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Re: Help with difficulty finding overtones or nodes?



Original poster: FIFTYGUY-at-aol-dot-com 

In a message dated 11/8/04 7:28:50 PM Eastern Standard Time, tesla-at-pupman-dot-com
writes:

 > The fundamental should be around 465 kHz without a topload,
 >  with the first overtone (3/4 wave) at about 1150 kHz, the
 >  next (5/4 wave) around 1680 kHz, and so on.

     No topload: ~400kHz resonance. With 3 different toploads resonance drops
to 278, 247, and 170kHz. If the overtones should then be within the 600kHz
range of my oscillator, I still couldn't pick them up - either by measuring 
input
voltage at the base or probing along the secondary for nodes.

 >  Set your generator to do square waves and try to excite
 >  the coil with harmonics.

     Hmmm... I think I'd need to obtain another generator. Vintage sine-wave
only vac tube oscillator here.

 > You would need to look for
 >  coil responses through the scope probe, not the oscillator voltage.

     Shouldn't operating at an overtone still cause a change in the
resonator's impedance?

 >  The Corums tried to suggest (erroneously) that 'lumped' or
 >  'transmission line' where two different modes of operation,
 >  over which the coiler had some choice, whereas in fact they
 >  are simply alternative *models* by which the coil may be
 >  analysed.  This appeared to fuel a silly debate which still
 >  flares up everytime someone comes across that dreadful 'Class
 >  Notes' paper :)

     From discussion here, I understand that everything is truly
"distributed", and "lumped" is a convenient model. You could probably argue 
that almost any
useful model for any system is "lumped" in some way, but that's why we have
models! On the other hand, if it doesn't behave "distributed" enough to 
warrant
worrying about it, the "lumped" model is just dandy.
     "Lumped" certainly seems good enough for the average coiler to get up and
running. Certainly good enough for Antonio's modeling, or Pspice simulations.
And the measured performance follows the "lumped" predictions. But I'm
curious about exploiting the "distributed" effects, if they are of any 
significance.
     I don't see how one has choice over the mode of operation, other than
building the secondary differently. I have a feeling that an efficient 
secondary
is going to be pretty "lumped".
     Speaking of efficiency, the Corums say to measure VSWR as a guide, not
spark length. But even if I concentrated on measuring SWR's, what relation (if
any) would they hace to spark lengths? And if they aren't related, what did 
the
Corums think the point of coiling was?

 >  For comments on this, see
 >    http://www.pupman-dot-com/listarchives/2000/August/msg01119.html

     Good job - very succint. I agree wholeheartedly. But the question is
always, "What is gained by going through the trouble of the more 
complex/accurate
model and is it worth the effort?"

 >  For some brief notes on how the overtones are affected by
 >  toploading, see
 >    http://www.pupman-dot-com/listarchives/2004/July/msg00409.html

     If I understood correctly when I first read that, when a topload is
added, it obviously drops the fundamental resonance, but the overtones are 
still
left way up there. The Corums mention a non-linear spread of overtone
frequencies (when the secondary is treated as "distributed"). Either way, 
does that mean
that even with my largest topload dropping the fundamental to 170kHz, the
overtones are going to remain out of the 600kHz range of my current oscillator?

 >  In recent years lots of study has been done on coil resonances.
 >  If you can give details of your toploads, we can calculate where
 >  the overtones will be found.

     Well, I've got three so far: 4x19", 6x24", 8x36".
     Thanks for the help!

-Phil LaBudde