[sdiy] RCA theremin antenna shape

[PRCamann at aol.com]

Given that the antennas used on a Theremin are there as plates of capacitors 
(your body or any other object being the other plate), the shape determines 
the variation in capacitance as a function of distance.  I haven't done the 
calculations in about 20 years, but if I remember from the physics classes I 
used to teach, a cylindrical capacitor (like the vertical pitch rod) has a 
capacitance that varies as the inverse of the radial coaxial plate separation 
(this assumes an infinite length; if the rod is long enough and you stay near 
the center, the approximation is good).  I haven't done the calculations for 
a loop-shaped capacitor (the volume antenna), but I note that empirically the 
capacitance is affected most greatly when the hand is relatively near the 
loop, so I infer that the behavior is perhaps more like an inverse-square 
relationship.

The 90-degree offset in orientation is there to minimize interaction between 
the antennae and the hand movements relative to each.

BTW, paul, if you're playing around with the Silicon Chip Theremin, 
substitute a loop antenna for the circular plate included with the kit.  I 
use an antenna of about 8" or 9" diameter from Radio Shack; it's the sort of 
antenna used by older NTSC television sets for ultra-high frequency (UHF) 
reception.  Don't know what an equivalent would be in Australia, but the 
radioshack.com site may still have them to give you an idea.  With proper 
mods to the volume circuit, I have a front panel pot that gives sensitivity 
control from least sensitive (hand has to be fully in the loop for the sound 
to stop) to most sensitive (the instrument cries at old romantic movies).

Paul Camann