Secondary resonance freq

From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Monday, June 08, 1998 12:20 AM
To:  Tesla List
Subject:  Re: Secondary resonance freq

Hi Marco,
            Very good question that deserves an answer:

> From:  Marco Denicolai [SMTP:marco-at-vistacom.fi]
> Sent:  Friday, June 05, 1998 1:11 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Secondary resonance freq
> I measured yesterday the resonance frequencies of my secondary. It is a
> 4.3" diameter PVC tube, winded for a length of 19.7" with AWG 26 wire.
> I got the following results:
> 324 KHz
> 813 KHz
> 1475 KHz
> 1766 KHz
> 2053 KHz
> These without any top toroid or sphere. I used a function
> generator (sine wave) and oscilloscope.
> I was waiting to measure the fundamental and all odd harmonics
> (x3, x5, etc.) but can anybody explain to me how I have got the
> above readings?

To All: the following is an attempt at an intuitive picture of what 
is going on. Feel free to correct/flame etc. This is right at the 
edge of what I know.

     I too have noted such odd spurii in experiments and this was 
with end feeding from a 7 Ohm resistive source. 

     Without a terminal you are right into free resonator territory. 
There is a less well defined boundary at the free end consisting of a 
bit more capacitance per turn than the bulk of the resonator than you 
get with a terminal stuck on the end. Other capacitances that would 
be swamped by a terminal if present are coming into play. You can see 
this effect also in bundle wound flyback transformer secondaries. It 
makes tuning a plasma globe supply using a driven oscillator an 
interesting experience. You can also see it in the analogue of an 
unbounded medium such as water in a lake. Ripples of many different 
frequencies can co-exist. NB - quartz crystals can also oscillate at 
many frequencies but I think most are harmonically related. 

     Perhaps I should defer to the RF experts on the list. I feel you 
have asked an excellent question which nobody seems to have addressed 
and I would be keen to hear more too.