On 26 feb, 02:09, Roy Lewallen wrote:
Jim Lux wrote:
breakdown? *Is there some distinction to "full?")

One often makes a distinction between a corona discharge which exists as
a steady state sort of thing and the streamers which precede a "spark".
*Both are air breakdown phenomena, but qualitatively different, and both
are different from a low pressure discharge like that found in a
fluorescent lamp or neon bulb, or from phenomena like St Elmo's Fire.

* . . .

Most interesting. I've always thought that St. Elmo's fire was a corona
discharge, and a quick web search indicates that it's apparently a very
widely held misconception. What's the difference?

Roy Lewallen, W7EL

Hello Richard,

A "full air" breakdown is the situation where there is a full
conducting path between the two conductors through air. So the
current that goes into the conductors is mainly supported by real
electron flow through air and not dielectric displacement current.
The effect is strongly non-linear and shows hysteresis.

Imagine you have an antenna and you increase the input power
gradually. At a certain point somewhere in the construction air will
break down. When you have a full breakdown in a uniform field, you
will notice a stepwise change in SWR. To stop the breakdown, you have
to reduce the input power significantly (hysteresis effect). As most
amateurs have an in line SWR indicator, you will notice full air
breakdown and will check the installation.

In an RF "corona discharge", air only breaks down in the high field
strength area. As you will probably know, field strength is highest at
air/conductor boundary with small curvature (edges, needle tips).
Outside that area air will not break down and current in that region
is supported by displacement current. This effect may show only minor
hysteresis and you may not notice this during normal operation.

The voltage required to establish a corona discharge may be far below
the level to get a full air breakdown.

At the tips of a HW radiator, you will have highest field strength.
However when you bring a conductor close to that tip (and retune if
required), field strength will increase. This is also the case with HW
end-fed radiators. At least the ground of your feeder, or the ground
of the matching network will be relative close to the end of the HW
resonator. For some formulas see the document referenced before.

I hope this clarifies the "full air breakdown" and "corona discharge"
issue as used in my postings.

Best regards,

Wim
PA3DJS
www.tetech.nl
the address is still valid, but don't forget to remove abc.