From: Paul
Date: Tue, 05 Jun 2001 00:59:19 +0100
Subject: Re: [TSSP] Interesting article on Medhurst, Wheeler, modeling,
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.