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# Re: Resonance

```That's a reasonable value, but how to calculate it is:

Determine your coil's inductance, using this equation:
L=mu*N^2*A/sqrt(l^2+4*a^2)
where mu is the magnetic permeability of the core (4*pi*10^-7 for
air-core),
N is the number of turns (count turns per inch),
A is the cross-sectional area of the coil in square meters,
l is the length of the coil in meters, and a is the radius of the coil
in meters.

Next, calculate its Medhurst constant using this formula:
K=.585-.25442*(h/d)+.15563*(h/d)^2-.02777*(h/d)^3+.00172*(h/d)^4
where h is the height of the coil, and d is the diameter of the coil
(units don't
matter, as long as they're the same)

Now, you can calculate its self-capacitance by multiplying the Medhurst
constant
by the diameter of the coil in centimeters:
Cself=K*d
where Cself is in picofarads.

Therefore, to determine its free-resonant frequency (frequency with no
topload),
use this formula, using the self-capacitance for C. If you want to get
its actual
resonant frequency, add the self-capacitance to the topload capacitance:
F=1/(2*pi*sqrt(L*C))
where L is in henries, C is in farads, and F is in hertz.

And to get the ideal frequency (quarter-wave resonance), use:
Fq=299800000/(Wl*4)
where Wl is the secondary wire length (not winding length) in meters,
and Fq is
in hertz.

Tesla List wrote:

> Original Poster: Philip <shadow42-at-totcon-dot-com>
>
> I have recently been trying to determine resonance of my coil. I dont
> really know
> what value it has, but ive seen that quite a few people have measured
theirs.
> Does 243khz sound reasonable for a 4.5 x 20" coil?
>     Thanks
> Philip Mac Duffie

--
--Mr. Postman (Doug Brunner)
<dabrunner-at-earthlink-dot-net>

```