Re: Fo shift issues with spark C loading? was, Tesla Coil Blunderbusses

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Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi John,
        I think you've made an excellent summary of all the questions:

On 14 Apr 01, at 19:55, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <FutureT-at-aol-dot-com>
> 
> Malcolm, Ed, Ken, all,
> 
> Some have questioned whether the spark's capacitive loading shifts the
> coil's required primary resonant frequency.
> 
> For max spark length at low power
> (27" spark), I had to tune my coil at 19 turns.  For max spark
> length at high power (42" spark), I had to tune at 21 turns.
> Clearly, something is affecting the required tune point at 
> different power levels and spark lengths.  Interestingly, this
> degree of frequency shift based on my tap point position
> agrees with the assumption of 1pF per foot of spark length,
> which Terry has suggested.  I think Greg Leyh found a similar
> figure for the spark's capacitance.  I forget how they determined the
> spark's capacitance.  Was it by back calc'ing the frequency, or was it
> derived some other way?
> 
> I remember that Ken H said that his frequency did not shift
> when his spark broke out of the toroid.  Considering the large
> size of his coil and toroid, the frequency shift would be only
> about 2% or so.  I'm assuming that his measurement method
> would clearly show whether a 2% frequency shift had occured?
> 
> I'm trying to create an explanation in my mind that would 
> satisfy both observations above.  If we accept that Ken's
> frequency did not shift with spark breakout, then the only two
> explanations I can think of are:  (1) the presence of the
> ion cloud around the toroid also shifts the frequency.  Thus,
> his frequency would have already shifted due to this cloud
> before breakout.  But if this is the case, then as the coil
> starts up, the frequency should shift as the ion cloud is
> formed and builds up.  I don't know if Ken checked for this
> effect?  (2) either the sparks do not have any capacitance,
> or if they do, it's not shifting the frequency.  I don't see why
> the sparks would not have capacitance though, or why it
> wouldn't shift the frequency if it does.  If the ion cloud does
> not cause a frequency shift, and if the sparks do not cause 
> a frequency shift, then what could be creating the need for 
> me to retune my coil from 19 turns to 21 turns as the sparks
> get longer?  (For each power level, I retuned for longest sparks.)
> 
> It is true that the primary may need to be tuned lower in
> frequency than the secondary for best power transfer.  But if
> this is the only reason to tune lower, why would the best tune
> point vary with power input and spark length?
> 
> My guess is that the primary needs to be tuned lower in frequency for
> two reasons; to set it to the lower split response, and to compensate
> for capacitive spark loading (or capacitive ion cloud loading).
> 
> I do see what Malcolm is saying about the spark breaking out after
> most of the energy has been transfered to the secondary.  This would
> suggest that the best tune point should not vary with spark length,
> yet in my tests, it did.  It is possible maybe that the ion cloud C
> loading adds just as much capacitance as the streamers? (This ion
> cloud persists between bangs and may affect the needed tune point for
> the primary.)  Alternatively, maybe the sparks break out sooner once a
> lot of ionization has built up along the streamer paths.  This could
> explain why the primary needs to be retuned. Still, if it's the sparks
> that have the greatest freq shift effect due to their capacitance,
> then Ken should have seen a freq shift when his sparks broke out.
> 
> Malcolm suggested that the lower primary tune point may make
> it harder for the energy to return to the primary.  Certainly if the
> lower tune point makes the sparks longer, (due to better energy
> transfer), the sparks will burn up more energy and leave less left to
> return to the primary.  Is there any other mechanism at work?
> 
> If this is the case, then a test could be done using no breakout from
> the toroid.  The coil would be tuned for max voltage or field strength
> from the toroid, at a low power level.  Then the power level would be
> increased, and the coil would be retuned if needed for max voltage or
> field strength.  If the coil needs to be retuned, this would suggest
> that the ion cloud C is affecting the needed tune point even without
> streamers.   I'm not sure if this is a perfect test though.  Does
> anyone have any other comments or insight into these issues? Am I
> missing some point?
> 
> Cheers,
> John Freau

I absolutely buy the 1pF/ft additional capacitance figure. Perhaps 
I've been guilty of muddying the waters. But I had to ask the 
questions considering both the erratic gap firing and extra output 
since there doesn't seem to be a solid explanation of either. At any 
rate, I intend to isolate the offset tuning problem and scope it to 
see whether there is a noticeable change in primary behaviour. I'll 
be glad if there isn't. I will plan an appropriate series of expts 
and carry them out in due course.

Regards,
malcolm