[TowerTalk] Buried HF/VHF feedlines

[jimlux]

On 7/25/20 5:41 AM, jimlux wrote:
> 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.
> _______________________________________________
> 
> 

correction, where the soil temp never goes below the dew point  (no 
coffee yet)