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.