Re: Hybrid Coil (was Coil Efficiency)

From: 	Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: 	Wednesday, July 30, 1997 11:10 PM
To: 	Tesla List
Subject: 	Hybrid Coil (was Coil Efficiency)

Malcolm Watts wrote:

> I came up with a guideline for good
> results that the height of a primary whose bottom turn starts at the
> same height as the bottom turn of the secondary  should be between
> 5 to 10% of the height of the secondary with a mean diameter around
> 2 to 3 times that of the secondary. That was the experimental result.
>     The theory: Using easily accessible mechanical analogy of the
> steel ruler in the vise and varying "coupling" height and noting
> effects on ruler movement, and looking at the concept of link
> coupling into a transmission line, it seems to me that what in effect
> we are doing is _tapping_ into the line at some way along its length
> (this applies to 2 coil systems only). Now if you do this in a
> transmission line, you are tapping into a particular impedance (with
> a particular impedance) since this is different along all portions of
> the line.

A very compelling theory, and observation!  That may explain an interesting
thing that happens in PSPICE, when you try to turn a standard coil into a
3 coil magnifier.  

If in PSPICE an appropriate capacitor (7.5 nF for my coil) is connected
across the lower 1/12 of the secondary on a standard coil, then the
operation of the coil is almost identical to that of a magnifier, according
to PSPICE.  This is because the K values between the primary and the lower 
1/12 of a secondary of a typical coil are around 0.4, whereas coupling to 
the upper 1/12 may be less than 0.01.  The point where the 7.5nF cap is
connected defines exactly where the 'tap' is along the secondary.

I propose calling this arrangement a 'hybrid coil', as it is not a pure 2 coil
system (due to the secondary tap) nor a 3 coil magnifier (finite coupling exists
between the primary and 'free resonator').

If in PSPICE you remove the coupling (about 0.1) between the primary and the
'free resonator', the output voltage improves only slightly.

The '1/12 point' seems to be the optimum place to attach the cap on the sec,
which tends to agree, to first order, with Malcolm's claimed 5% to 10% values.