Re: Calculating and Measuring Resonant Frequency / Inductance of Secondary Coil (magnifier)

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

Original poster: "Robert Jones" <alwynj48-at-earthlink-dot-net> 

Hi Dan,

For your set up to detect the 1/4 wave resonance the output impedance of
your analyzer would have to be very high (say >50,000ohms) and the lead
capacitance would probably pull the resonant frequency significantly
(depending on the ratio of self C of the coil to lead capacitance).  The
actual output impedance of your analyzer is probably fifty ohms so you
probably measured the 1/2 wave resonance.

Assuming fifty ohms output impedance of your analyzer you should be
measuring the secondary by connecting the signal lead of your analyzer to
the lower end of your secondary and leaving the top end open circuit.  The
ground lead  ( I assuming its a single ended output) should be connected to
earth and preferably connected to a local ground plain or primary coil.  For
accuracy ( better than 10% or perhaps 5%) try to reproduce the conditions
(including top load) in which the coil will be used. Remembering that the
drive is with respect to  the ground/earth (and any near by grounded
components such as the primary coil) and in to the bottom of the coil. If
the analyzer has separate outputs and inputs terminate the drive at the base
with a 3db attenuator and use a x10 probe on the input. If the output is
floating and the input is differential, ground/earth the reference of the
probe and the case/ground of the attenuator, preferably as a star point.
This ensures the drive is wrt the ground/earth, keeps the input within their
common mode range and reduces errors due to poor common mode rejection.

Yes it is hard to only connecting one lead but that's how it works in
practice i.e. similar to a series tuned circuit but the series C is the
capacitance of the coil (and any top load) to ground.

Bob

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, January 11, 2004 7:23 PM
Subject: Calculating and Measuring Resonant Frequency / Inductance of
Secondary Coil (magnifier)


 > Original poster: dhmccauley-at-spacecatlighting-dot-com
 >
 > In the midst of my magnifier design, I came up with a discrepancy.
 >
 > Secondary Coil Specifications:
 > Coilform Diameter:  16"
 > Wire Gauge:  14 AWG (0.064" wire diameter)
 > Turns per Inch:  9T (based on my finished magnifier)
 > Winding Length:  24"
 >
 > When I calculate the inductance and resonant frequency of my secondary
coil
 > (in magnifier coil) using either calculations, or TeslaCad, or WinTesla, I
 > get the following
 > numbers:
 >
 > Inductance:  9.76mH
 > Fres:  365kHz
 >
****************************************************************************
 > *
 > Now, when I measure this secondary coil using an Impedance Analyzer, I get
 > the following:
 >
 > Plot of Impedance vs. Frequency
 > http://www.easternvoltageresearch-dot-com/images/mag1_sec_response.jpg
 > (Note:  Top vertical line of graph is 14mH and each vertical division is
 > 2mH)
 >
 >  >From graph, the flat portion of impedance curve is about 9.5mH which
 > corresponds to what I calculate.
 > However, when impedance is minimum (-2mH), the frequency is about 606kHz.
 > I'm using this minimum impedance point as
 > where the resonant frequency occurs.
 >
 > These frequencies don't match one another, yet I would expect them to.  Am
I
 > missing or doing something incorrectly here?
 > (The secondary coil was placed directly across the two probes of the
 > impedance analyzer when measuring.)
 >
 > Thanks
 > Dan
 >
 >