[TowerTalk] Buried HF/VHF feedlines

[Michael Tope]

On 7/25/2020 11:17 AM, jimlux wrote:
> 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.
>

Here is a picture of one of the splices we did at W6VIO. IIRC, this was 
an existing splice that didn't have any strain relief to keep the two 
halves in axial alignment. As a result, the center conductor cylindrical 
splice contact split open and shorted to the inside of the hardline 
shield. The kludgy fix was to strain relief the cable to a "splint" so 
the center conductor splice contact wouldn't be side loaded:

https://www.dropbox.com/s/prc9vx929v7rmp7/P8210023.JPG?dl=0

Here is what the center conductor splice contact looked like after repair:

https://www.dropbox.com/s/ha4ilkpxjr2akd6/P8200017.JPG?dl=0

I remember the water ingress problem, but I don't recall the cause. At 
one time those hardlines were pressurized, but whoever was maintaining 
the pressurization system retired or moved on to other things and nobody 
picked up the ball, so the water ingress may have been the result of 
simple neglect.

73, Mike W4EF..............