[TowerTalk] Buried HF/VHF feedlines

[David Gilbert]

I understand all of that.  Water vapor will penetrate the walls of the 
conduit and condense inside, or as you say, get inside via air flow 
through the conduit.  But why is that a problem for the Heliax inside 
the conduit?  Assuming the conduit is below the frost line, of course.

73,
Dave   AB7E


On 7/25/2020 6:01 AM, jimlux wrote:
> On 7/25/20 5:41 AM, jimlux wrote:
>>
>> 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.