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Re: Self Capacitance formulas

Hi Ed,

	Mark R. posted the self capacitance formula due to Medhurst
in a previous post. If you use this formula in conjuction with standard
inductance formulas for soleniods (like Wheeler's), you can compute 
the self resonant (no top terminal) frequency of most coils. I have found
that it works very well for predicting most of Malcolm's measured 
frequencies. 

	If you want to read a little more of Medhurst try:
"HF Resistance and Self-Capacitance of Single-LAyer Solenoids" by
R.G. Medhurst, Wireless Engineer, Vol 2, p35, Feb 1947
*Note: the range of frequencies he used does not allow his Q formulas
to work at normal Tesla coil frequencies.


	If you are just interested in computing self-resonant frequencies
there is another method which I have found useful and generally accurate
to about 10% for all coil sizes -  space wound or not. Its limitation is that
it probably shouldn't be used for aspect ratios (Height/Diameter)<1.

The formula is:
                 (1/5)
    29.85 x (H/D)
F = -------------------
     N x D

(hope the ascii came out)
where
F= self resonant frequency in Mhz of an isolated coil 
H= coil height in meters
D= coil diameter in meters
N= total number of turns

Make sure the top line reads " (H/D) to the 1/5 power"
Note that the frequency is a very weak function of the
aspect ratio (H/D), but a fairly strong function of the number of turns
and the diameter.

	This is an adapation of the formula for Helical Antennas found
in Reference Data for Radio Engineers. A form of this equation also
appears in both of the Corum brother's books:
"Vacuum Tube Tesla Coils" and "TC Tutor"

Incidentally, they incorrectly attribute the analysis of the helix to 
Kandoian and Sichak. These guys actually just made a simplification of the 
formula derived earlier by JR Pierce (1947) and Franz Ollendorf (1925).
------------------------------------------------------------------
I have a list which has more complete references.

-Ed Harris