Seeking Advanced Spark Gap Information

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Sunday, April 05, 1998 7:20 PM
To:  Tesla List
Subject:  Re: Seeking Advanced Spark Gap Information

Tesla List wrote:
> ----------
> From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
> Sent:  Saturday, April 04, 1998 11:19 PM
> To:  Tesla List
> Subject:  Seeking Advanced Spark Gap Information
> Hi all,
>         I have been examining the voltage and current waveforms across a
> spark gap with an advanced 30MHz bandwidth fiber-optic probe system.  What I
> see is amazing!!  Honestly, I don't believe it.  I will elaborate more when
> I know this is real.
>         I am seeking any information from any one who may have connected a
> spectrum analyzer to an antenna near a coil or who has any knowledge of very
> powerful high frequency signals being generated by Tesla coils.  These
> signals would be at a fairly constant frequency in the 10-100MHz range and
> would be pulsed in sync with the gap firing.  They would probably be more
> powerful than the fundamental frequency signals and may appear as a set if
> Fourier spikes on an analyzer.
>         Also, anyone who has advanced knowledge of how the spark in the gap
> is generated and maintained during the firing cycle please let me know and
> I'll bounce this off you to see what you think.
>         I may be seeing something that is not real and I am looking for
> independent information that would confirm what I see.  If it is real, this
> whole spark gap business just got much more complex!
> Thanks and I'll explain more when I know I haven't goofed. :-)
>         Terry
>         terryf-at-varinet-dot-com
> or
>         terryf-at-peakpeak-dot-com


I'm very interested in what you have measured. Some thoughts (you
probably already know all this...):

When the gap fires, it has a very pronounced negative-resistance
characteristic - as gap current climbs, the voltage drop across the gap
markedly decreases. Negative resistance devices make excellent
oscillators when coupled to [unintentional parasitic] wiring inductance
and capacitance. A common example of an intentional negative resistance
device is a tunnel diode. When the sparkgap fires, these parasitic L and
C elements are shock-excited, and driven into oscillation, typically at
multiple-MHz frequencies. 

The negative resistance of the gap may permit relatively large amounts
of energy to be coupled into these unwanted parasitic oscillations. The
oscillations and the harmonics account for the realtively broadband RF
"hash" associated with sparkgap operation. BTW, it's these parasitic
oscillations that HV chokes are attempting to suppress when protecting
neon sign transformers. These VHF parasitics also account for much of
the broadband interference seen as the interference "hash" on local TV
recievers whenever the gap fires. 

-- Bert --