1/4 Wave Theories - What now?

From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent:  Tuesday, June 02, 1998 5:26 PM
To:  Tesla List
Subject:  Re: 1/4 Wave Theories - What now?

At 11:51 PM 6/1/98 -0500, you wrote:
>From:  D.C. Cox [SMTP:DR.RESONANCE-at-next-wave-dot-net]
>Sent:  Monday, June 01, 1998 12:30 PM
>To:  Tesla List
>Subject:  Re: 1/4 Wave Theories - What now?
>to: Terry
>I think most of the capacitance is turn to turn but it would be very easy
>to test your theory.  Measure the cap of a secondary inductor and then
>elevate it approx 25-30 feet above ground (in a sense isolate it from any
>nearby ground plane) and then measure the capacitance again.  If the cap is
>turn to ground the value should diminish considerably --- if the cap is
>turn to turn as I suspect the value should not change very much.  You could
>elevate it on some plastic spools, a wooden pole, or even a bunch of
>cardboard boxes stack up high.  A simple test that will validate your
>theory or invalidate it.

I believe that the self capacitance is made up of two capacitances.  The
turn-to-turn capacitance and a space-charge capacitance around the top of
the coil.  

First the turn-to-turn capacitance:

The turn-to-turn capacitance is the standard capacitance that accounts for
the resonant frequency of any simple inductor.  If we look at the model for
an inductor we see that it suggests three elements. An inductor, series
resistor, and a parallel capacitance that is generally assumed to be the
turn-to-turn capacitance (I won't attempt ASCII art here :-))  This is easy
to measure, as follows, on a Tesla coil.  Simply remove the top terminal and
ground connections from the secondary inductor.  Then add about a 1K
resistor from the top of the coil to the bottom.  This will swamp the
resonance of the coil's external capacitance.  Then measure the Fo frequency
and calculate the parallel capacitance.  This will be the turn-to-turn
capacitance and will be surprisingly small.

Space charge capacitance:

The space charge capacitance is caused by an object being charged in free
space or near a ground plane as we typicaly have in a Tesla coil.  This is
the energy stored by the electrostatic fields around the coil.  This is
measured as we always do but with the turn-to-turn capacitance subtracted.
Since the turn-to-turn capacitance is so small compared to the space charge
capacitance, it is usually not worried with.

I don't have a simple means to do your test (perhaps you have already done
it?) but my first experiment "Non-Liner Coil Winding Experiment" Had in
interesting side light that was not emphasized.  In this experiment, three
coils were compared and the electrostatic fields around them were mapped
(roughly).  These three coils were wound with different and varying pitches.
The two non-linear coils had a vary interesting property they have very
different Fo frequencies and thus different self capacitances depending on
if they were right-side-up or up-side-down. In other words, if you simply
invert these coils the self capacitance changes drastically.  This basically
shows the effect I was talking about.  The description of the coils and
experiment is at my page at:


I will also add Mark Rzeszotarski's analysis of these coils to the page
tonight.  These coils do not match the Medhurst equation as he notes!

Mercifully,  This is a pure HTML document and Mark's notes are in simple
ASCII text format.  There is no Word97 version :-)) 

All the best,

        Terry Fritz   

>> From: Tesla List <tesla-at-pupman-dot-com>
>> To: 'Tesla List' <tesla-at-pupman-dot-com>
>> Subject: 1/4 Wave Theories - What now?
>> Date: Sunday, May 31, 1998 11:21 PM
>> ----------
>> From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
>> Sent:  Saturday, May 30, 1998 3:09 PM
>> To:  Tesla List
>> Subject:  1/4 Wave Theories - What now?
>> Antonio and All,
>> Here is my pet theory about resonances capacitances and such. 
>>         I believe that the self capacitance is the capacitance of the
>> windings to ground.  Each turn has a capacitance to ground along the
>> of the coil.  Each capacitance is separated from the rest by some
>> depending on where they are located.  This produces all kinds of local RC
>> networks.  As you tune around the coil you happen upon points were the
>> resonances add up and cause a peaking effect.  This accounts for the
>> resonance points we see during tuning.  The real key here is that all the
>> currents seem to be in phase along the coil.  No big standing waves as we
>> have all thought in the past.
>>         I should also point out that there is a voltage node at the base
>> the secondary simply because it is grounded there :-)  This zero voltage
>> point has nothing to do with any standing waves.  The currents in my
>> clearly show that the top and bottom secondary currents are in phase.  Of
>> course, the current and voltage are 90 out of phase as in any LC network.
>>         The current at the top of the coil is about 65% of the current at
>> the base.  I believe this is due to the current splitting between the top
>> terminal and the self capacitance.
>>         This is all rather new so there are still details to be worked
>> I have not seen any big problems with this so far.
>> Regards,
>>         Terry Fritz