# Re: Formula for true self capacity of a coil.

```Hi Dick,
actually there were slight differences between the inductance values
calculated by Malcolm and those by myself, even though we both used the
Wheeler formula. In the end I just posted Malcolm's original values, but
probably used my own in the actual calculations. Even if yours ahd been used,
I guess the difference would not be much.
However, I have personally run into an interesting little problem with the
Wheeler formula, but this I shall describe in a separate posting.

As for the formula for the intrinsic capacity of a coil (Ccox), I have a
little question to pose; does this formula only work for an upright coil, what
it the same coil were horizontal to ground instead or even far removed from a
ground plane?

Regards,

Gavin

Tesla List wrote:

> Original Poster: "Richard Kircher" <richard.kircher-at-worldnet.att-dot-net>
>
> Gavin,
>        Great posting!  One problem though is that on Coil 1 I can compute
> the exact same coil inductance.  Using the same formula and your number of
> turns and dimensions, I get different inductance for Coils 2 and 3, i.e.,
> 28.82 and 42.67 mH.  This would change your comparison of resonate
> frequencies.  Am I missing something?
> Thanks, Dick
>
> At 12:27 PM 5/29/00 -0600, you wrote:
> >Original Poster: "Gavin Dingley" <gavin.dingley-at-astra.ukf-dot-net>
> >
> >Hi Bob, Terry, Malcolm,
> >I think I may have found a formula for true self capacity of a coil,
> >well this is it:-
> >
> >
> >C = (  (11.26 * H)   +   (  (16 * R) + ( 76.4 * R^(3/4) )  )    /
> >(sqr H)
> >
> >C is the coil capacitance in pF, R is the coil radius in meters and H
> >the coil height in meters.
> >
> >I hope that makes sense?!
> >
> >It came from a TCBA article written by D. C. Cox. Pres., Resonance
> >Research.
> >
> >Measured Values for Capacitance of Spheres and Toroids
> >by:  D. C. Cox. Pres., Resonance Research
> >Volume 17, #3 TCBA News 1998
> >
> >A photocopy of the article was sent to me by Don Butler, a fellow coiler
> >in the U.K.
> >I had sent the article to Bob Jones, but there appeared to be
> >complications in reading it, so I tried the formula my self to see what
> >would happen, the result looked good.
> >
> >I wrote a program that calculated coil inductance, Medhurst C and "Cox"
> >C, along with the lumped LC resonant frequency using Medhurst and
> >resonant frequency using the Jones formula with Cox's self capacity of a
> >coil.
> >I then plugged Malcolm's coil data from an earlier post into the program
> >and this is what came out:
> >
> >Coil 1)
> >
> >Hs = 21.75",     Ds= 4.75",     N approx 870t,    L = 17.87mH
> >1/4 wave = 394.5kHz
> >
> >Cmed = 8.8pF
> >
> >Ccox = 22.18pF
> >
> >Lumped LC (using Cmed) fr = 401.43kHz
> >
> >Jones (using Ccox) fr = 397.07kHz
> >
> >-------------------------------------------------------
> >
> >Coil 2)
> >
> >Hs = 22.75",     Ds= 5.95",     N approx 910t,    L = 29.3mH
> >1/4 wave = 269kHz
> >
> >Cmed = 9.9pF
> >
> >Ccox = 24.64pF
> >
> >Lumped LC (using Cmed) fr = 298kHz
> >
> >Jones (using Ccox) fr = 296.678kHz
> >
> >-------------------------------------------------------
> >
> >Coil 3)
> >
> >Hs = 16.2",     Ds= 9.05",     N approx 650t,    L = 42.25mH
> >1/4 wave = 212.7kHz
> >
> >Cmed = 11.19pF
> >
> >Ccox = 33.6pF
> >
> >Lumped LC (using Cmed) fr = 230.41kHz
> >
> >Jones (using Ccox) fr = 208.78kHz
> >
> >-------------------------------------------------------
> >
> >The Jones formula is;
> >
> >fr = 1/ (4 * sqr(L*Ccox) )
> >
> >Well it appears to work, and using Medhurst C in the Lumped formula also
> >seems to work two. Of course, the Jones formula is more versatile when
> >calculating wavelengths other than 1/4-wave.
> >
> >Regards,
> >
> >Gavin, U.K.
> >

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