Today I replaced the loading coil at the top of my mast and added some extra
wires close to the coil as a capacity hat (rather like the cross-wires that
sit on top of the coil on a centre-loaded mobile whip). These were stood off
from a metal spreader bar at the top of the mast. I insulated them by using
some plastic (I assumed it to be Perspex/Plexiglass) rods from an old Decca
loading coil. I have been using another type of ex-Decca Perspex as
insulators for more than a year.

When I finally got it all tuned up again - the new coil resonated
differently from the old one - the antenna current rose with extra power,
then suddenly fell. I investigated and found the "insulators" sparking
across. I rapidly removed the RF, but the plastic caught fire and burnt for
about 15 minutes, eventually disintegrating so the extra wires now hang down
and wave in the wind.

If anyone has these Decca bits - they are the square-section pieces with a
slot in - beware they are not good insulators!

The attached picture shows one of the stand-offs still on fire and the wire
from the other one dangling.

The good news is that after the fire the system was stable, so I am back on
the air again.

Mike, G3XDV
http://www.lf.thersgb.net
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