From: Paul
Date: Wed, 06 Sep 2000 10:40:40 +0100
Subject: Re: [TSSP] Low Q Values
Mark S. Rzeszotarski, Ph.D. wrote: > 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. ... Wasn't aware of that. I'd really like to see Malcolm's conclusions - are they posted on the net somewhere perhaps? Did he look at the higher overtones also? I wonder if he would donate his measurements to extend our reference database? If someone has put such effort into a series of measurements we really should be making full use of their results. > 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. I think we're doing a good job of quantifying the capacitance of the coil - the frequencies come out right. Mark - do you disagree with any of the following assumptions which are constraining my thoughts at the moment: 1/ Assuming negligible loss factor for the E field due to operating over a good groundplane - equiv a couple of ohms only. 2/ Must be strong coupling of B field to ground plane - evidence is reduction of coil L by 3% from the Nagaoka value. 3/ A large part of the 'missing loss' may be eddy current losses, evidence: when coil raised, Q goes up. If it were E field loss the Q would go down. > 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. I get 220 unloaded and 147 loaded. > ... effective resistance of 228 ohms for the unloaded coil > and 226 ohms for the toploaded coil ... I get 175 ohms and 160 ohms respectively. Can we compare numbers? Consider the toroided case, I get: (1000 turns, 0.255mm wire radius, winding 0.762m by 0.260m) DC resistance: 70 ohms. Skin depth (97.36 kHz): 0.213 mm Wire area used: 97% Straight conductor AC resistance: 71.8 ohms spacing ratio: 0.67 phi factor: 2.23 (from Medhurst table VIII) Wound resistance: 71.8 x 2.23 = 160 ohms. against your 226 ohms, so one or both of us are wrong. Mark, can you check where our arithmetic diverges? > Lots to ponder here. Yep! Not least that our calculated Q factors go up when toroid removed, the measured value goes down. To me thats another indicator of eddy current losses. I've been trying to calculate the impact of Terry's foil ground plane. Thought I had all the component formulae but its not working. I'm integrating the effect of nested current loops to get silly answers. One reason: I've not taken account of mutual coupling between the nested loops in the foil. Right now I'm throwing my hands up in despair and looking for a handbook formula. Any tips or references anyone? Regards, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.