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
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