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[Towertalk] Burying Coax in Conduit Trench

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Subject: [Towertalk] Burying Coax in Conduit Trench
From: (Steve Katz)
Date: Wed, 28 Aug 2002 07:14:40 -0700

> I am considering running a 30 foot trench from the base of my tower to my
> ham shack for my coaxial cables, and would like to here comments /
> suggestions from anyone that has done this before, on things to do / not
> do,
> etc.
> All of my cables are 9913, and I have a bundle of about 8 feedlines.
        [Steve Katz]  The biggest problem there is that 9913 isn't very
flexible, so one must be very careful during the "pull" process (getting the
feedlines through the pipe) to assure the cable isn't bent or kinked to a
damaging point.  9913 should only be bent on a 10x diameter radius, which is
about four inches, and I'm sure you know it cannot be repeatedly bent at any

> I am thinking about a 4 (four) inch conduit pipe, and wondering about a
> few
> things:  (BTW,
> I live in Southern California)
        [Steve Katz]  8 runs of 9913 (.405" O.D.) should fit, but I'd grease
them before pulling them through the pipe to assure they slip right through
and don't try to kink in the process.

> How deep should the trench be below ground?
        [Steve Katz]  In southern CA, it doesn't matter because there's no
frostline to worry about, unless you're up very high in the mountains.  I'd
make it shallow, just deep enough to prevent potential landscapers/gardeners
or others from hitting it when servicing the property.  I'd also use PVC
pipe, with multiple elbows glued to each end to make inverted spouts to
prohibit water entry.  We don't get condensation here, although even if a
small amount did occur, it could still vent out the inverted pipe openings
at the ends.  (Frankly, I've never seen a drop of water in any of my
underground pipe runs for cables in southern CA.  Even though I have a 7
days/week automatic sprinkler system that literally "rains hard" on the
openings at the ends, no water gets into the pipe because it would have to
defy gravity to do so.  I had such an installation 120' long (schedule 40
PVC pipe) in Chatsworth for 7 years, and opened it up twice for service, and
both times it was dry as a bone inside, even in winter.)  I plug each
inverted end opening with high density foam to keep out varmints but still
allow the pipe to breathe.  It would not be a nice surprise to open the pipe
for servicing and find it full of black widows.

> What kind of pipe is best to use?
> Should I drill holes in the bottom side of the pipe so water can drain
> out?
        [Steve Katz]  I wouldn't bother.  If you do it right, there'll be
nothing to drain.

> Should I place some gravel on the top of the trench before I lay the pipe
> on
> top, and cover it, to help with drainage?
        [Steve Katz]  See above.

> Anything else I should know?
        [Steve Katz]  Nah.  Be careful handling 9913 prior to, during, and
after the installation, though.  It's fragile cable, not to be stepped on,
driven over, mishandled or kinked, ever, during its lifetime or that
lifetime will be cut very short.  -WB2WIK/6

> Thanks for your help.
> Herb - KG6OK
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