[Top] [All Lists]

[TowerTalk] Coax

To: <>
Subject: [TowerTalk] Coax
From: (W0UN--Signal Hill Ranch)
Date: Sat, 28 Apr 2001 07:08:26 -0600
ONE of the direct burial problems has been touched
upon, but not the other half of the issue.

As has been mentioned MOST PVC and PVCII cables are
not designed for direct burial.  There can be microscopic
pin holes in the jacket that can wick in moisture and
degrade the cable over time.  And the problem is that it
is impossible to detect the degradation from the shack.
The VSWR tends to get even better with time as the
cable loss goes up.  The only real way to tell if the cable
has deteriorated is to do a "before and after" watt meter
test at the far end of the cable to see just what the loss in
the cable is.

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.
When I have had to bury cables and wanted a 2 inch pipe
rather than a 4 inch sewer pipe, I have used regular plastic
pipe and used a Skil saw to cut slots (along the axis of the
pipe) every foot or so.  This allows the moisture to drain
out the bottom, yet protects the cable from above.  (Just
be very careful with the Skil saw, or set up a jig on a
table saw.)

If you are going to direct bury or even place coax in a
pipe without drain holes then you must use hardline or
sealant filled coax designed for the purpose.  Unless you
are on the east coast, then the extra dBs of attenuation
that you will incur over time with improper direct burial
ARE APPRECIATED by those of us out west!       ;-)

Just remember, plastic pipe is like a diode--moisture flows
in (humid air) but never flows back out.

John  W0UN

List Sponsor: - Trylon self-supporting towers,
safety equipment, rigging gear, LOOS tension guages & more!

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>