[TowerTalk] Blackened Sheild on Belden Coax?

[Ian White, G3SEK]

James Sawyer wrote:
>
>It sounds as if you where hit by lightening.
>
>Jim
>

That's a possibility...

As Tom said earlier, by feeding in RF from a transmitter you couldn't
possibly heat the shield enough to blacken it, without totally melting
the PE insulation inside (that's because most of the heat is released in
the center conductor) Anyhow, the 21-22ft repeat pattern doesn't tie up
with a high SWR at 14MHz.

On the other hand, if it really was RF heating damage (and the presence
of a repeat pattern argues that way) then lightning would probably be
the only way to get large shield currents in a repeating pattern,
without overheating the inside. 

The only other way to get a 21-22ft repeat is if the cable was at some
time manufactured/stored on a 6ft drum.


>-----Original Message-----
>From: owner-towertalk@contesting.com
>[mailto:owner-towertalk@contesting.com]On Behalf Of Guy Olinger, K2AV
>Sent: Tuesday, September 19, 2000 5:27 PM
>To: Ian White, G3SEK; towertalk@contesting.com
>Subject: Re: [TowerTalk] Blackened Sheild on Belden Coax?
>
>
>
>Hmmm...
>
>>
>> Tom Rauch wrote:
>
>> >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!
>>
>
>Well, I split the coax jacket (some old RG8U, not foam) for over a
>hundred feet to check it out. The black gradually reoccurred every 21,
>22 feet, with clean copper in the middle. What do you think caused that?
>
>I figured I did it during an early mis-informed period when I was
>running 400 watts on 20 meters to a tuner, then thru that coax to a 40
>meter ground plane. It had a 1.5 to one swr on 40 cw straight through
>and took a kW matchbox to tame it on 20. I worked people on 20.
>
>Maybe heat to some degree accelerated the contamination process?
>
>- - . . .   . . . - -     .   . . .     - - .   . - . .
>
>73, Guy
>k2av@contesting.com
>Apex, NC, USA
>
>
>
>
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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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