# Re: Capacitance of a coil

```Ok, for all you diehards who want to try to best the Medhurst formula, I found
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
days).
I have NOT used this or even tried it out, so someone can tell me if it is
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
cosine).

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 of the
winding to the diameter of the bare wire.  The capacitance is practically
independent of the number of turns.  Of course, the wire insulation thickness
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.
Mike

```