[TowerTalk] Buried HF/VHF feedlines

[jimlux]

On 7/25/20 1:02 AM, David Gilbert wrote:
> 
> 
> Heliax is essentially plumbing.  Quite possibly I simply don't 
> understand, but why is water a problem?  How does it penetrate the 
> copper sheath?
> 
> And if water is a problem, why can't you have a hole in the conduit at 
> the low spot to drain off the condensation (assuming you're below the 
> frost line)?  The condensation should develop rather slowly, I would 
> think, because it requires water vapor to penetrate the conduit.
> 
> Like I said ... maybe I just don't understand the situation.
> 
> 73,
> Dave   AB7E
> 
> 

Daily temperature fluctuations cause air to move in and out of the 
conduit - that's where the condensation comes from.  Unless you live 
where the dew point never goes below the soil temperature, water will 
accumulate.  Barometric pressure variations do the same thing, but a 
lesser effect. Wind causing a small pressure differential between the 
two ends also pushes air through the conduit.  You'll see this when one 
end is outside and another is inside a building, especially if the 
building has HVAC with outside air input.

One way I've heard of, but have not seen in person, to fix this is to 
run sufficient DC or AC current through the coax to make it slightly 
warmer.  This sounds like one of those ideas that might work, but then, 
it's hard to calculate that it will, and if you've got a commercial 
installation, you're more likely to go with something tried and true 
(fans, nitrogen purge, etc.).  Or in a broadcast environment where 
there's significant power flowing through the coax 24/7.

Someone probably tried it in the 30s or 40s, but it didn't work "well 
enough"

Running power through an antenna to melt the ice off.. that's been done 
a lot - the big Canadian SW broadcast station at the north end of the 
Bay of Fundy on the border between Nova Scotia and New Brunswick did 
that. Resistive heaters on dishes is also a standard thing.