Re: Quarter Wavelength Frequency

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: Paul Nicholson <paul-at-abelian.demon.co.uk>
 >
 > Ed wrote:
 >
 >  > As L/D gets large the ratio goes to 1/2, just what one would
 >  > expect with a straight wire.
 >
 > The fact that it tends to such a round number with mathematical
 > precision suggests that the figures you give are simply
 > revealing some mathemematical relationship inherent in the
 > equations you are tabulating.
 >
 > For us to say more, you would have to describe the steps in
 > the calculation, and in particular how you are determining
 > lambda.
 > --
 > Paul Nicholson
 > Manchester, UK.
 > --

	Answer to last question first; it's a very simple program.  For lamda I
use c/Fr, where Fr is the self-resonant frequency calculated using
Lundin's approximate expression for inductance and a power series
approximation to Medhurst's data as given in an old handbook I have.  In
other words, the program calculates L, calculates (estimates?) C, and
computes the parallel resonant frequency of the combination.  I'm sure
the accuracy of the calculations doesn't justify three significant
figures or maybe even two!!!!!  However, I have made the calculation for
several different coils and compared the calculated frequencies and was
quite surprised to find agreement within a couple of percent.  I attempt
to measure Fr by feeding the bottom of a coil, isolated as far as
possible from nearby objects, with a low-impedance source and observing
the "top voltage" with a small plate remotely located and again "as far
as possible" from the coil.  Crude but works OK.  Of course, it should
also be possible to observe the voltage at generator port and looking
for the frequency giving a dip, but that seemed a little artificial to
me.  The signal generator I'm using is a standard "Hewlett Packard"
circuit, since it's the only one I have putting out significant voltage
in the region above 100 kHz.  It doesn't go high enough in frequency to
observe any overtone responses.

Ed