TSSP: List Archives

From: "Mark S. Rzeszotarski, Ph.D."
Date: Tue, 05 Sep 2000 13:02:22 -0500
Subject: [TSSP] Low Q Values

Greetings All:

        I am new to the list so please excuse this if it has already been
discussed.
        I see Q values being reported in the 20-100 range for conventional
tesla secondaries and do not agree with this finding.  I expect to see
values in the 200-350 range for typical tesla coil secondaries, based on my
experience.  
        Some years ago Malcolm Watts conducted a series of careful, detailed
measurements on close-wound and space-wound coils, examining the effect of
toploads, H/D and winding spacing on coil Q.  Most of his results were in
the Q=200-350 range.   He did his tests in a large empty classroom with the
coils well separated from metal objects.  As I recall, he built a low
impedance amplifier (emitter follower) for his signal generator to take the
signal generator out of the picture.
        I can see changes in the input impedance (mostly phase changes)
simply by waving my arm six feet away from a base-fed coi connected to my
vector impedance meterl.   I think the Q measurements are being swamped by
the nearby ground planes and are not indicative of the coil Q  (but perhaps
are indicative of the system Q).  The coupling between the coil and ground
plane is probably more capacitive than inductive, since mutual inductance
falls off quickly with distance and is small if the coil is raised at least
two coil diameters above the ground plane.  Capacitive effects are not so
easily sorted out, however.
        My modelling of proximity effects predict Terry Fritz's large coil
to have a Q of 305 unloaded, and 204 with an operating frequency of 97 kHz
(about what one would expect from a 45 inch toroidal topload).  This is
based on an effective resistance of 228 ohms for the unloaded coil and 226
ohms for the toploaded coil (drops slightly due to lowered operating
frequency).  The expected DC resistance of the wire is about  70 ohms,
yielding a ratio of (Rskin+Rproximity)/Rdc of 3.25, which is similar to what
was reported by Medhurst years ago for a 3:1 coil.
        Lots to ponder here.
Regards,
Mark S. Rzeszotarski, Ph.D., MetroHealth Medical Center,Radiology
Department, Cleveland OH 44109-1998


Maintainer Paul Nicholson, paul@abelian.demon.co.uk.