Building half a Theremin (Vol) - how to get long range (several feet) sensitivity

Hi,
I m new here and appreciate to be able to find so much information on this topic in one place!
My contact with Theremin has been 'light' so far - I am not a musician.

I am planning an installation where I would like to use tube based Theremin volume control circuits to control another audio sources. The sensitivity being such that objects (people) can be sensed several feet away. It is stationary and space is not a problem. I remember seeing old movies with dancers controlling the Theremin on a small stage. What kind of antenna could achieve something like this? Would it have to be two opposing systems between which the person moves?
Thanks for any pointers, Wolfgang

http://www.thereminworld.com/Forums/T/34001/special-project-need-a-theremin-with-a-big-radius-of-sensitivity-and-midi-output?Page=0

 The technology which is used in the volume part (resonance circuit + oscillator) is even less sensitive than in the pitch ones.

Ilya, 
thanks for the link. Gives me some hope to try it and experiment with it and see what kind of sensitivity I can achieve. Ultrasound can be a backup.
Cheers, Wolfgang