> I understand, but I'm trying to model an existing device that
> deliberately set them up with minimal coupling. Most designs would use a
> single large inductor, but LARGE is the word, at about 3.5" diameter,
> and maybe 14-20 inches long.

10mH coils have a lot of parasitic capacitance no matter who makes them
(even if pelerine pitch wound with litz wire). I also think that your
measured 700kHz self resonant frequency is due to multiple sections
(layers) resonating while being tightly coupled. This corresponds to a
much higher self capacitance than you calculated. 10 to 30pF should be
about right for that size of coil, if it is good.

>http://www.dvanhorn.org/TankCkt.gif

Is the tank at the right a part of the oscillator ? If so, and you are
trying to tune the osc. I think it would be easier to use a capacitive
divider to insert the antenna and not use L1 at all. I.e. replace L1 with
a small capacitor and see with what Ctrim value it works best.

My limited experience with Theremin lead me to quickly abandon direct
tuning of the oscillator and use indirect tuning. I had an XO that was
weakly coupled to two identical tanks using twiddlecaps. Each tank had a
short antenna, one for pitch, one for volume. Each tank also had a JFET
peak rectifier. The pitch rectifier drove a varicap in the 'real' pitch
osc, which beat against the XO, and the volume rectifier drove another
JFET mounted as volume control. So I had only one critical coil to tune
(the one for the beat osc). I used 6 transistors and a voltage regulator
(78L05). There was a lot of rf leakage into the (external) audio amp but
the amp did not mind. It all ran on 10mA or less from a 9V battery. The
circuit was built 'dead bug' style on a small piece of weissblech. I used
a 10 MHz crystal I think. After I finished playing with it I dismantled
it.

I also think that you can control the pitch without an antenna by putting
the osc in a plastic box and just putting your hand on or near it (the
bottom would be shielded with foil), if the frequency is high enough. 27
MHz should work great.

A high Q antenna usually causes trouble since the transmitter can see a
complex impedance that changes even with modulation sidebands. It can
easily self-oscillate like this.

good luck,

Peter

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