On Wed, 25 Feb 2009 04:04:39 -0800 (PST), wrote:

The advantage of the half wave is exactly for its high impedance in
relation to the loss of ground. The far ground still dominates low
angle launch characteristics, but if (like the large number of radials
offers) you lose less to ground, you have more in the air in all
directions.

....
this leads to 1100Vp voltage (at 1500 Ohms). Without careful
construction, E-field at sharp edges will exceed 3000V/mm easily.
....
This
will not result in full air breakdown (due to strong nun-uniformity of
E-field, but will result in undesired corona discharge.

I would have thought they were the same. (Corona discharge is NOT air
breakdown? Is there some distinction to "full?")

However, this is not an exclusive disadvantage of a half-wave radiator
when a quarter-wave radiator can exhibit similar problems through
similar poor building practices. The same high potential "problem"
exists at the quarter-wave's distant end where the half-wave's is at
the near end, feed point. As the original poster posed this as a
single band antenna, a half wave has practical solutions if the
additional gain is deemed sufficient for the effort.

On the other hand, going from quarter-wave to 5/8ths does yield
benefit that exceeds +/- 10% or 1dB.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Wed, 25 Feb 2009 04:04:39 -0800 (PST), wrote:

The advantage of the half wave is exactly for its high impedance in
relation to the loss of ground. The far ground still dominates low
angle launch characteristics, but if (like the large number of radials
offers) you lose less to ground, you have more in the air in all
directions.
...
this leads to 1100Vp voltage (at 1500 Ohms). Without careful
construction, E-field at sharp edges will exceed 3000V/mm easily.
...
This
will not result in full air breakdown (due to strong nun-uniformity of
E-field, but will result in undesired corona discharge.

I would have thought they were the same. (Corona discharge is NOT air
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.

Field uniformity is only part of it, of course.

Bazelyan & Raizer, "Spark Discharge" from CRC Press, 1998, is probably
one of the better books on this.

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

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.

On Thu, 26 Feb 2009 03:24:28 -0800 (PST), wrote:

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

Hi Wim,

Fair enough in the short description where both require the ionization
of air.

73's
Richard Clark, KB7QHC

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
Indeed.. but St. Elmos fire has many characteristics not shared with
corona.. (the extent of the glow, for one.. corona tends to be a very
small sheath or longer leaders/fingers)

Most likely, it's glow discharge off small water droplets shed from the
surface from which is charged. Basically the process is like an
electrostatic sprayer.. small bumps in the liquid surface form from
whatever cause, and the surface charge tends to make the droplets come
off. They're charged to the maximum charge for the diameter, and as the
droplet evaporates, it gets smaller, causing the glow discharge to shed
charge. Or, a big droplet splits into smaller droplets because of
electrostatic forces.


You can set up a nice demo in a dark lab with something like a cork wet
with sal****er, a HV power supply, and some cookie sheets for electrodes.

Jim Lux wrote:
Most likely, it's glow discharge off small water droplets shed from the
surface from which is charged. Basically the process is like an
electrostatic sprayer..

One very foggy night on Hwy 1 between Carmel and Santa Cruz,
I came up behind a mobile radio in operation. The glow
off the end of the antenna was bright orange. An oncoming
CHP officer stopped the vehicle for having a "red light"
visible from the front of the vehicle. When I left the
scene, the two were arguing whether it was really a "red
light" and whether the operator deserved a ticket or not.
I don't know what radio service was involved.
--
73, Cecil http://www.w5dxp.com
"Government 'help' to business is just as disastrous as
government persecution..." Ayn Rand

Cecil Moore wrote:

One very foggy night on Hwy 1 between Carmel and Santa Cruz,
I came up behind a mobile radio in operation. The glow
off the end of the antenna was bright orange. An oncoming
CHP officer stopped the vehicle for having a "red light"
visible from the front of the vehicle. When I left the
scene, the two were arguing whether it was really a "red
light" and whether the operator deserved a ticket or not.
I don't know what radio service was involved.

I've read that W6AM used to drive the route between Long Beach and San
Francisco. His mobile station is legendary around here. The operator
was no doubt explaining to the officer that a glow discharge in air is
more of a salmon than a red.

Did the officer look like Broderick Crawford by any chance? :-)

I couldn't help but fix the subject line.

ac6xg

On Thu, 05 Mar 2009 08:11:33 -0800, Jim Kelley wrote:

Cecil Moore wrote:

One very foggy night on Hwy 1 between Carmel and Santa Cruz, I came up
behind a mobile radio in operation. The glow off the end of the antenna
was bright orange. An oncoming CHP officer stopped the vehicle for
having a "red light" visible from the front of the vehicle. When I left
the scene, the two were arguing whether it was really a "red light" and
whether the operator deserved a ticket or not. I don't know what radio
service was involved.

I've read that W6AM used to drive the route between Long Beach and San
Francisco. His mobile station is legendary around here. The operator
was no doubt explaining to the officer that a glow discharge in air is
more of a salmon than a red.

Did the officer look like Broderick Crawford by any chance? :-)

I couldn't help but fix the subject line.

ac6xg

Rumor has it that W6AM had a 1 KW Swan amp in his trunk. Plus some
really big Rhombics.

--
73's
Ken Slimmer, WA0SBU