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STUPID SIMULATIONS


From: 	Mark S. Rzeszotarski, Ph.D.[SMTP:msr7-at-po.cwru.edu]
Sent: 	Monday, September 22, 1997 2:46 AM
To: 	Tesla List
Subject: 	Re: STUPID SIMULATIONS

Hello Malcolm and all,
        Malcolm said in part:
<snip>
>I would be interested if you did a run on a 6.6" coil, Hs about 21" 
>with a topload of 7pF. On what basis are you choosing the wire size?
        The simulation is one of many I am playing around with.  This
particular one tries different wire sizes and coil heights to maximize Q
with several constraints:
1.  primary surge impedance = 30 ohms (fixes Cp and Lp, since Fres is known)
2.  secondary surge impedance between 25k and 35 k ohms.
3.  make the wire length close to the operating 1/4 wavelength.
        These parameters are based on the experimental observation that
several excellent coil systems I have studied all have these common factors.
When I plug in your 7 pF top capacitance with 6.6" diameter, it says to wind
the coil 10.5 inches tall using 22 AWG wire, which is probably a bit short,
if you want to keep the thing from arcing down its sides.  To compensate for
the short coil, you need to add more top capacitance according to this
model.  The program suggests that one should use a coil height of around
18.5 inches with 14 pF of top capacitance, using 22 AWG wire.  An even
taller coil would need more top capacitance.  You can either try your coil
with some more metal on top (raid the kitchen for pots, pans and metal
bowls), or suggest a better constraint for this model!  The high top
capacitance is typical for some of Richard Hull's coils (55 pF on his 14"
diameter Nemesis coil).  If you can achieve breakout, you get really long
sparks.  Wire sizes of 21-23 AWG maximize Q for coils operating at these
frequencies.  (The program tries all AWG wire sizes from 4 through 40.)
        Note that by constraining the surge impedance of both the primary
and secondary, I am essentially fixing the secondary voltage rise to around
34 times the primary voltage.  This is a tradeoff, but appears to yield nice
long sparks.
        The matching of the quarter wavelength was discussed at length on
the list a while ago.  Skip Greiner has a spacewound coil that has a H/D of
1.37.  It is wound to match quarter wavelengths, and it really puts out long
sparks.  I still don't have a good reason to follow this practice except
that several coils I have built also work nicely with this constraint so why
not!  In addition, coils with a H/D of around 3:1 closewound with the
correct wire gauge and large top capacitance seem to naturally resonate
close to the 1/4 wavelength.  (Let's not discuss this again on the list
right now!)
Regards,
Mark S. Rzeszotarski, Ph.D.