In a message dated 98-06-16 08:26:37 EDT, mlowell@noclant.navy.mil writes:
> I have two friends with flat-top towers and the coax loops always seem to
> tend to bear on the edges of the top plate at some point of rotation. We
> are concerned that this will eventually bite through the jacket and
> shield. We have glued pieces of split nylon air hose or garden hose over
> these edges to help the problem, but I'm sure some of you must have some
> better ideas or rules of thumb for making these loops.
>
> In my mind, it would seem that a scheme that applies the least amount of
> twisting per unit of coax length would fend off fatigue the best.
There are actually two ways to bring the feedlines off of the mast. The
first is the classic "big rotation loop". I think this is what you're
describing with the cables sweeping off of the mast/antenna boom and making a
big loop until they attach to the tower leg. Yes, it's common to have the coax
damaged over time by rubbing against the tower hardware. Yes, you can
sacrifice some hose or other material. There is a split plastic flexible
tubing that you can buy that'll do an excellent job of protecting the cables.
I think you can find it at bigger hardware stores. You might also try trailer
repair shops - they use it for trailer wiring. This method puts the most
strain on the cables.
An alternative method is to bring the coax (a jumper or pigtail makes it
much easier) down the mast and then wrapping it (them) around the mast 2-3
times between the bottom antenna boom and the top of the tower and then
attaching them to the leg. What you've got is enough loops of slack that the
cables actually move very little during rotation. If you install the cables at
the center of rotation, then the cables are only subject to +/- 180 degrees of
rotation. With a couple of wraps of cable, they actually move very little.
This is the same idea as the cables that you see on your local TV/radio
station remote truck with the hydraulic mast and microwave dish. The feedline
is just wrapped around the mast and allows not only for rotation but also
hoisting so you can see that there is enough slack for ham rotation
applications. It is also the easiest on the cables.
In all cases, stay up on the tower to observe the cables during rotation
before final taping and make sure the rotator is at the center of rotation
before proceeding with either method.
>
> In a related matter, what's the best way of attaching the coax runs to the
> tower? Are a few turns of good ol' Scotch 33 good enough (If I don't flag
> the last turn!)?
I prefer taping the cables to the tower leg. Running them down the
center of the face is how professionals do it. I don't know that it's any
better.
Tape is cheap and works fine. Another thing you can use are 6-10 inch
pieces of #14 house wire. Just twist them around the bundle and you're in
business. Easier to remove for cable adding or subtracting than tape. DO NOT
use white tie wraps - they'll turn useless in UV in a short time. Black UV-
resistant tie wraps are more expensive and are one-use but they have their
place. If you work for the phone company and can get them for free - go nuts.
If you are in a lightning prone area, you might consider running them down
the inside of the tower. That way, the tower acts as a Faraday shield and
protects the cables. Installing and removing cables inside a tower are a pain
in the butt but might be worth it for the above reason.
>Does having the shield running so close to another
> conductor for so long have any effect on the coax?
What kind of effect? The other cables will have RF induced in them and
may re-radiate your signal or have some other unforseen consequences for
multi-op environments but for the average station, I don't think it's much of
a problem.
Cheers and GL, Steve K7LXC
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