Hi,
last week I found following: Metal near the theremin has big influence to the pitch.
I had a toolbox (30x80x30cm) and placed it (caused by other reasons, we repaired something) about 1,5 m away from the Theremin. And the result was: The area of the Theremin widened. The pitch was higher in frequency, and I did not reach the very low frequencies. But the area for high frequencies widened significantly.
Another experience: I wanted to show the theremin to colleagues in a room where about 3 computers were working. I did not get a clear sound. It was impossible to adjust the theremin properly.
So in the future, I will try whether the instrument placement is ok - before showing it anybody.