Re: Quarter Wavelength Frequency

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

Original poster: "Antonio Carlos M. de Queiroz" <acmdq-at-uol-dot-com.br> 

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

 >...
 > 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.

A very sensitive method to observe the resonances of a coil is to
drive it through a tuned primary circuit, as in a Tesla coil.
In place of the gap place a low-impedance square-wave generator,
generating a frequency of about 1/100 of the expected resonance
frequencies, and tune the primary circuit to find the resonances.
At each of them you will see full beats in the voltage over the primary
inductor. The observed oscillation frequency is the resonance
frequency of the secondary too.
There is no need then to observe what is happening in the secondary.
The setup may be this, with a square wave generator made with a
555 and a buffer:
http://www.coe.ufrj.br/~acmq/tesla/tuner.gif
Change C1 (easier) or a tap in L1 for tuning. I use a few capacitors
and a large veriable capacitor, all in parallel connection, as C1.
When you find one of the resonances, slide a finger along the
secondary coil, and you will see clearly where are the voltage
nodes (zeros) along it, places where the presence of the finger
causes less perturbation in the waveform.
It's a funny experiment. It's easy to see many resonances in this way.

Antonio Carlos M. de Queiroz