From: Paul
Date: Wed, 21 Feb 2001 17:00:35 +0000
Subject: Re: [TSSP] Problems with high elevation/small radius
Hi Mark, Wasn't sure if you were working away at the moment. > I have recently acquired a nice Pearson current transformer Yes, perhaps you can tune for max Ibase so that you don't have to probe anywhere near the top of the coil. > Also, the snow is letting up so I may be able to work outside soon. In view of the results for your coil mz3012-1: bare d=0.089m h/d=3.18 sr=0.88 b/h=0.07 turns=622 f1 647.8kHz 696.2kHz +7.5% f3 1575.4kHz 1656.0kHz +5.1% f5 2264.1kHz 2369.4kHz +4.7 it seems reasonable to persue indoor measurements on these small coils at low elevation. I think that, providing the coil is low over a good ground plane, the presence or absence of a surrounding room would only make on the order of 1% difference, if that. > Can you spell out explicitly what you want/need to sort this out > more fully? You sent me (in mrz123000.txt), data for a 3.5"x11.125" coil at 5 heights above a concrete floor. The 'mz3012-1' above is this coil at its lowest position. The ambiguity comes from reliance on the concrete floor. Perhaps you could re-measure f1,3,5 of this coil at a winding start height of 0.75" above a tinfoil surface, with the foil extending out say around 18" radius from the coil? The tinfoil would connect to the feedline screen. Could you also make a note of the DC inductance of the coil, in-situ over the foil, as a check on the extent of eddy coupling. In your mrz123000 measurements you used a plane antenna 30" above the coil and displaced 32 inches to the side. Could this add up to 1pF to the top of the coil, I wonder? If you use the CT to find the point of max Ibase, then you can eliminate the plane E-field antenna and this potential source of disturbance. It would be reasonable to dispense with the 3.2 ohm shunt resistor for now, since we're just interested in the frequencies, and not Q or Vbase. Somehow I've a feeling that the results will still be several percent below the predicted value, which would confirm that we have a 'small radius' problem. In which case I wonder if you have another piece of acrylic tube which will fit inside the existing tube. If so, a measurement of f1 with the inner tube inserted might give us an idea of the effect the acrylic's relative permittivity (circa 4 I believe) on the total effective capacitance. (Or maybe a wider tube, surrounding the coil, would do?). > What kind of ground plane do you expect? For now, just use tinfoil without any consideration for eddy currents. Eddies will tend to raise f1 a little, but nothing like amount of error in question. > I have, or can wind coils of pretty much any h/d in the 2-4" > diameter size. The coils for which you've already supplied results are proving quite enough of a challenge, so no need to introduce any more. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.