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Re: [TowerTalk] Buried HF/VHF feedlines

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Buried HF/VHF feedlines
From: jimlux <jimlux@earthlink.net>
Date: Sat, 25 Jul 2020 05:41:46 -0700
List-post: <mailto:towertalk@contesting.com>
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.
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