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

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

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