Re: Capacitance of a coil
Tesla List wrote:
> Original Poster: Hollmike-at-aol-dot-com
> Ok, for all you diehards who want to try to best the Medhurst formula, I
> this equation in an ooooold electrical engineer's handbook(copyright 1914,
> 1922, and 1936 John Wiley and Sons). I have the 1936 edition.
> This equation was tested on inductors ranging from 1 - 200uH and matched
> the measurements at 30MHz with good agreement(whatever that meant in those
> I have NOT used this or even tried it out, so someone can tell me if
> worth anything. Here goes:
> Cself = pi*D/(3.6cosh^-1(s/d)) in pF
> where D = the coil diamter(in cm), s = pitch of widing(in cm) and d = bare
> wire diam (in cm)
> In excel, that trig function would be ACOSH (or the inverse hyperbolic
> A further note in this book states that the important parameters in the coil
> capacitance are the diameters of the winding and the ratio of the pitch
> winding to the diameter of the bare wire. The capacitance is practically
> independent of the number of turns. Of course, the wire insulation
> and dielectric constant and the form on which the coil is wound will alter
> the capacitance of the coil.
> If you try this, Let me know how it works.
The winding length of the coil is not a parameter in this equation??
When I use Medhurst's formula on my 10" coil, I come very close (15.88
pF) to the measured value (based on reverse calculating from Ls and Fo).
However, if I use the above formula, I only get 3.88 pF. Is there an
error in the algebraic representation above?
BTW, is the book Pender and Delmar's, "Electrical Engineers' Handbook :
Electric Communication and Electronics"? I've got volume IV (Power) but
not volume V. Is volume V worth getting, and how does it compare with
Terman's Radio Engineers Handbook?
-- Bert --