Of the Theremin and Futurist Stringed Instrument Making

I can assure you that the use of large coils, which necessarily need to be wound by hand (hundreds of meters of 0.2 enameled copper wires) is infinitely preferable to using the small ready-made commercial inductors because their larger diameter increases the capacitance between one winding and another, and this helps to correct linearity.
In order to ensure that a large inductor works correctly, it must be vertically positioned below and in line with the pitch antenna because the wide electromagnetic field it produces must in some way be in phase with that radiated by the antenna because the action of the hand in the electromagnetic field of the antenna has a simultaneous effect on that radiated by the inductor, and probably contributes towards providing maximal linearity. In the case that the inductor is positioned horizontally, linearity is considerably reduced and leads to a certain instability in the central octave.

In the case of Big Briar’s Etherwave, the antenna inductance is created by four standard 10 mH elements, each of which is mounted on the printed circuit. This considerably increases pitch extension (5-6 octaves), but the effect is linear only in the case of the central octaves, whereas the lower and upper octaves are shrunk.
Other commercially available theremins, such as PAIA’s Theremax, do not use antenna inductance and simply increase the working frequency of the oscillators; but this does not ensure linearity.
The theremin of Lydia Kavina is something of a puzzle: the case is too small to contain large inductors but, after having seen it in action, I have to admit that its linearity is optimal: its builder probably identified presumably complex alternative solutions in terms of circuitry.
I have no information concerning the Ethervox but, given that it is an essentially digital instrument, I imagine that its linearity is optimal. Furthermore, the fact that it is possible to switch off the continuous glissando in order to take advantage of the automatic pitch control means that the problem should not arise. My only doubt is that, although it is undoubtedly a theremin, it tends to be more similar to a movement-controlled computer – something like passing from a traditional guitar to the MIDI guitars introduced by Roland and Casio ten years ago.

The use of antenna inductances is also valid in the volume control circuit, but this has less to do with the need for linearity in the circuit response to hand movements and is largely due to the need to ensure a wide range of oscillator frequency in such a way that the corresponding control voltage is capable of having a profound effect on the VCA.