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RE: Shorted turns, primary proximity - Add a sheild???

Subject:  RE: Shorted turns, primary proximity - Add a sheild???
  Date:   11 May 97 08:53:34 EDT
  From:   Alan Sharp <100624.504-at-CompuServe.COM>
    To:   "INTERNET:tesla-at-pupman-dot-com" <tesla-at-pupman-dot-com>


William Noble wrote:
>It would
>seem 
>to me that the tesla secondary would very much like to have a nice sheet
>of 
>metal right at it's base to make a nice close ground plane.  Yet that
>same 
>thing would also look like a shorted turn to the primary, right.  Well, 
>perhaps, unless you put a cut half way through it,
>Now, if you were to do this (put a properly cut piece of metal under the
>coil) 
>you wouldn't have a shorted turn "on axis", but there is still plenty of
>metal 
>"off axis" that could act as a shorted turn.  On the other hand, the
>thing 
>will shield anything below it pretty well, and it will act as a E field 
>reflector as well.  An interesting question of course is what would
>happen if 
>it were iron/steel versus aluminum because it would increase the primary 
>inductance.
>Someone has probably already tried this and can say what the truth is

I recently demonstrated my solid state coil to a men's group. I couldn't
get
more than 7" out of it - normally 14". Then I realised that the wooden
table
had a steel frame. Metal sheets near the primary - even slit as you
suggest
will steal power (steel/iron especially).

Duane Byland did experiments with copper sheets - very small benefit,
and
using a cointerpoise made from hundreds of copper wires - earthed in the
centre
and laid out as a big star. It gave some benifit and significantly
reduced the 
ground currents.

I would have expected a large surface area to be required to act as a
capacitor
plate against the toriod - but wire seems to work just as well
presumablely
because it is "a reflected wave thing". 

I did consider using copper printed circuit board - etching a pattern of
fine
copper strips centraly connected, lots of surface area, minimum eddy
currents.
Cost has held me back though.

have fun,

Alan Sharp (UK)