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[TowerTalk] Coax

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Subject: [TowerTalk] Coax
From: (J. G. Kincade)
Date: Sat, 28 Apr 2001 10:44:36 -0500
John is correct. My experience with telephone company buried cable is the
same. Unless there is something besides air to physically consume the
remaining space in the conduit (phone company cables are injected with a
waterproofing gel substance under pressure during manufacture) moisture
absolutely will build up inside. In my installation, my tower base is about
60' uphill from my shack. The difference in elevation is about two feet. I
used 3" grey electrical conduit because I wanted to run several coax lines
in it, with the first run to the tower base serving as a jumpoff point for a
second run to the base of my groundmounted vertical. All I had to do was put
a loose wad of fiberglass screen in each end (to keep out critters). The
conduit has a natural slope to it, so standing water is not a problem. It is
still "wet" inside, though, so using direct bury type coax for ANY conduit
installation is something I'd recommend. My conduit is only buried about 6"
deep, just enough to keep me from damaging anything with the tractor or the
riding mower. I'm putting up a larger new tower soon a little further from
the shack, and I think I'll go with John's idea of perforated 3" or 4"
plastic drain pipe for that one, because the last 40' will be on level
ground. The right size conduit to use is twice as large as you think you'll
ever need. Otherwise, Murphy's law will prevail in the future, when you want
to add a line for that new coax switch two years from now, or whatever. The
difference in materials cost between 2" and 3", for instance, is minimal
compared to the possible future aggravation of having a conduit that's too
small for your needs.
73, Jerry W5KP

> So the answer is to put the cable in a conduit of some sort--
> typically plastic pipe, right?  WRONG, wrong, and more wrong.
> During daily temperature cycling, the pipe will pull in warm,
> moist air and the moisture will condense out when the pipe
> cools.  Over time the moisture will build up inside the pipe
> until the coaxial cable is guaranteed to be laying in water.
> The worst possible situation for a pin-hole filled PVC jacket.
> I have seen this EVERY time I have taken cables out of
> PVC pipe that has been buried for some time--even in the
> dry climate of Colorado.
> The secret to burying cable (if you must) is to use a pipe
> with drain holes.  If you want a relatively large pipe then
> use standard sewer drain pipe with holes every few inches.

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