Round vs flattened primary tubing

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Wednesday, June 10, 1998 11:46 PM
To:  Tesla List
Subject:  Re: Round vs flattened primary tubing

Tesla List wrote:
> > You are absolutely
> > correct about maintaining the conductive skin area (this
> > configuration may actually be slightly better in this respect than a
> > round conductor because the RF currents will tend to concentrate on
> > the inner side) and how the round edges would be better for corona
> control
> > than the sharp edge of copper ribbon, but I think the real advantage is
> how you
> > could gain more inductance for a given sized flat pancake while
> maintaining the
> > same generous turn-turn spacing.  I prefer to make my flat primaries
> > smaller in diameter than say the length of the secondary or the
> > diameter of the topload.  This allows the coil base to roll through
> > more doorways than would otherwise be possible, and helps reduce
> > primary strikes by keeping this component _tucked-in_.  My 15-1/2 inch
> > diameter secondary MTC-1 system rolls through a normal 31 inch wide
> > man-door.

> Robert:  
> RE the increased inductance for a given sized primary.  Since my
> post, I played around with the Wintesla2 program to see the effect of more
> compact primary windings.  Interestingly, the inductance was about the
> minimum for wire diameter equal to the turn to turn spacing for 1/4 inch
> wire or tubing.  The inductance goes up a little as turn to turn spacing is
> either decreased (mutual coupling effect?) or increased (wire length gets
> longer).  However, the change in inductance only amounted to that which
> would be gained by increasing the total number of turns by a tenth of a
> turn!  (I have no idea if Wintesla2 is producing completely correct
> results.)  Except for fitting through a doorway, the other benefits don't
> seem worth the effort.
> --Steve


There can be a significant advantage! Let's build a pancake style
primary from 3/8" copper tubing with 1/4" clearance between turns. Let's
also specify an inner diameter of 10", and a maximum outer diameter of

3/8" round tubing:
In order to have 1/4" clearance between turns, we'd need a turn-turn
spacing of 5/8". This limits us to a primary with 14 turns, resulting in
about 72 uH of inductance. 

Flattenned 3/8" tubing:
Now lets make a primary using the same tubing that's been pre-flattenned
to form a copper "strap" that's approximately 1/2" wide by 1/8" thick.
Using this material, we can decrease the turn-turn spacing to 0.375"
while still maintaining 1/4" clearance between turns. We can now fit 20
turns in the same space, resulting in a primary with about 201 uH of
inductance, or about 2.8X the inductance!

-- Bert --