[TowerTalk] Water in Coax

[Roger Parsons]

I have a 300' run of LMR400 going up a tower - at the top are a 900MHz bi-directional amplifier and antenna. The system stopped working properly a while ago, and I eventually (duh) found that the problem was water in the coax. There was a joint at the 220' level which I initially suspected, but decided to change the whole run to a single length of cable and this was done last Saturday. Everything now works fine again.

I was interested to see what had caused the problem and proceeded to cut 20' pieces off the cable until I saw a nice shiny outer conductor. This happened at about 100' - proving (I think) that the problem was below that point. I then inspected each 20' length very carefully and could not find any cut or defect in the jacket!

Am I right in thinking that if water has penetrated I will see corrosion of the outer conductor? Or could just a flow of water not cause corrosion? Which would mean that the failure point could be above the corroded section. (Although I have looked at the whole 300' I have not inspected the top part to the same extent as the lower section.) I am sure that capillary action could allow water to rise up the cable - anybody know how far this could happen?

The cable is a cheap(er) rip-off type rather than genuine. It is conceivable that the sheath is permeable - but if that is so it would only have to be the lower part - which seems terribly unlikely. And it sure looks and feels like polyethelene.

The problem happened over a long period (more than a year) so I guess it would only need a very small hole - but if there is one I can't find it.

I am very puzzled. Can anyone enlighten me?

73 Roger
VE3ZI