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Re: Formula for true self capacity of a coil.



Hi Gavin,

On 29 May 00, at 12:27, Tesla List 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.

Thanks for doing that. I have to stress that those measurements 
were taken under less than ideal conditions.
      I just tried the formula on my ~10" coil and got a Ccox of 41.2pF 
which seems believable (perhaps a bit on the low side so I will 
recheck my computation). Obviously, trying to take meaningful C 
measurements at home or in the workshop here is a total waste of 
time. I will simply have to get the room.

Regards,
Malcolm