From: "Malcolm Watts"
Date: Mon, 11 Dec 2000 09:08:39 +1300
Subject: Re: [TSSP] Coils at high elevation
Hi Paul, all, On 9 Dec 00, at 7:26, Paul wrote: > 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? Here is how it was done: I started with two coils of identical winding lengths and diameters wound on 6.5" gaspipe (might have been 6.6" actually- I'd have to go back to the notes). Anyway, each coil was placed on some insulating material (can't remember what) so that the windings started at the 26" off the floor. After measuring the largest h/d, I removed the appropriate number of turns from the bottom of the coil and lowered the insulating platform to keep the starting height constant. The h/d=1 coil then was wound near the top of the pipe and the bottom of the pipe must have been on the floor by that stage. It is a pity that those msmts are not fresh ones that we could have consulted over while being taken. > 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! I had the strap running vertically from the bottom of the winding down to the floor and then along it to a very short piece of coax coming from the generator. I would like to do the msmts again but am not in a position to do so this year. Very disappointing from my POV. I always like to revisit and re-measure to make absolutely sure. > 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. Very much so. There was a minimum clearance of 6' including the scope probe from objects other than the support (pipe). Any less and proximity effects were plain to see. The clearances could have been better had I been in a larger room. > 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. As soon as the new generator is up and running I should be able to take such msmts. Regards, malcolm
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.