[TowerTalk] Blackened Sheild on Belden Coax?

[Ian White, G3SEK]

Tom Rauch wrote:
>
>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.
>
Pretty much the same here. 

>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!
>
That's certainly true, from having melted quite a lot of 9913-type cable
with UHF QRO. Even then, the melting was due to the center conductor
getting hot (because it has a smaller surface area than the shield). It
was the centred conductor that heated up the polyethylene, not
dielectric losses. The shield was not involved at all... at least, 
not until the center conductor moved across and touched it!

>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.
>
If the outer of braid-over-foil type cable is blackened, you might be
lucky enough to find that the inside of the foil (which is what really
matters) is OK.

Like Tom, I'd suspect water first, a bad batch of cable second, and
electrical causes last of all.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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