[TowerTalk] Buried HF/VHF feedlines

[jimlux]

On 7/25/20 10:00 AM, David Gilbert wrote:
> 
> 
> 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.
> 

Oh, for heliax, no problem, except there was a comment about a splice.
And, it's not unusual for there to be a crack in the heliax shield - it 
won't have any detectable effect on the loss (any more than the gaps in 
braid).  If you pressurize it, you'll see the slow pressure loss, maybe 
- the outer plastic jacket holds pressure pretty well, and you might 
just attribute it to leaks at the connectors.  It's also a well known 
problem with flex waveguide.

You might see it on a TDR (or a synthetic TDR from a swept measurement) 
if you flex it to open the crack a bit. But you might not.

OTOH, there are plenty of people who directly bury heliax (and similar 
cables) and have no problems.

At W6VIO (the JPL club station) we did have a situation with our heliax 
going from the shack up to the antennas on the hill behind the shack 
winding up filled with water. But I don't know if that was leaky 
connectors (atmospheric breathing) or cracks in the shield, or what. 
They were on the surface of the soil, and were exposed to rain, 
sprinklers, etc. as well as potential animal gnawing.

Mike W4EF might remember the history of the water in the cables.




> 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.
> 
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