[Lowfer] Re: Meltdown

[James Moritz]

Dear Lowfers,

At 04:01 09/07/2002 -0400, you wrote:
>At about 2155 CDT this evening, two of the three plastic insulators
>supporting the transmitting antenna literally burned up, causing the
>antenna wire to fall to the roof of the house.  Luckily, this so detuned
>the antenna system that no further arcing occurred, and the roof of the
>building did not catch fire.

This has been a problem here too - once they get going, plastic insulators 
can burn like candles, dripping burning plastic onto anything below, so be 
careful! It is hard to see the corona - at night it is a faint purple glow, 
with little yellow or white flashes when it finds something combustible and 
if it is very quiet you can hear a hissing sound. It also causes small, 
rapid fluctuations in the antenna current. It's a good reason to avoid 
indoor loading coils.

After having this happen several times with my LF antenna, I found the 
problem could be cured with "corona rings", which are just hoops of stiff 
wire about 100mm diameter attached to the ends and sharp corners in the 
antenna wire, a bit like what you often see on high voltage power line 
insulators. These reduce the field gradient at these points and are quite 
effective in eliminating corona. I found the same treatment was necessary 
for the feed-through insulator on the housing of my loading coil.

Another thing to watch out for is wet timber within a foot or so of the 
antenna wire - little arcs can spontaneously form on the surface of the 
wood, which dance around in a fascinating way, forming charred tracks which 
set the wood smouldering.

Cheers, Jim Moritz
73 de M0BMU