[Lf] corona effect]

[Andre Kesteloot]

Vernall wrote:

> 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