[TowerTalk] another subject: coax and cables buried or elevated above ground?

Thu Jan 28 09:48:50 EST 2016

Gary,
1. Keep on postin' you have a significant interested audience and the 
others have delete buttons/keys or can program their email client to 
send your stuff directly to the trash.

2.  I get the same concern expressed by otherwise normal folk.  They 
tell me you will  always get condensation in conduit.  I have plenty of 
4 inch PVC conduits and they have no water problem except one that was 
broken due to improper backfill.  I sleeved cables inside flexible 
irrigation tubing before pulling in that conduit.

As the environment in which your conduit is located experiences heating 
and cooling as well as changes in barometric pressure air is alternately 
drawn into the unsealed ends and expelled.  Moisture in that air may 
condense inside the conduit  and build to a level that would cause 
problems.  The really detailed worriers are concerned that even if the 
conduits are sealed when a vacuum is created in the conduit air might 
flow in the braid of coax or whatever and get into the conduit somehow 
(or by other mysterious means.)  I've heard a lot of low probability 
mechanisms brought up as potential means of failure by folks often 
recommending holes in the conduit to drain the water underground. To me 
that is almost like leaving a hatch open in a submarine to drain out the 
inevitable condensation. Later in this post I will detail an incident 
involving a conduit with subterranean "drain holes."

With no holes in the underground portion of the conduit the rate of 
condensation buildup would not be substantial in most cases.  There are 
various trade names for desiccants, Dryzit I believe is one I have 
used.  The little paper packages of silica gel that comes in so many 
items can be saved, recharged in the oven and reused indefinitely.  You 
can buy silica gel in bulk.  Take a small can with a compression lid 
(small enough to fit in the conduit.)  Poke a jillion small holes in it 
(smaller than the granular desiccant or wrap with window screen material 
with holes smaller than the silica gel) tie a string to it, fill it with 
silica gel and insert it down into the conduit.  Then seal the conduit. 
If the seal works there is no problem.  If the seal doesn't work, there 
is no problem as the desiccant will absorb the moisture.

Since we all  leave a string or rope run through the conduit in case we 
want to pull another wire, use it to pull a towel through the conduit as 
a moisture detection/removal measure. If it comes out wet or damp you 
can then take action.

With Dryzit and similar products special containers must be used as the 
dry chemical gets wet and becomes a liquid solution.  I prefer silica 
gel for a couple reasons.  Not dissolving is one and you get it free in 
packaging is another. Alternatives to a desiccant include blowing warm 
dry air through the conduit.  It takes very few cubic feet per min of 
warm dry air to completely eliminate condensation problems. Of course 
this requires you to not totally jam pack the conduit with cables and 
leave a little space for air circulation. Unless you have a severe 
condensation problem the warm air could be placed on a timer and only 
run  a small duty cycle for energy conservation.

Hot humid climates can have significant condensation problems and taking 
preventative action is prudent.

Oh, about my cracked conduit due to my laziness and sloth re backfilling...

My wife was in the barn where this conduit terminates and came to tell 
me water was coming out of it.  I went and to my surprise there was 
about 4-5 gal/min flowing out of the end of the conduit which is about 2 
ft above the concrete floor. she kept placing 5 gal buckets under it to 
catch most of the water.  She would lug one to the door, dump it, and 
return to find the other 5 gal pail nearly full. I realized this wasn't 
a good solution so went back to the house for a cordless reciprocating 
saw.  Outside the barn where the conduit came up out of the ground at a 
45 degree angle to penetrate the wall I cut a notch out of the conduit, 
careful to avoid hitting any wires (sleeved in 1 inch irrigation 
tubing.) This let water exit the conduit outside the barn,  Yahoo 
success.  Next day... same problem.  Water is gushing out the notch I 
cut and overflowing the end of the conduit 2 ft above the barn floor. I 
cut a larger notch at ground level and that permanently solved the 
problem of water running into the barn.

I had previously detected the fact that the conduit was compromised by 
noting the water on the pull rope (and rag) which I had run by using a 
shop vac to blow a balled up rag through the conduit. So I still use 
that conduit but cable bundles are sleeved in the irrigation tubing 
which  is 1 inch in diameter and good for 100 PSI (way beyond 
condensation.)  I recommend the flexible irrigation tubing to use as 
sleeving on wire bundles in conduits where condensation or water ingress 
could otherwise be a problem.  Silicon caulk or similar could be used to 
seal the entry/exit points where the wires enter and leave the 
irrigation tubing for really strenuous cases.

Lowes has the irrigation tubing in 100 ft rolls for $12.

Patrick        NJ5G

On 1/28/2016 7:22 AM, StellarCAT wrote:
> My ongoing tower/antenna projects... hopefully I’m not boring others out there...
>
> I had planned on burying all lines to the towers (2, one at 150’ out and one at 300’ out) ... at first I thought in solid joined (sealed) PVC which I can get for $9/10’ ... and I’ve also thought about direct burial which for the coax and control cables has the added benefit of increased capacitance to ground and thus a reduction of energy (lightening event) that reaches the shack... but control cables aren’t normally rated for direct burial... or at least I don’t think so (researching that now)...
>
> Then this latest QST article on coax says DON’T bury in any type of conduit be it plastic or otherwise. It says condensation WILL get inside and ruin (flood) the cables.
>
> So .... if the cables are underground – below the frost line – say minimum 1’ down (upstate SC) ... how will condensation develop? Or will it? I’m an engineer (HW/SW development) and reasonably intelligent but find it difficult to wrap my head around this issue... will it or won’t it – flood that is? if it were always under ground – including both ends then I’d think not.... but it isn’t always under as the ends come up to and above the surface... does that introduce a temperature variant enough to induce condensation? And I don’t like the idea of holes in the plastic – that just means mud will indeed eventually work its way inside.
>
> Maybe use corrugated tubing (again – no cuts or openings) ... this would allow whatever moisture there might be, and I can’t imagine its going to be a great deal, to settle in the lower points of the corrugation.
>
> Comments from others that have been through this - in the south with similar conditions would be appreciated.
>
> Alternatively I could use a leader line and go from say 15’ on the first tower to the second tower and then that same level to the shack with supporting 4x4 poles along the way... but this seems iffy as well as first the cables are ‘leaving’ the tower above ground potential so that might mean a higher voltage (common mode – probably?) on the lines (again: lightening type event)... and the first tower is a rotating tower ... although thinking about it that might work quite well to go from the tower with a ‘swing-arm’ of hanging coax/cables to allow rotation.
>
> thoughts?
>
> Gary
> K9RX
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