From: Paul
Date: Sat, 09 Dec 2000 07:26:48 +0000
Subject: [TSSP] Coils at high elevation
I've been tackling some resonant frequency measurements sent in some time ago by Malcolm Watts and Mark Rzeszotarski which apply to coils at a high elevation above the ground. Attempts to predict the measured frequencies for these coils have consistently failed, the predictions being around 10% to 20% high. This has been an ongoing issue for quite a while and some difficulties with the laplace solver were identified and cured by discarding the program. The replacement software, which uses a completely different method to determine the capacitance matrix is still not complete but is currently able to supply results to about 4% accuracy for bare coils, so I've been doing some preliminary trials. Using the new program the f1 error for H/D=1 in Malcolm's catagory (a) coil 1 measurements is +7.7%, and for Mark's 875 turn coil, +16.3%. These errors can no longer be attributed to defects in the capacitance determination software and must be due either to an inadequate representation of the environment, or more worryingly, a problem with the application of internal capacitance within the model. I'd like to find out a little more about how the two coils in question where situated. Mark's coil was standing on an insulated platform. Malcolm, could you say how your coil was supported at 26" elevation? I've been assuming that the base feed to the coils was via a thin wire, and that the wire rose vertically to meet the coil base. Now this may not be the case, since Malcolm was aiming at Q factor measurements (and mentions a strap feed) and Mark was making simultaneous measurements of f,Q, and Zin. Modelling the effect of a thicker feed wire suggests it would take quite a wide strip feed line coming out horizontally for some distance to achieve the extra capacitance required, and I suspect you'll both tell me that's not the case. Any other suggestions as to what may be introducing extra capacitance to the coil are most welcome! This is quite an important tesla secondary configuration as it emphasises the effects of internal capacitance, has the highest transimpedance, and is of particular interest to bipolar coils. For the same reason, it is also particularly proximity sensitive. I wonder perhaps if anyone is in a position to take measurements of the f1, f3, and f5 frequencies on a small suspended coil, outdoors, with the coil suspended on a fishing line or similar and fed with a very thin wire rising vertically to the coil base? While this hardly represents a practical setup for operation, it would, if there were no other objects nearer to the coil than say, two or three times the base elevation, enable the capacitance to be determined unequivocally. Regards, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.