From: Paul
Date: Fri, 15 Jun 2001 21:06:55 +0100
Subject: Re: [TSSP] Cint experiment
I wrote: > Related to the Cint issue - does anyone fancy building a small > h/d coil with around a dozen or so tapping points in order to > measure a current profile? The point being that a direct current profile measurement would provide unequivocal evidence of the elevated current maximum caused by current circulating through Cint, thereby demonstrating beyond any doubt that longitudinal capacitance is having a big effect. Has been done before - Breit in his 1921 paper shows some current profiles, but doesn't give much detail. Surely it has been measured and since, but I've not yet turned up any references. So it would be very nice if we could measure our own quantitative current profile. The other way to prove Cint is by making a coil on a former thin enough that its material dielectric does not add too much capacitance, so that the higher mode frequencies agree to say, 1%. This is a less direct method - I'll say more about the prospects for this in another post. Finn Hammer wrote: > Yes, I will do it, but can I complete the measurement without a > digitizing scope? A measured profile to match the computed profile shown in pn2511, figure 6.1, which is h/d=1, would be ideal. A smaller h/d would make the profile harder to measure, and a larger h/d would take the current maximum further towards the bottom, and it would be reduced in amplitude. A digitizing scope is not necessary for this test. The coil must be constructed with about a dozen or so series tapping points, distributed around the circumference so that the taps are roughly evenly scattered over the whole coil. They don't have to be a fixed number of turns apart. Two methods of measurement: a) Insert a current sensing device into each tap, one at a time, small and self-contained. Only has to give a relative current indication. Adjust base drive voltage to get the same current reading at every tap. Take a final measurement with the current sensor in the base lead. b) Insert a resistance into each tap. Measure the Q. Bypass the resistance and adjust a variable resistor in the base circuit so that the Q is the same. Only needs a relative Q indicator, but it needs to be fairly high resolution. Probably worth doing both methods, having gone to the trouble of making a special coil. > I would like to do it on a 200 mm dia. former, since I have special > jaws for the lathe to chuck them without making them unround. 20cm might be a little small - depends how small and unobtrusive the tapping points were, and how many. Could be just about useable, but will need a bit of thought. > Pls. choose H/D ratio and turns, I'll have a think about that. We need to make sure the coil Fres will be in range of your instruments. > I will resume measuring on the low H/D ratio coils as soon as the > frequency counter arrives (thanks, Kurt!) A fresh set of measurements on your 500 turn coil would be most useful. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.