[TowerTalk] Blackened Sheild on Belden Coax?

[Tom Rauch]

> Your blackened braid is a result of old cable where ultra violet light of
> the sun has contaminated your coax jacket.    It will happen usually at
> about 10 years old.   Best to just not use that stuff.   Not worth the
> problems you will have when things "just dont seem to match" if not now,
> later.   A common problem with older coax.
> 
> 73
> Mike
> K0BUD

Hi Mike,

Any time I've had "blackened braid", it has been from water. It 
doesn't matter if the cable is one month old, if I get water in the 
cable the braid turns black and loss goes way up. 

I have plenty of ten year old or older coax that has been exposed to 
the sun and is as good as new, as long as it didn't get wet inside.

As for current causing it, I've even melted some lines that had the 
unfortunate luck of being in the wrong SWR at the wrong time and 
never seen the braid change color. I can't imagine the braid getting 
so hot as to discolor and not having the dielectric melt!

Coax depends on the weave making a solid connection in the 
braid. Once it gets tarnished, loss goes up. This is especially true 
on higher frequencies. Remember skin effects try to keep the 
current on the inside of the braid and on the braid surface. When 
the woven strands can't make good electrical connection, losses 
increase.

That's one reason why braiding should never be used for lightning 
grounds, another is it already has several times the RF resistance 
of a smooth conductor of the the same dimensions.     

I wonder what other people's experiences are? 
73, Tom W8JI
w8ji@contesting.com

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