Rik,
I have some comments to make on corona:
I just received a question about the corona effect with 136kHz antennas.
Maybe it's an interesting item to add to my LF antenna webpage, therfore I
would like to know if someone already experienced this effect on 136kHz
(with what antenna and power/antennacurrent).
Some years back I had a fire start on feeder insulation that I later
attributed to starting by corona discharge. The "mistake" I think I made
was to use insulating tape that presumably had a high dielectric constant,
and the tape winding had a fairly sharp edge. The fire appeared to start
from the edge of the tape. I was running around 500 watts on 182.7 kHz SSB
and I happened to hear a "funny noise" outside the shack, so I stopped
transmitting and rushed outside to find the feeder ablaze. A nearby hose
was used to put out the fire. The XYL was not impressed.
Some investigation showed that corona discharge is basically a voltage
gradient (local field strength) phemomena. My feeder was in a thousands of
volts per metre situation. It pays to avoid sharp edges, even in dielectric
material. Where nuts and bolts are used on an antenna wire, protection from
breakdown can be achieved by using a wire ring around the nut and bolt, with
a wire bonding it to the hardware. Such a "corona ring" reduces rather than
initiates corona discharge! The highest field strength region of a
top-loaded vertical with a ground-mounted loading coil is at the top of the
loading coil and the first part of the up-wire.
73, Bob ZL2CA
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