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Re: [TCML] Twin coils primary lead design

Hi Peter,

The stray inductance of your primary circuit is reducing the effective
coupling of the twin system. There are several possible fixes, You've
already  tried one (reducing primary tank capacitance and reducing stray

You could also try increasing the coupling by elevating the primaries
(only when running as twins). Stray inductance is reduced by reducing
the diameter of the current loop in the primary circuit(s) by bringing
supply and return current paths closer together so that their combined
magnetic fields tend to (mostly) cancel. Some possible options include
running them closely side by side (like lamp cord or as a twisted pair),
making a sandwich HV stripline structure with copper strips separated by
a HV dielectric. One of the best configurations would be to make the
equivalent of a high voltage/high current hardline coax (as you
suggested), using a solid outer copper pipe and heavy gauge silicone HV
wire inside or and inner conductor of 1/4" tubing with insulating
spacers. This configuration should minimize corona losses versus stripline.

You're not looking so much for impedance matching, per se, but canceling
the stray inductance that messes with your tuning and coupling.

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Peter Terren wrote:
> I am working on a small twin coil system with a cylinder primary and 
> small spherical topload. Using a single coil only, run by a single MOT 
> SISG, it puts out a 60 cm spark for a 50 cm secondary winding.
> I am finding results a bit limited when run as a twin system due to the 
> length of the primary leads.  With a series primary arrangement I should 
> tune to 3 turns on the primary but with loose leads this drops to about 
> 1.5 turns which is not efficient and spark length is less than 30cm 
> despite double the power.
> With parallel primary windings, I should tune to 6 turns but it is 
> reduced to 3 turns with the lead in wire inductance. It seems like I am 
> wasting half of my inductance.
> Best performance of 90cm sparks with 2 MOT SISG is with a reduced tank 
> capacitance to allow greater inductance in the primary using 8 turns in 
> parallel connected primaries with lead in wires taped together sort of 
> transmission line like.
> My question: How can I minimise the effect of primary lead inductance. 
> Should I use two close parallel conductors (sort of transmission line 
> like) or would a coax arrangement be better?  I can make up a coax 
> system with copper pipe and plastic tubing covering 1/4 inch tubing. I 
> don't really understand the concept of impedance matching as applied to 
> Tesla coil primaries though or whether it is even likely to be relevant.
> Any thoughts?
> Peter  www.tesladownunder.com
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