Next Topic. A physics based simple** formula for hand capacitance** which is reliable in practice of theremin is probably not to be found. So I can add a simple approach for tests and discussion.

After searching many of published material I did not found anything reliable about this topic. Physical formulas to calculate capacitance of rods, plates ore between them overestimate the capacitance because they always assume, that the whole electric field consists only between them. No experiment validates such fomulas in the far distance. However, plate to hand area gives better results as rod section to hand area.

So I try to make another approach for longer distances. An old theremin grand dame is supposed to help me soon.

# basic experiments

Now comes the next step to confirm the formula. Before I start, I will make a trip to another simplification. The well-known resonance formula is:

If the decrease of the frequency due to the hand capacitance is very small compared to the oscillator frequency itself, you can use approximately the first derivative:

Just a few cm and more away from the rod antenna results in most cases an error of less than one percent.

Ah is the front area of the conductive surface (hand) in direction to the rod antenna. I used my "one liter bottle" results from previous experiments and varied Ah a bit to fit the curve. The overestimation near the rod is recognizable because the front area is not concave, as assumed in the cylinder-based formula. I think its still pretty good.

In the next topic I will address the question of the true role of EQ coils and their influence on pitch frequency.

What is the advantage using an additional linearization-coil for the pitch oscillator? I give a simple explanation here. Without active electronic elements necessary for oscillator condition some importend L and C estimate the result mainly. C1 may contain more C resulting from transistor or tube.

The sum of inductances L1 and L2 (if not magnetic coupled) and their resulting parallel capacitance determine the proper pitch oscillator frequency. Here please find an excel calculation. Of course can also be done a LTspice simulation.

But how much is the down shift of all the resonances in case of a definit hand capacitance for example of 1 pF? How is the effect on pitch linearity by using the "linearization" coil? Next question, answer is coming soon.

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