Antenna response range

I'm currently testing the pitch section of the em theremin on a breadboard but the pitch signal doesn't seem to change much until the hand is really close to the antenna. Even then the frequency is only about 1kHz. Any advice on what could be wrong.

The oscillators are working fine at 264kHz but antenna response is lacking.

Your oscillators are running on a too low frequency. They should run at about 285kHz. There are two resonant circuits on the var osc side, the parallel resonant circuit with the variable 100uH inductor and the 3.3nF capacitor and the series resonant circuit with the linearization coils (4 x 10mH) and the static antenna capacitance. The latter's resonant frequency has to be slightly lower as the one of the oscillator itself. There is a complex interaction between both which influences antenna sensitivity, pitch range and linearity.

Possible causes may be

-bad tuning of the variable inductors

-too much stray capacitance around the 4 linearization coils

-bad choice of the linearization coils with a too low SRF (one should use the recommended 3pi wound 10mH inductors by Bourns-Miller or their Coilcraft equivalents)

-bad positioning or wiring of the linearization coils (too much coupling between them)

-too much parasitic capacitance seen by the antenna.

The var osc without the antenna coils connected should run at about 275kHz. As soon as you connect the antenna circuit, its frequency should go up by ~10kHz. When touching and grasping the antenna now, the frequency should go down by 5kHz. If that works, you may tune the fixed pitch oscillator for zero beat with your hand at about 50cm/20" from it. Now you should get a pitch range of about 2.5 kHz and all is fine.

"The var osc without the antenna coils connected should run at about 275kHz. As soon as you connect the antenna circuit, its frequency should go up by ~10kHz. When touching and grasping the antenna now, the frequency should go down by 5kHz." -- Thierry

Ok I set the var osc to 275kHz then connected the antenna coils but there was no change in frequency. Is this a clue that there might be something wrong with my antenna circuit?

I talked about connecting the whole antenna circuit, that means the 4 coils in series with the 3/8" or 10mm diameter and 16" or 40cm long pitch rod. And no other metallic parts in a 6' or 2m radius around, above and below the pitch rod, out of the theremin circuit and the antenna fixing itself.

If that doesn't raise the var osc's frequency by 10kHz check the 4 coils with an RLC tester, each one should have a DC resistance of about 30 Ohms and an inductance of 10mH. 

The 4 coils should be soldered on a circuit board with at least 1cm or 3/8" distance between them and at least 0.5cm or 3/16" above the circuit board to reduce parasitic capacitance. The wiring between them should be done in "Z" form, not in "S" form to prevent annihilation of the inductance by counter-field coupling.

I think the fact that I'm testing on breadboards and working in a lab with stuff everywhere might be affecting the response of the antenna. So I think it will be a good idea to get everything on pcb and test with a portable oscilloscope in a clear area to see better results.

Ah!!!!! I just put every component on a pcb and i'm still getting the same issue. The frequency response from the pitch antenna is doesn't change sufficiently until the hand is almost hold the antenna.  I'm using the suggest linearization inductors recommended by moog (actually bought them brand new) and still no dice.

The pitch tuning tunes the frequency nicely but the antenna is not very responsive. I've set both oscillators to 285kHz, reduced below 260kHz and still NOTHING. what am i doing wrong?