[Top] [All Lists]

[Towertalk] Burying Coax in Conduit Trench

To: <>
Subject: [Towertalk] Burying Coax in Conduit Trench
From: (Kimo Chun)
Date: Tue, 27 Aug 2002 21:37:50 -1000

There are a number of considerations. The usual guideline is to
put the pipe at least 12" below grade. You could consider less if
you don't feel you need to worry about accidental pick-axes or
other things penetrating it, or a car driving over the line.

At least in Hawaii, where it is quite wet, it is almost impossible
to consider keeping the cable dry even in a sealed conduit.
Water has a unique way of finding its way in, not to mention=20
condensing water.

California is much drier but "when it rains, it pours", yes?
9913 cable has a hollow tube center dielectric. This makes it
a wonderful water pipe should any water ever get in. I would
rather consider the Cushcraft or Belden equivalents to 9913
that use a foam center. I also don't like 9913 as the center
conductor likes to travel under certain circumstances pulling
a connecting pin out (for type-N non-captive). I've used the
Cushcraft TL cable series for years.

Unless you have a straight 30 foot run, 8 cables in the 4"
conduit may prove difficult to pull unless you pull them at the
same time. The cable will not lay flat like more flexible cable
might so it will start getting "crowded" in there. You could
consider four at time but by first "lacing" them together in a
bundle. You should leave a 3/16" or larger nylon rope inside
as a future "pull string".

ABS plastic is stronger but regular Schedule 40 PVC should
suffice. The EB and DB PVC liners available from electrical
houses are rather thin and could crush or crack. I think they
are used in concrete lined vaults by the power company.
I use them for lighter shipping tubes for large antenna pieces.
They DO get cracked in shipping, sometimes.

Our soil here tends to be packed pretty hard. A trench lined
with a bed of gravel will tend to act more like a water trough
than a drainage ditch. You should consider if it is worthwhile to
put in a "french drain" and grade your trench down to it.  You
want the water to drain through the gravel and fill the larger
"drum" of gravel where it has time to leach into the surrounding
soil. Of course, this whole exercise  depends on just how much
water it is going to have to get rid of. Granted, you're not trying
to keep the conduit from getting wet "at all" but to not let it sit
in water for a period of time where it could leak and soak in.

Holes in the bottom of the pipe would be a good idea as I've not
seen many here that didn't get water in them anyway. Doing this
would then prompt me to make my drainage system even better....

Don't use the standard ninety degree elbows that are usually
sold as they are for liquids. Way too tight a bend for cables...
especially if you're trying to pull them through the turns. Get
factory sweeping bends. more than two bends
before a pullbox. Personally, with that stiff cable, I'd have
only one bend between pull boxes.

Consider this. The pipe isn't all that expensive. If you think
one run is enough. Put in two. It can't hurt, much. Next you'll
want AC power out by the tower and you shouldn't run it in
the same conduit as lower voltage cables. So consider
another 3/4" to 1" line for that (sealed - no holes) and use
either direct burial type wire (even inside the conduit) or
perhaps regular ROMEX 12-2 (plus ground). Use a weather
proof outlet box and a Ground Fault Interrupter (GFI) outlet
(or better yet a GFI breaker in your panel for that circuit).
Just a thought.

Your conditions may vary.

Sorry for the LONG answer.

73, Kimo Chun KH7U

I am considering running a 30 foot trench from the base of my tower to =
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 =

All of my cables are 9913, and I have a bundle of about 8 feedlines.

I am thinking about a 4 (four) inch conduit pipe, and wondering about a =
things:  (BTW, I live in Southern California)

How deep should the trench be below ground?

What kind of pipe is best to use?

Should I drill holes in the bottom side of the pipe so water can 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?

Anything else I should know?

Thanks for your help.

Herb - KG6OK

--- StripMime Report -- processed MIME parts ---
  text/plain (text body -- kept)

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