Fwd: Re: Terry's New Plane Wave Antenna

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Terry Fritz" <terry@xxxxxxxxxxxxxxxx>

From Bert Hickman

Tesla list wrote:

> Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>
>
> Tesla list wrote:
>
>> Original poster: "Dmitry (father dest)" <dest@xxxxxxxxxxx>
>>
>>> Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx> Hi,
>>>  It is well known that coils tend to charge up the surroundings
negatively due to the fact that negative voltages arc "easier"
>>> than positive ones.  Ie, mercury arc rectifier.
>>
>> ? B&R "Spark discharge", page 284: "6.11.1. Negative leader. [...] it`s
more difficult to make the breakdown by the negative voltage,
>> we need higher voltages to do that."
>
>
>
> The book is wrong. Corona really forms more easily on negatively
> charged bodies. Or it appears to do so. In an electrostatic machine
> producing symmetrical voltages, longer sparks are obtained when the
> positive terminal ball is smaller than the negative, so the positive
> side breaks down first. Otherwise the negative ball is observed to
> emit corona while nothing happens in the positive, and sparks do not
> form (why, I don't know). Probably, this is more related with spark
> generation than with breakdown voltages, and I don't remember seing
> actual measurements of breakdown voltages with positive or negative
voltages, that really don't have reasons to be different.
>
> Antonio Carlos M. de Queiroz
>

Antonio, Dmitry and all,

Actually the B&R book is correct. It is true that initial visible corona
breakdown begins at a slightly lower E-field with a negative
polarity than with a positive polarity (in a strongly divergent
E-field). However, it's also true that, during longer spark formation,
streamers form and propagate at a significantly lower terminal voltage
for positive polarity than for negative polarity. That's why Antonio's
empirical observation is correct AND why B&R's book is also correct for
long sparks. Some graphs showing these polarity effects for corona
through streamer discharges can be found here:
http://www.imp.gda.pl/ehd/el_disch.html

More detailed descriptions of positive and negative corona can be found in:
"Positive and Negative Point-to-Plane Corona in Air", W. N. English,
Phys. Rev. 74, 170-178 (1948) and "Point-to-plane corona: Current-voltage
characteristics for positive and negative polarity with evidence of an
electronic component", G. F. Leal Ferreira, O. N. Oliveira, Jr., and J. A.
Giacometti,  Journal of Applied Physics -- May 1, 1986 -- Volume 59, Issue
9, pp. 3045-3049.

A good discussion of polarity effects for streamer and leader
propagation can be found in "Gas Discharge Physics", by Yu P. Raizer,
Springer, 1997 (corrected edition), ISBN 354019462-2, section 12.8.2,
pages 361-362. As an example. Raizer mentions that in a rod-plane gap
geometry the breakdown voltage for a negatively polarized rod is roughly
twice that of a positively polarized rod.

Numerous similar comparisons exist in the literature, including EHV
power transmission and lightning studies. BTW, this asymmetry appears to
be true for breakdown phenomena within liquid and solid dielectrics as well.

Bert
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