From: "Mark S. Rzeszotarski, Ph.D."
Date: Wed, 06 Sep 2000 06:04:07 -0500
Subject: Re: [TSSP] Low Q Values
Hello All:
At 10:40 AM 09/06/2000 +0100, you wrote:
>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.
Malcolm did not post them to the net. I have a copy of his results,
but he asked that they not be distributed without his permission. He
measured 1/4 wave only. Contact him directly if you want the results. I
computed skin effect, proximity effect, etc. from his data some years back
to see how good Medhurst's tables were. They looked pretty good.
>> 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.
Thinking about it, eddy current losses are probably the culprit. I
measured Q for my small 32 AWG coil last night at 1/4 wave and got Q=193 and
189 without a topload (two independent measurements, Fres=644,191) and Q=179
and 177 with the toroid (Fres=395372), using an HP-606A signal generator (50
ohm output) with a scope. These Q values are in the range I would expect
for 32 AWG wire. They, too, show a downward trend as a topload is added,
suggesting that eddy current losses may be present.
BTW, all of Malcolm's data also demonstrates a falling Q with added
toploads.
>> 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?
I was using less wire spacing than you. I, too, use Medhurst Table
VIII. Are you sure the spacing is .67? It seems a bit off.
>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.
Sure looks that way.
>
>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?
I can compute the mutual inductance between coils using several
geometries. I use a numerical integration technique which gets me answers
within 5% of what I can measure experimentally. There are several series
approximations for estimating mutual inductance but they work poorly with
tesla coil geometries because you end up subtracting terms with large values
in the series. Can you describe the geometry you are looking to solve?
Regards,
Mark S. Rzeszotarski, Ph.D., MetroHealth Medical Center,Radiology
Department, Cleveland OH 44109-1998
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.