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

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi All,

I would like to mention, that if one is trying to measure very subtle
frequency shifts that temperature, operating frequency, and voltage also
have some effect on a capacitor's actual capacitance.

http://www.wima-dot-com/fkp1.htm

At high current, voltage, and frequency the properties of polypropylene
vary a bit.  So our capacitor's value my be shifting a bit from the meter
measured value in high accuracy full power testing.  The URL from Wima
shows the temperature dependance but I can't find the other graphs right off.

Cheers,

	Terry



At 05:02 PM 4/14/2001 -0400, you wrote:
>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
>