Re: Medhurst

Tesla List wrote:
> Original Poster: Parpp807-at-aol-dot-com
> Hi all,
> Would someone kindly show me the oft-mentioned Medhurst equation and its
> derivation?
> Is there a text reference to it that is not out of print for a century?
> Cheerful thanks,
> Ralph Zekelman


	Medhurst's "equation" should be in any good electronics or "radio"
handbook.  I have some references which wouldn't help you unless you
have access to a library which can get reprints of old articles.  The
attached text file, which is a QuickBasic program for estimating the
self-resonant frequency of a solenoid, has an expression for calculating
self-capacitance .  (Marked by ******************.)  It (and the
program) should be quite accurate enough for any TC secondary which you
are likely to build. I have found the program to give an accuracy of 5%
or better in predicting the SRF of typical small secondaries.  Note that
the capacitance depends on the diameter and length of the winding, NOT
on the number of turns or the spacing between turns.

	Way back in the 1930's Medhurst (and others - Palermo, for example)
made many measurements of the resonant frequency of coils and derived
strictly EMPIRICAL expressions for capacitance, both in the form of
tabulated constants and expressions which approximate the capacitance
calculated from them. The expression I use is an adaptation of one given
in the RADIOTRON DESIGNER'S HABDBOOK, 4th Edition, 1953, Chapter 11,
DESIGN OF RADIO FREQUENCY INDUCTORS, pp 451-453.  As you can see, my
reference books date back to the days when designers worked with slide
rules and their wits, not computers.