Cecil Moore wrote:
chuck wrote:
My understanding is that a strong electric field ionizes the air and
leads to formation of a corona.

Yes, but according to the references, fairweather conditions are
not a strong enough electric field to cause corona in the absence
of some other energy source.



That is probably based on the notion
that below a field strength of ~300
volts/meter (Paschen), voltage breakdown
in air is not possible. Fairweather
fields tend to typically be weaker than
that although 200 volts/meter is
sometimes observed.

But Paschen's number assumes parallel
plates, and not sharp electrodes, and
Llewellyn demonstrated sharp electrode
breakdown at 100 volts/meter, a strength
often found in fairweather fields.

There are many other factors that
influence electrical breakdown, such as
ionization of air by naturally occurring
radioactivity (e.g., radon, which would
be another energy source but one which
is usually present near the earth),
atmospheric pressure, etc.

So yes or no? Paschen's law says no. But
Jonassen admits that Paschen's law has
never been tested for non-parallel
electrodes of really small dimensions
and he found it doesn't hold for small
spherical electrodes at separations of a
few cm. Llewellyn reported measurements
below the predicted Paschen minimum for
sharp electrodes.



I can't comment on that, Cecil. I was trying to keep the geometry
simple with a single vertical wire.

Well, a single vertical wire would have its ground plane grounded
so it might also build up a charge between the inner coax conductor
and outer conductor from wind-driven charged particles.

I would think so. And the coax looks
like a capacitor to DC, FWIW.

But wouldn't
a ball on top reduce the possibility of corona just like it does
on other vertical antennas?

It would reduce the possibility of
corona if it has a greater radius of
curvature than whatever it replaced.

73,

Chuck


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