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Message-ID: <5.1.0.14.0.20021031140531.00af9ff8@gemini.herts.ac.uk>
Date: Thu, 31 Oct 2002 14:26:08 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
In-reply-to: <000801c280b2$7262b240$216868d5@oemcomputer>
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Subject: Re: LF: Re: Mike's insulators ??
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Dear Mike, LF group,

At 07:52 31/10/2002 +0000, you wrote:
>I don't think it was corona. Corona tends to be quite small and erode
>slowly. What I saw was a yellow arc some 8cm long, running the length of
>both insulators. This is not the normal property of a good insulator.

I had this kind of problem quite a bit when I first started using kw power 
levels - what happens is corona appears near sharp ends and corners of 
conductors first, and chars the surface of the insulator. The carbon that 
forms on the surface of the insulator is conductive and acts a a new site 
for corona to form and so tracks slowly along the surface. If you catch it 
in time, you can often see intricate branched patterns of burnt tracks on 
the surface of the plastic. If you watch it closely, you can see each track 
slowly extending with little whitish arcs dancing around at the ends of the 
tracks. But of course, this usually sets fire to the plastic at some point. 
Of course, glass or ceramic insulators won't burn, but will get eroded or 
cracked in time.

For some reason, it seems to happen more when the surface is wet. I have 
seen it occur on pieces of damp wood that were near but not actually in 
contact with the antenna wire. As Alan said, fitting corona rings to the 
ends and corners in the antenna wire solved the problem. These are just 
stiff wire hoops about 100mm in diameter, which reduce the field gradient 
around the pointy ends, and suppress the formation of corona. I found a 
good corona warning was given by a VHF broadcast radio left on while 
operating on LF - if corona appears, lots of loud crackles and pops are heard.

Cheers, Jim Moritz
73 de M0BMU