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

Thu Jan 28 16:13:08 EST 2016

Take those articles with a grain (or two) of salt.  I see many of them 
as a good guide for those who understand the circuit, how it works, and 
what it's supposed to do.  Over the years, I've found many errors in 
construction articles.

   Wires may not be rated for immersion, but coax, rotator control 
cables and cables to remote antenna switches are hanging out in the 
elements. Rain, Ice, snow, extreme heat and cold temps don't hurt them. 
Ever look at a piece of CAT5 that's been on the tower for a while?  Even 
in the Northern climates, after a few years, the jacket becomes so hard 
and brittle you can crush it just by squeezing it in your hand unless it 
was specifically rated for exterior use.  Now there is some cheap 
cable!  OTOH, I've had some rotator control cable do the same thing, but 
the wires inside remained fine. It was purchased so long ago, I have no 
idea as to its origin, other than it didn't come from a swap.

One time, one of the local hams picked up a lot of very flexible RG-8 
size coax. (source?)  The jacket was a gray, smooth, almost slippery, 
soft plastic. It was almost as flexible as 8X, but after a few years in 
Michigan's sun (43deg 37min N) it looked like it had a bad skin disease 
and was covered with dark scabs. The chemical smell was very strong 
(plasticizer?)   I still have a few short pieces that were used indoors 
and they appear to be fine, but they do have an odor, although faint.

Yes, with the pressure changes, mainly due to temperature changes, there 
can be substantial condensation due to "breathing"  (voice of 
experience), but WHY CARE? What condenses, will eventually evaporate.  
Sealing conduit is likely to cause ingestion of moisture unless all 
joints are 100% sealed.  I leave both ends open, yet after nearly 15 
years, moisture has never been a problem.  I used to drill  little drain 
holes at low spots, but I no longer do that.  It seems to me to be a 
waste of time.  Besides, In the spring, around here a conduit a foot 
down is likely below the water table. Sump pumps get a real workout in 
the spring!

What difference would it make if the conduit were full of water as long 
as there are no connectors, or splices in there and the coax is a 
trusted brand?

73

Roger  (K8RI)

On 1/28/2016 Thursday 9:17 AM, Al Kozakiewicz wrote:
> QST is not peer reviewed.  All kinds of nonsense can make its way into print.
>
> The notion of copious condensation in buried conduit has always seemed like ham urban legend to me.
>
> First off, there are literally millions of miles of buried electrical conduit in the world filled with wire not rated for immersion. The failure rate for this infrastructure does not seem to be anything exceptional.
>
> More important, condensation requires a constant supply of moist air. Where is this supply coming from? Not convection, unless someone changed the laws of physics while I wasn't looking. If there really is enough air flow from the pressure difference between the ends of the conduit sufficient to bring in the required moisture (unlikely), then seal the ends. Simple.
>
> Al
> AB2ZY
>
> ________________________________________
> From: TowerTalk <towertalk-bounces at contesting.com> on behalf of StellarCAT <rxdesign at ssvecnet.com>
> Sent: Thursday, January 28, 2016 8:22 AM
> To: tower
> Subject: [TowerTalk] another subject: coax and cables buried or elevated        above ground?
>
> 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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-- 

73

Roger (K8RI)

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