Re: Capacitance of a coil

> I do think there is a closed form equation for
> such a thing but it has not yet been found.  
There is a closed form equation of sorts.. after all, you are basically
calculating the capacitance of a cylinder against an infinite ground
plane, both of which have analytical descriptions, so you can write an
explicit integral for the capacitance. However, I don't know that the
integral itself has a *convenient* solution. it is sort of like
integrating y  = 1 / ln (x) from x=0 to something. Sure, there is a
solution, but it doesn't make solving the real problem any easier.
> The Medhurst equation is very good and it is a standard any other method
> can be measured against.  However, a good program could be used to generate
> massive amounts of data that could be helpful in finding a closed solution.

Those empirical formulas (Wheeler, Medhurst) etc that come from the NBS
Circular are based on lots of experimental data taken by people who are
truly obssessed with precision and accuracy. I am always amazed by the
thoroughness of those kinds of measurements (I don't have the patience
to account for every speck of dust that may or may not be there, and to
determine its potential effect on the measurement, etc.).

> Does anyone know the claimed accuracy or limits for the Medhurst equation.
> It would be interesting to know were the regions are that it starts to
> loose accuracy?

I would take a look at the famed NBS Circular (C54, I think it is). I've
been meaning to scrounge up a copy since it is sort of the "bible" of
this sort of discussion, but have been busy on other things. I am sure
that they address the errors in the approximations to excruciating
detail. They certainly do on their circulars and bulletins on antenna
gains and noise powers.

Jim Lux                               Jet Propulsion Laboratory
ofc: 818/354-2075     114-B16         Mail Stop 161-213
lab: 818/354-2954     161-110         4800 Oak Grove Drive
fax: 818/393-6875                     Pasadena CA 91109