Seeking Advanced Spark Gap Information

From:  Greg Leyh [SMTP:lod-at-pacbell-dot-net]
Sent:  Sunday, April 05, 1998 3:37 AM
To:  Tesla List
Subject:  Re: Seeking Advanced Spark Gap Information

>         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.

Can you post a waveform?

>         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.

Since your msmt bandwidth is about 30MHz, could the probe/front end
possibly be ringing around its freq limit?  This could be tested on 
the bench with a fast pulse gen. 
If that's not the cause, then the gap may be generating pulsating 
leaders as the spark closes the gap.  Are you seeing a fast chain 
of high current spikes, just before the gap fires?

>        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 captured some waveforms of the arc current at the top electrode
on my coil a couple of weeks ago, just before we tore it down.  
These waveforms can be viewed at:


Perhaps the most interesting one with regards to arc propagation
is the 2uS/div one, where dart leader action is clearly visible,
with time structures around 400nS, or 2.5MHz.  This type of dart
leader generates a lot of the RFI in natural lightning strikes,
and may be producing some of the HF spectrum that you are seeing.

Another noteworthy point is that the advancing dart leaders appear 
to exist only on _positive_ voltage swings at the top electrode!  
This highly asymmetrical nature of the main arc current may explain 
some of the DC electrostatic phenomena that R. Hull and others 
have observed around TC's.