[TowerTalk] Coax in Conduit and WATER!

[John Brosnahan]

>They specifically advised not to ever use conduit.   A local friend who
>services commercial radio stations antenna sites, pager sites, and cell
>sites, also recommended against using conduit.   He states that when he
>pulls the coax for replacement after problems develop or a lightning hit
>-- he has never found the conduit encased coax dry.  It is always wet.  

In most situations I recommend direct burial without conduit.  My experience
with 100,000 ft of buried hardline (radio astronomy observatory) was that the
only places that had problems were where a previous engineer had placed
coax splices in ammo cases to "protect" them.  All of the ammo casses
were filled with water--immersing the splice in water to constantly test the
quality of the waterproofing.   It wasn't that good and we ended up
locating all
of the splices with the help of a TDR, digging them up and redoing them .

>He is adamant that regardless of how the conduit joints have been sealed
>they leak.   Similarly the direct buried conduit splice done with
>connectors always leaks without regard to the manner or amount of
>sealing performed.   

I don't think the conduit joints leak--if made of typical PVC pipe used in
water suppies.  The problem is that, with temperature and barometric
pressure variations, humid air is pulled into the conduit from the ends
and the moisture in the air condenses--but it never re-evaporates.  The
equivalent of a water diode--conducts water (moisture) one way but not
the other.  Whenever I do use a conduit for cables I use a Skil saw
to put many slits on the downward side to allow for drainage.  This works
well in Colorado where the soil is typically sandy, the water table is
low (or non-exisitent!), and the air has a low moisture content.  It would
not work with a high water table.

Conduit does protect the cables from damage due to sharp stones and
rodents, even though it guarantees that your cable will be immersed.
Depending on the cable, under some circumstances it may be a valid
tradeoff.  Unspliced hardline with a good jacket will probably survive
without bringing in moisture into the dielectric.  RG-213 and RG-8 will
be full of water in a short period of time.  But that can also happen when 
the cable is above ground!  I once removed a 300 ft line of RG-17 from a
40M beam at an unnamed station in the midwest, where the cable ran
down hill about 10 ft from the house and then 140 ft up the tower.  When the
cable was dropped on the ground the water ran out like a hose!

My problem is with rodents and coyotes, who think coax and cables in
general are the country equivalent of the Nyla-bone dog toys.  They all
chew on it--unless suspended in the air.  Burial just hides the chews
of the pocket gophers from sight!  A backfilled trench is like a super-
highway to an underground rodent.  All the hard work has been done--he
just has to push a little dirt around to build off-ramps!

73  John  W0UN





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