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Guy wires near an FM antenna

T

TXengineer

Guest
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?
 
> 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>
 
> My guess is that this is not sheathing; rather a
> non-conductive section of the guy assembly. <clippage>
> 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,...
_____________

Both observations are correct, but metallic guys affect the net radiation pattern of the antenna far less then the tower does, itself.

Paper #6 at http://rfry.org develops this in more detail, and shows some of the pattern changes that towers can produce.
//
 
I've observed this too, and from the ground a consultant on one occasion asked if that was fiberglass above. It clearly was not but looks white near the tower by the antenna aperture.

The only difference I can see in this part of the guy wire is that the strands of wire are woven differently around the eye at the attachment point and then back the other direction (away from the tower). The guy wire becomes thicker because there are twice as many strands and the "grain" of the strands may change direction and reflect light differently given the appearance of being white - the entire guy wire is metal though in this case. This is called an "eye spice" in rope?

> 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?
>
 
An aside on guy assemblies

> The only difference I can see in this part of the guy wire
> is that the strands of wire are woven differently around the
> eye at the attachment point and then back the other
> direction (away from the tower). The guy wire becomes
> thicker because there are twice as many strands and the
> "grain" of the strands may change direction and reflect
> light differently given the appearance of being white - the
> entire guy wire is metal though in this case. This is
> called an "eye spice" in rope?

Similar to an "eye splice" but not entirely so. I've been trying
to remember the name for the grooved steel inserts that form the
interior of the "eye"....I keep coming up with "thimbles" but it's
been a long time....

Meanwhile, the steel cable used for guys is rarely, if ever,
spliced. The cables, themselves, are generally straight lengths,
just cut and taped at the ends. The "eye" is made by using what's
called a "pre-form". It's a group of wires which are laid flat
with the center point formed into a loop and the two "legs" forced
into opposing spiral secttions. The interior of each spiral is
coated with an abrasive substance. The loop is placed at the
end of the cable and first one leg is wrapped around the cable,
then the other so that the spirals interlock. The two spirals
then serve to force each other tighter as strain is put on the
eye and the abrasive really bites solidly into the guy cable.

It's exceedingly rare for a pre-form to "slip"; a break in a guy
cable usually happens (and that, too, is rare) due to a failure in
the cable itself.

BTW...on directionalization....

Yes, the tower itself is a major factor in directionalization, for
which reason a good installation takes geography into consideration
in deciding upon which tower leg to use to support the antenna. One
wants to make the tower augment, rather than block, the signal from
the main population density. Of course it can also be used in a
"negative" way to partially deflect signal from the direction of
a co- or adjacent channel. To control tower directionalization you'll
note that FM antenna elements are almost never mounted directly to
a tower leg; rather stood off on steel brackets some distance defined
by a fraction of wavelength for the particular frequency.<P ID="signature">______________
"environmentalism is collectivism in drag."
--George Will (or won't)</P>
 
Phillystran is often used, but that is black--at least the section we have up there.

There are also pre-formed fiberglass guys, usually abt 12~10 ft in lenght, that can be used on smaller towers The ones I've seen are grey, but might look whitish. However, they are solid, and wouldn't look stranded.

There are probably other products out there.

Our tower is close guyed (220 ft. on a 1/2 acre) & we have 2 6ft dishes on the top section. So we have to have guys above our 4 bay antenna (17 kw ERP.

Hence the need for insulated guy sections.
 
I noticed something similar the first time I visited my FM site. We have solid sections about 15' long, which look to be pre-formed. Since those sections connect to the top of the tower right above the the FM antenna, I just assumed it was to prevent the wires from affecting the pattern. Our tower is just a baby though, as far as FMs go. Only 140 feet tall. Obviously atop a big mountain!
 
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