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*To*: tesla-at-pupman-dot-com*Subject*: Re: Formula for true self capacity of a coil.*From*: "Gavin Dingley" <gavin.dingley-at-astra.ukf-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)*Date*: Thu, 01 Jun 2000 07:58:18 -0600*Approved*: twftesla-at-uswest-dot-net*Delivered-To*: fixup-tesla-at-pupman-dot-com-at-fixme

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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