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I want to use the concept of theremins for determining the presence and (approximate and not individual) distance of approaching people. The sensors should roughly cover radii of 2.5 meters. The measured distance does not have to be precise but it should be stable (noiseless) without being too delayed. Can I just take the same circuitry of a theremin and connect a bigger antenna to it in order to increase range? What kind of objects would affect the sensor readings apart from humans? It will be used in a very shielded environment, but there will a metal object (without any electricity though) and the data and power lines of the sensors nearby (please see figure).

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    \$\begingroup\$ I'd check out some of the documents that discuss filtering capacitive touch systems and how to deal with drift and bias. Different sensors for sure, but the long term and short term drifts might be very similar from a data analysis perspective, and could make your system more robust and require less shielding. \$\endgroup\$
    – Bob
    Nov 13, 2013 at 17:51
  • \$\begingroup\$ @Bob The environment is underground and as far as I know the walls are not wired. The next electrical device is about 4 meters away from both antennas. Do I have to worry about shielding anyway? \$\endgroup\$
    – Lenar Hoyt
    Nov 13, 2013 at 17:58
  • \$\begingroup\$ I'm no expert in shielding, I'm just thinking ahead to data processing and signal analysis techniques. Traditional DSP doesn't always offer the best solutions for some of these systems. \$\endgroup\$
    – Bob
    Nov 13, 2013 at 20:54

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Theremins operate on near field effects. There are essentially two antennas and as your hand approaches them, the additional loading capacitance produced serves to detune them. This detuning is detected and the volume/tone are accordingly adjusted.

Near field effects could be exploited for proximity detection, however the distances for effective loading are actually quite small and dependent on frequency. The higher the frequency, the closer the loading object needs to be to the antenna. This implies that a low frequency would be ideal, however any stationary metallic/conductive object within the near field zone would serve to load it. Particularly for low frequencies, if there are existing loading object such as filing cabinets or even daily differences in ground moisture (earth conductivity) the loading will be different. Its a moving target. If the field is loaded with static objects and a dynamic one enters the system, the additional effect due to the dynamic one will be less effective.

Higher frequencies could be used and the shadowing/ghosting and reflections could be measured between the two antennas. Any conductive object within the space which is capable of reflecting/absorbing the field will distort the maxima/nulls of the standing pattern which can be measured as shifts in the received signal strength (RSSI) on both the antennas. The principle is analogous to moving several inches left or right for a better signal when speaking on your mobile phone except the environment is being altered rather than moving the antennas. The pattern will change on a daily basis due to the same effects, however you can determine if an object has entered the vicinity by examining the rate of change of the RSSI on both antennas.

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