length would be
too small for what I suggest below.... But, I've seen them in
much greater lengths as solid rod stock. In fact, 10-footers are
common in folded (may the folks who invented THEM rot in hell)
unipoles.
In the case of a tower unique to FM, It would be undesirable to have
conductive guys near the aperture because of the directionalization they
might impart to the signal, especially since the guys on FM or TV towers
generally are (otherwise) one continuous length and are effectively
grounded at the bottom.
There are, of course, guy cables made of non-conductive material;
"Philystran" is one example that leaps to mind. They ARE very expensive,
so not as commonly used as one might guess.
Let's hope there's an expert on board who can correct me if I'm wrong
about this....or perhaps add more.
<P ID="signature">______________
"environmentalism is collectivism in drag."
--George Will (or won't)</P>