How to Tune a Flat Spiral Coil

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Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Hi Paul,

Today I used a Ballantine model 6310A test oscillator and connected the
output to an HP465A amplifier.  I connected the output of the amplifier to
the outer lead and terminal of the wye coil.  Using my digital voltmeter I
measured the voltage across the outer lead and terminal.  The wye coil
resonates at 4.54KHz.

I have found that the primary capacitance is directly coupled to the
secondary inductance.  So if you want to tune a flat spiral secondary, you
have to figure your resonant frequency, measure the inductance of the
secondary coil, and then choose a primary capacitor that will tune the
circuit to the resonant frequency.

So what I'm saying is that if I have a 35mH coil and the coil resonates at
4.54KHz, then the primary capacitance would be .035uF.  The capacitance I
have been using is .026uF.  I chose this capacitance by trial and error
because standard theory didn't work.  Using this new formula that I have
discovered, the correct capacitance for the primary should be .035pF.

So I made up a .0324uF capacitor, and sure enough, the coil works much
better.  The vortex that I was getting in last night's experiments is now
much more defined and appears to be rotating.

This is a significant discovery in flat spiral coils and I'm not aware of
any other publicly available information to this end.

What I haven't figured out yet, is the relationship of the primary coil to
the secondary capacitance.

Dave