How does a Theremin “antenna” work?

Posted: 10/26/2016 8:16:07 AM

Joined: 8/25/2016

I always assumed that the shape of the antennas was important; a vertical monopole for pitch and a horizontal loop for volume, thinking that this minimized their interference with each other. But it seems they actually operate more in the range of 200-500 kHz. At these frequencies a good antenna would be hundreds of meters long, and the use of different frequencies for each antenna is sufficient to prevent interference.

On the other hand, the Moog Etherwave schematic has a bunch of coils in series with the antennas, which could be electrical lengthening?

Most descriptions I've read explain that it's just the human's capacitance to ground that detunes the oscillators, so any shape of metal will do, since it's just acting as a capacitor plate.

This page describes something different, though, which I don't understand:

Beyond 4 inches (10cm) RF heterodyne theremin pitch variations are caused from changes in the "radiation resistance". This is the total RF electro-magnetic power radiated from the pitch antenna divided by the square of the net current flowing into the pitch antenna. The pitch field is a dual electrical/magnetic equilibrium, not just a capacitive field as so commonly state.

Posted: 10/26/2016 12:55:09 PM

From: Colmar, France

Joined: 12/31/2007

You are right, you should not see the theremin antennas as antennas in the classic way but rather as electrodes (as Leon Theremin himself called them) or, still more logical, as capacitor plates who will find their counterparts preferably in the player's hands.

The shapes (vertical pitch rod and horizontal volume loop) are there for mostly historic reasons and until now, they represent the most ergonomic and precise way to play. Different efforts have been made to bring theremins with plate antennas to market, but without success.

The inductors in series with the antenna are not for "lengthening" in the rf technology way. They form a series resonant circuit with the antenna, and this series resonant circuit is to be seen in parallel with the parallel resonant circuit of the oscillator. If everything is well dimensioned (that's very complex calculus), the resonance frequency of the series (antenna) circuit (even without the pitch hand) is slightly lower than the one of the parallel resonant circuit and goes even much lower when the pitch hand comes into game. Since the oscillator operates the antenna circuit above its own resonant frequency, the latter will behave like a variable inductor in parallel to the oscillator's parallel resonate circuit. That makes that the frequency of the oscillator will jump up as soon as you connect the antenna circuit and then go slowly down as the pitch hand approaches, but limited in an asymptotic way to the initial frequency from which it jumped up. That limits the effect of the hand when playing close to the antenna which is sensed by the player as a wider (or at least less progressive shrinking) tone spacing. That process is called linearization. It works only on instruments with good LC-oscillators, not on cheap CMOS circuits with RC oscillators!

The "radiation resistance" stuff on the old temecula website is, although I highly estimate its author in general, rather esoteric stuff without scientific base, IMNSHFO...

Posted: 10/26/2016 6:13:12 PM

From: Northern NJ, USA

Joined: 2/17/2012

This is an excellent topic, thanks for bringing it up!  I believe there many ways to look at it that are not mutually exclusive (though as Thierry points out they may not all have equal merit).

I've done a fair amount of research regarding coils and resonant LC circuits.  My Spice simulations generally show a much larger oscillation amplitude than the real thing, and the difference cannot be accounted for via wire skin effect resistance and the like, so I assume the lower Q of the physical oscillator is due to RF transmission.  For instance, trying to make the coil Q over 100 (or so) at resonance doesn't seem worth the effort because you're entering an area of diminishing returns due to other losses which seem beyond one's control.

I would very much like to quantify these losses, if for no other reason than to see how closely I can get the simulations to match reality.  I recently did a couple hours of google searching along these lines, but came up empty handed in the math department.  Of all the practical Ham radio stuff I ran across, the high Q resonant antenna construct seemed the most applicable:

There is a loading coil at the base, and a capacitive "hat" on the whip, and the whole thing is designed to be highly resonant (high quality factor) at a specific frequency.  They make versions that can mechanically vary the inductance of the loading coil in order to tune the structure to other frequencies.  This is all fairly "Thereminy".  The way I think about it working in terms of RF is: you have a rod that is "whipping back and forth" at a very high voltage due to the resonance, which somewhat makes up for the rod being much smaller than the transmission / reception wavelength.

In general, using a plate antenna shape for Theremin use will give you higher capacitive interaction between the hand and the circuit (compared to a rod shaped antenna).  It will also give better (more linear) response if you are working backwards from the LC resonance equation in a digital Theremin, because the capacitance seen by the LC tank is almost exactly proportional to the inverse hand distance.  In an analog heterodyning Theremin your needs are different, because you want the best fit between the hand distance and an exponential pitch response from the difference frequency, which can never be more than an approximation over a certain playing zone (it usually deviates most in the near and far fields).

"On the other hand, the Moog Etherwave schematic has a bunch of coils in series with the antennas, which could be electrical lengthening?"  - sleepycc

An interesting thing about coils is that the inductance is highly related to wire length, so I would say yes.  Clearly Theremins with an EQ coil are tuning that coil with the series antenna capacitance to a frequency very near the tank LC resonance.

I think I read that RF antennas which are too short act capacitively.  And they generally aren't very good at radiating RF.  Both of these things however are beneficial for Theremin operation, because we want the interaction with the hand to be capacitive, and we don't want energy bleeding off as RF because we want the selectivity, noise immunity, and low power operation that high Q resonance provides.

Posted: 10/27/2016 5:43:00 PM

Joined: 3/26/2010

I thought the "Historic reason" for the horizontal loop antenna is that it less likely to snag

on clothing or poke a little person in the eye.   

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