30MHz Spark Gap Testing
From: Dale Hall [SMTP:Dale.Hall-at-trw-dot-com]
Sent: Tuesday, April 07, 1998 8:20 PM
To: Tesla List
Subject: RE>30MHz Spark Gap Testing
RE>30MHz Spark Gap Testing - Is this real??
Terry & List, Your scope observations are very interesting. Good observations.
Would like to see a detailed schematic dwg w/LRC values of set up, incldg load
equivalency the probes, where & how they are connected.
Of particular interest is the value & number of interconnects - looks like you may
be using copper strapping everywhere - what lengths, widths, thickness' ?,
likewise about any RFC bypass caps to gnd (NST centertap). This portion of the
circuit is the most likely to contain HF damped osc from the sparking impulse
excitation ***. The Tesla primary is a highly selective filter to all but the
fundamental resonant frequency, turning any energy into a common resonant
sinusoid (effect of capacitive and inductive resonance at moderate Q)....even with
spark gap opening and closing in series with a previously stimulated LC it
would merely serve to add energy at the resonant freq to the resonating LC -at-
f=1/(2PI*sqr(LC)). The placement of the gap changes little (parallel w/ NST or
series w/ Lpri given the RFC's provide good isolation).
Perform some Lrfc & Crfc calcs for likely activity around 50MHz.
Have you scoped the Igap alone ?, Current between RFC and gap (each side) ?,
and the current between gap and Cpri ? What was Cpri, Lpri for your tests ?
For your 50 ohm terminated antenna test, can you describe it in more detail ?
Are you using transmission line matching to antenna impedance ?
(1/2 wave 50 MHz ~ 9.36', 1/2 wave 200kHz ~ 2340')
I typically find ~2mHz oscillation after spark channel conduction in my coils.
Next time I set things up I'll look closely at the gap = vacuum.
I use TDS 540 usually BW limited to ~100MHz.
*** sparking impulse excitation Ref: HV Engr Fundamentals, Kuffel, Zaengl pgs 375,6
ISBN 0-08-024212-X 1986 Pergamon Press.
Fig 5.34 Trichel pulses: regular pulses before but close to each event of gap breakdown
Trichel studied this effect using static conditions for observation & control for research..
Date: 4/6/98 11:43 PM
To: Dale Hall
From: Tesla List
From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Monday, April 06, 1998 3:36 PM
To: Tesla List
Subject: Re: 30MHz Spark Gap Testing - Is this real??
> From: terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
> Sent: Sunday, April 05, 1998 11:44 PM
> To: Tesla List
> Subject: 30MHz Spark Gap Testing - Is this real??
> Hi All,
> I believe that I have made a new and significant discovery regarding spark
> gap operation. My experiments, which deal with the measurements of voltages
> and currents in the primary circuit, have shown that the operation of the
> spark gap is not that of a simple switch.
> It is generally believed that when the voltage across the spark gap reaches
> a certain level that current passes through the gap and super heats the air
> creating a virtual short across the gap. This short remains in place until
> the primary circuit losses energy and the super heated air region can no
> longer be maintained. At this point, the resistance is believed to rise and
> the gap "quenches" and the resistance returns to a very high level. Typical
> measurements performed with relatively low bandwidth equipment more or less
> have demonstrated this phenomena. Once the gap has closed, the current
> through the gap is assumed to be a simple decaying sine wave.
First of all, one might approximate the gap as a short. In reality
and at "low" frequencies, it isn't really a short at all although it
can approximate a voltage sink, albeit not a super stiff one. It
does consume a considerable amount of power as evidenced by light,
heat and sound. One can also measure losses at some particular
current and width.
> When equipment capable of much higher bandwidth is employed this picture
> seems to change dramatically. When one terminates a quality antenna into
> the proper 50 ohm impedance the fundamental signal seems overwhelmed by
> heavy noise spikes. This explains why a simple wire is often used as a
> scope probe to receive primary waveforms as opposed to a higher quality
> antenna system. The simple wire and its very poor impedance matching to the
> input of an oscilloscope attenuate these noise signals so that only a nice
> clean signal is left.
The antenna is primarily picking up e-field noise isn't it? What does
a high BW current probe say?
> If one uses a high bandwidth properly terminated antenna, a series of noise
> spikes are seen (my testing was done without the secondary inductor in
> place). Careful examination will show these spikes appear as a series of
> noise bursts that occur at the peaks of the fundamental frequency. The
> power of these noise bursts is vastly higher than the fundamental waveform.
> In my testing, I have found that these burst consist of ~50MHz signal bursts
> that persist for about 100nS and then fall off to a much lower level. The
> power of these bursts is remarkable. Early testing has shown that, even at
> low voltage levels, these bursts may reach many hundreds of amps at the
> ~50MHz frequency. By far the most powerful burst is the very first one.
> Typical scope photos often show this as a vertical line that occurs just at
> the beginning of conduction. My equipment has not been able to measure the
> level of this spike with great accuracy but it appears to be around 400 amps
> which is remarkable considering the relatively low 2000 volt spark gap
> setting. What is even more remarkable is that the current outside this
> burst appears to be close to zero. In other words the full current in the
> primary seems to be conducted only in these short bursts.
> Others and I have measured nice sine wave currents in the primary system
> before. So how can the preceding be true? I fear we have been tricked by
> the low frequency response of the equipment we used and the classical
> impulse response of those systems. These energy bursts can easily act as
> pure impulses which will excite a low bandwidth system producing sine waves.
> We may have only been seeing the heavily damped response of these current
I for one have seen these bursts on quite modest equipment and in
greater detail using a 100MHz scope. It might be worth pointing out
that if you see this kind of phenomenon while scoping the secondary
under no breakout conditions, be very suspicious about what exactly
you are monitoring.
> What are the implications if this is true? If the primary circuit is doing
> all of its work at ~50MHz and at hundreds, if not thousands, of amps at that
> frequency, everything changes! The conductors must be short, wide, copper
> strips. The capacitors must be able to withstand even greater stresses than
> we ever imagined. And the spark gaps... who knows? This would imply that a
> much better spark gap or other switching system may give much better
> performance. EMI becomes a major concern. At 200KHz it is easy to
> disregard EMI. At 50MHz all kinds of problems can arise.
But again, does this manifest itself as similar changes in primary
inductor current (not just over exceedingly short wiring paths)? I
suspect not for the simple reason that the inductance is rather
significant at such high frequencies. Still, I could be wrong.
> I have written a paper on all this. It is available in Word 97 format and
> as an HTML web document at:
> Look at the 30MHz Primary Circuit Measurements section. You may want to
> save the graphics and view them with a viewer to get the highest resolution.
> The main page is:
> I realize this is a rather dramatic change in the way we all look at
> primary circuits. I have tried my best to confirm this. Unfortunately, I
> seem to be rather alone in my ability to probe into these regions so
> independent confirmation is difficult. However, I am confident that the
> truth will be determined quickly. As far as I can tell everything appears
> to be working properly and all the results I have seen make sense and appear
> very real.
> Comments are very welcome.