TSSP: List Archives

From: "Terrell W. Fritz"
Date: Mon, 04 Jun 2001 19:05:04 -0600
Subject: Re: [TSSP] Interesting article on Medhurst, Wheeler, modeling,

Hi Paul,

As I look at it more and more, you are indeed correct.  He got the program
to do the mutual coupling and the capacitances and got the values pretty
close but he really did not see the "light"...  Not too much really new
stuff there but it is just packaged nice :-)  I read this thinking back
when "I" was going to write the same wonderful paper :-))  However, I
realized that something was all screwed up, but did not know what...  The
program he uses just gives as many harmonic peaks as there are sections in
the program.  Since his model is loss less, they will go on forever as you
add more sections, the amplitude will stay the same.  A quick check of a
real coil looking at the harmonics quickly demonstrates that they diminish
drastically as the frequency goes up which is hard to explain with the
model given.  His transmission line elements probably have losses but he is
never going to get it to really work adding resistors here and there (been
there, done that :-)))

Cint is very key in all this and is what finally really made this all work.
 However, he (nor I) had a nice canned program component to do that ;-))
Malcolm's test coils probably did a far better job in this area but I don't
think they had the mutual inductances...  So close, yet, so far...

Forgive if you have talked about this before (I get a LOT of mail and loose
track...) but are their any areas in your present models that seem weak to
you or are giving odd results?

Cheers,

	Terry

At 12:59 AM 6/5/2001 +0100, you wrote:
>Terrell W. Fritz wrote:
>
>> He seems to have a pretty unique perspective on all this.
>
>I don't know,  his approach in these two papers is a pretty
>conventional treatment of a solenoid as a transmission line.
>
>The odd thing is that in the intro, the author expresses
>surprise that a solenoid exhibits a spectrum of resonances,
>which surely is common knowledge?  
>
>His attitude to the internal capacitance is quite common,
>and was my own thinking too, briefly. See the first couple
>of posts in the tssp list archives.  Robert Jones wrote to
>me saying he thought the Cint could be safely neglected, on
>the grounds that turn-turn storage is small.  He had me 
>convinced for about half a day, until I thought it through.
>It all seems pretty clear now. The temptation to drop Cint
>is pretty strong, because it simplies the math quite a bit,
>and you get (almost) the regular x-line theory, but if you
>do that, you get distinctly wrong answers for the higher mode
>frequencies.  The answers improve steadily as h/d is increased,
>so for long solenoids, you'll get away with it.  Below h/d=10
>you need to consider Cint to even get in the right ballpark
>with f5 and above.  Down at h/d=1 and below, his answers will
>be even more wrong, as Cint begins to dominate down there.
>
>There's a dip in the capacitance at around h/d=1, as Cext
>hands over the lead to Cint, the exact min C h/d depends on
>the locale around the solenoid, walls, etc, not to mention
>the topload.  Medhurst spotted this dip, but hadn't separated
>out the C contributions in a way that would make sense of it.
>
>Cheers,
>--
>Paul Nicholson,
>Manchester, UK.
>--


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