[Lf] [Fwd: LF: Re: Mike's insulators ??]

[Andre Kesteloot]

James Moritz wrote:

> 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