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Message-ID: <5.1.0.14.0.20020313113544.00ab42c8@gemini.herts.ac.uk>
Date: Wed, 13 Mar 2002 12:07:33 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: Re Antenna experiments
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At 03:13 13/03/2002 -0800, you wrote:
>BTW I have had some problems with burning at the top of the fibre glass 
>mast and am wondering whether the filler might be carbon black??
>73, John, G4CNN
Dear John, LF Group,

I think the carbon is just what is left after the resin bonding the 
fibreglass has been decomposed by the discharge. I'm sure a chemist could 
tell you more, but I think it is true that virtually all resins are organic 
compounds that leave a carbon residue when burned. I have had similar sooty 
residues after arcing on various types of plastic, as well as fibreglass 
and wood - usually the discharge leaves an interesting branched pattern of 
the black residue. The process seems to be a vicious circle - once the 
residue is formed, it provides a conductive path for the discharge to 
spread along - much the same as the "tracking" you sometimes get on car 
engine ignition components. I have actually seen it happening - little 
white arcs move across the surface of the plastic, leaving a smouldering 
trail behind them.

On my antenna I have largely stopped this happening by fitting "corona 
rings" - these are just hoops of wire about 100mm in diameter attached, and 
electrically connected to, wherever the highest field gradients are - the 
ends, corners, and junctions in the wire normally. They reduce the 
potential gradient and suppress the discharge.

Cheers, Jim Moritz
73 de M0BMU