"- I have to follow particular measures for loop and pitch antenna lengh?
- It's ok to use chrome fittings?
- I have a piece of 3/8" copper tube, can I use it or I have to prefer brass or alluminium tubes?"
1) I don't see where the manual (www.paia.com/manuals/docs/9505_pages_200.pdf) specifies the antenna lengths, probably because they expect you to buy the flimsy wires from them. Your best bet might be to start off with experimental small diameter rods (3-5mm) bent in the shapes and approximate sizes shown in the picture (see below), with the intention of changing them to the sizes and forms that you want later. If this is your first theremin, it is better to not deviate too far from plan until you get it working, and then experiment from there. Someone here on Theremin World did describe using a 17" antenna on the pitch side, and commented that the Theremax did not seem terribly sensitive to antenna length.
2) Chrome is fine, as is bare brass, copper, or aluminum. The metal doesn't doesn't matter all that much except that you want to avoid things that rust or oxidize heavily.
3) If you want to make nice looking curves in your volume loop, avoid using hollow tubing and instead use solid metal rod - it takes more force to form but it bends smoothly without kinking. I have bent up to 3/8" (9.5mm) diameter solid brass (though I don't recommend starting with that) into a nice loop by bending a rod around a circle of wood that is somewhat smaller than the radius of your loop to allow for spring back. The circle of wood and a stop block should be screwed and glued to a piece of plywood that you can clamp to a bench or have someone else stand on while you do the bending. Aluminum rod in 6 - 7mm diameter is very easy to bend and will form nicely rigid antennas, although you would probably not find plumbing fittings to fit.
Here is a nice article showing a bending jig and describing how to make an antenna for a Theremax.
