At 03:13 13/03/2002 -0800, you wrote:
BTW I have had some problems with burning at the top of the fibre glass
mast and am wondering whether the filler might be carbon black??
73, John, G4CNN
Dear John, LF Group,
I think the carbon is just what is left after the resin bonding the
fibreglass has been decomposed by the discharge. I'm sure a chemist could
tell you more, but I think it is true that virtually all resins are organic
compounds that leave a carbon residue when burned. I have had similar sooty
residues after arcing on various types of plastic, as well as fibreglass
and wood - usually the discharge leaves an interesting branched pattern of
the black residue. The process seems to be a vicious circle - once the
residue is formed, it provides a conductive path for the discharge to
spread along - much the same as the "tracking" you sometimes get on car
engine ignition components. I have actually seen it happening - little
white arcs move across the surface of the plastic, leaving a smouldering
trail behind them.
On my antenna I have largely stopped this happening by fitting "corona
rings" - these are just hoops of wire about 100mm in diameter attached, and
electrically connected to, wherever the highest field gradients are - the
ends, corners, and junctions in the wire normally. They reduce the
potential gradient and suppress the discharge.
Cheers, Jim Moritz
73 de M0BMU