Re: First Light for 10" Coil.

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

John,

Measurement with and without topload is one method to obtain the affects of 
the effective topload capacitance each situation, but not the "only" 
method. There are a few "highlights" I would like to note on your reduction 
factor so that coilers are not mislead.

1) The freespace or exact capacitance of 47pF assumes no ground plane. If 
we were to look at the topload without the secondary, but at the same 
position above ground (about 84"), the 12" x 42.5" toroid would have a 
capacitance of about 50.4pF.

2) The capacitance of 34pF you showed (topload installed and based off 
Fres), is a back-calculation "assuming" the secondary's self capacitance is 
100% unchanged (the DC capacitance or uniform current). We know the 
secondary capacitance changes when operating at Fres. Once the coil is 
operational with the topload installed, "all external and internal 
capacitance" is affected as the currents are displaced. Also, the proximity 
of toroid to coil affects one another. Not only does the topload 
capacitance change, but also the secondary capacitance changes (it would be 
silly to think only the topload capacitance is affected).

3) Calculating for topload with and without is fine to show that the 
topload changes. I have no problem with that. And for any coilers own 
calculations, via program or pencil n' paper, reducing the topload by some 
factor (reduction %, whatever) is fine to arrive at a loaded Fres value.

But I think it is "misleading" to state "the toploads capacitance is 
reduced "this much" when placed on the coil. It does change, but not by the 
factor stated (the factor is a change from the DC capacitance and is what 
should be made clear to coilers).

Solving for C with say C=1/(4pi^2*F^2*L)

F is the resonant frequency
L is the inductance "at the resonant frequency" (=Les, not Ldc)
C is the capacitance "at the resonant frequency" (=Ces, not Cdc)

Imagine if you used F with Ldc to solve for C. You are using a DC value (L) 
with a frequency that does not represent DC values. Would you find Cdc or 
Ces? The answer is "neither" capacitance. You would calculate some value 
"between" Cdc and Ces. This is what your using with your topload reduction.

The factor your using for the effects of the topload is a factor from "an 
exact topload capacitance without ground plane" (which doesn't exist on 
this planet) to a capacitance value that is somewhere between Cdc and Ces. 
As mentioned, I don't have an issue with using a factor in this way to 
arrive at a correct loaded Fres value (that is the goal). But I think  it 
should be clearly stated that the factor involves this method and does not 
represent "actual" topload change in C.

In the old Javatc, I also used the reduction factor for the same purpose. 
With the new Javatc now using Paul's Geotc code, I no longer need to use a 
factor. Now the topload represents the "true capacitance" of the topload at 
it's position and with the effects of the entire system to the degree of 
accuracy of the program.

For a check, you could do this.

1) Install the 12" x 42.5" topload at a height of 84". Run the program. 
You'll find Javatc will show the "exact capacitance" without groundplane 
and will agree with Inca at 47.35pF. Note, the height is really unnecessary 
since there isn't a groundplane involved.

2) Now install a ground plane, say 50 inch radius. Run the program. You'll 
find that the topload capacitance will change to 50.38pF.

3) Install the coil (see below). Run the program. You'll find the topload 
capacitance will change to 42.5pF. Note, this is "not" 34pF as you have 
shown. The difference is Javatc is using Les, Ces, etc.. and "not" the DC 
values.

coil inputs for above: [ r1=5, r2=5, h1=31, h2=72, turns=1150, wire size 
(not awg)=0.035433 ]

In this light, you'll find the reduction factor is:

10.24% : based on exact Ctop [47.35pF] to Ces Topload [42.5pF] at Fres
28.19% : based on exact Ctop [47.35pF] to Cdc Topload [34pF] at Fres
or
15.64% : based on Ctop w/groundplane [50.38%] to Ces [42.5pF] at Fres
32.51% : based on Ctop w/groundplane [50.38%] to Cdc [34pF] at Fres

I sure hope this helps with the understanding of "where" the reduction 
factors are coming from and "how" they are being derived.

I would like to add that I feel strongly that the "true" C of toroid at 
Fres is the difference of the topload above a groundplane at it's position 
with and without the coil running at the resonant frequency. In this case, 
that would be a reduction of 15.64%. But, that is personal preference. For 
any program that cannot yet pull in the distributed current affects at RF, 
then a reduction factor is required to make up for the "unknowns", and in 
this case would be 28.19% as John has shown.

Why? Because Ctop above ground is not known and the programmer must resort 
to C exact without groundplane. Also, because Ces is not known, the 
programmer must also resort to Cdc. There's nothing wrong that. The only 
thing required at that point is finding (preferably by imperical data) 
factors to approximate so that Fres loaded is as close as possible to 
reality. This is what John has done recently with his Ctop calcs.

Take care,
Bart


Tesla list wrote:

>Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
>
>John:
>
>I think it's simple physics.  The top closed turns of the coil act to shield
>the electrostatic field from the underside of the toroid.  This will cause a
>10-20% reduction in the toroid's measured isotropic value.
>
>D.C. Cox
>
>
>
>Dr. Resonance
>
>Resonance Research Corporation
>E11870 Shadylane Rd.
>Baraboo   WI   53913
> > toroid capacitance when placed on the TC secondary. This capacitance is
>much
> > less than the toroid capacitance calculated by the standard equations.
>Why??
> >
> > For example with your coil the 12"x42.5" toroid capacitance using the
> > standard toroid equations is about 47.4 pf. This will not agree with the
> > actual toroid capacitance when it is located on the secondary. The actual
> > toroid capacitance is 34 pf which gives the 84.8 KHz (you measured). The
> > only way to find this actual toroid capacitance when it is on the
>secondary
> > is by measuring the operating frequency with and without the toroid after
> > the coil is built. You then calculate the resonant frequencies with and
> > without the toroid. All of the surrounding conditions that affect the
>toroid
> > and secondary capacitances will then be accounted for.
> >
>
>
>
>