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