Stacked toroids

I have been meaning to respond to this subject for a couple of days but have
been busy with inventory at work and it finally snowed and my snowmobile was
not running well.  I fixed the snowmobile, it now runs great - and the snow
has all melted.

Let's say we have two toroids that are 3 x 20" each.  This is equivalent to 20
pf.  Now let's stack them by laying one on top of the other.  In my opinion,
this adds little capacitance to the system - about 10% or two pf in this case.
We now have an effective toroid that is 6 x 20" which is about 22 pf.

If we raise the second toroid up above the first by using a hollow metal
cylinder (such as rolled up aluminum roof flashing, beer can, etc.) then we
have almost the sum of the two individual toroids.  Since they still have
facing sides and share some space, the equivalent total will not be the sum of
the two but about 70% of that number which in this case would be about 28 pf.
This is about 40% more than the value of one toroid.  Based on my testing, the
minumum separation between the two toroids needs to be around 1/3 of the
toroid diameter.  For a 20" toroid this would be about 6".  For my 33 and 40"
diameter toroids, I use a 14" tall cylinder that is 20" in diameter.

Certainly, much more testing could be done in this area.  It would be nice to
have a variety of toroids and cylindrical spacers and test the different
combinations while measuring the resonant frequency of the secondary / toroid
combination with a signal generator and scope.  Then calculate the value of
the toroids.  It would be nice to know the effect of having the spacer too
tall.  The diameter of the spacer also should be considered.  I expect the
larger the diameter, the larger the total capacitance.  What would be the
equivalent capacitance of the two 3 x 20" toroids used above with a 6" high x
4" diameter spacer?  Now change the diameter of the spacer to 14".  I would
expect the capacitance to increase but have not confirmed this with testing.

Ed Sonderman