From: "Terrell W. Fritz"
Date: Wed, 06 Sep 2000 12:52:32 -0600
Subject: Re: [TSSP] Low Q Values
All, So the consensus sounds like my Q measurements have something wrong. :-| I used a low Z signal generator based off Malcolm's so that should be ok. I will play around and see if I can find to source of the error. I have removed the plane now but I can also try taking it out side. I will review my measurement techniques too to be sure I have not made some basic error... Cheers, Terry At 01:02 PM 9/5/00 -0500, you wrote: >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.