From: Bert Hickman
Date: Tue, 26 Jun 2001 07:42:29 -0500
Subject: Re: [TSSP] Some Considerations
Malcolm and Paul, I agree with virtually all of Malcolm's previous post except the part about recommending a spherical topload terminal. My thought process is as follows... It would seem that the key to getting an accurate measurement is to have a method of calibrating the divider ratio, or accurately computing the effective capacitance between the topload and the probe plate above to determine the divider ratio. In addition, the topload must be large enough to shield the resonator's E-field from the measurement plate, and that the measurement plate must be far enough away to avoid being struck or from forming corona on the plate. Clearance may need to be several leader lengths (or more?). There are some things we can do to improve measurement accuracy. For example, suppose we add a grounded plate (a "guard ring") around the sides of the capacitive probe and also behind it. This will virtually eliminate fringing fields or other extraneous E-fields, and should improve measurement repeatability once the unit has been calibrated with a given toroid size and spacing. If the top of the toroid had a flat section tangent to the outer curved surface, the toroid and measurement plate would form an (almost) ideal flat plate capacitor, the value of which should be computable with fairly good accuracy. However, the E-field drop off near the center of a regular toroid may actually be desirable. I suspect that using a toroidal topload would be more desirable than using a sphere especially for measuring voltages after breakout. As Paul mentioned, once breakout begins, the presence of leaders and streamers (particularly in the region between between the top terminal and measurement plate) will significantly distort the E-field and our measurements in changing and unpredictable ways. A spherical topload may actually cause leaders to concentrate precisely where we don't want them - at the top of the terminal - due to thermal effects. Suppose instead that we use a toroid of relatively large outer diameter combined with a comparatively small radius of curvature, or we purposely add a calibrated breakout sphere to stimulate breakout at a known e-field. In this case we should be able to direct most of the discharges laterally, or downward, minimizing E-field distortion between the toroid and measurement plate, and improving accuracy. This is getting very interesting... :^) -- Bert -- -- Bert Hickman Stoneridge Engineering Email: bert.hickman@aquila.net Web Site: http://www.teslamania.com
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.