" One of these mistakes was using shielded cable everywhere and as a result the pitch increases markedly when the theremin is boxed up because of the extra capacitance." - patricks
I dont think the above problem is caused by shielded cable - its probably just the box capacitance.. if anything, shielding the antenna lead (a BAD idea!) will probably reduce the effect of the enclosure, so I doubt that putting this 'right' will fix that issue..
That said..
1.) Unless one specifically accounts for screened RF cable capacitance, or designs the theremin in a special way*, Dont use screened cable at the RF side - use thin hookup wire - ideally single conductor ..
1b.) The above is particularly true if you have a equalization coil in the antenna circuit - The better theremins generally have this - and when fitted, the total antenna capacitance must usually not exceed about 15pF (usually between 10pF and 15pF) - screened cable can easily add 5pF.
1c.) Simpler theremins (like the SC/Jaycar) have the antenna in parallel with the tank capacitance - this capacitance is large (180pF for the SC) so the extra capacitance of screened cable is not so significant.... BUT... Its still not a good idea - the capacitance of screened cable is extremely variable and temperature dependant.
2.) Apart from the RF side (antennas and all signals before the mixer) it is my opinion that screened cable is a good idea - particularly on all audio leads and this includes leads to potentiometers.
3b.) I would even advise using screened cable for power supply connections.
3.) IMO, access to the tuning presets without having to open the theremin should be compulsory - It is just insane to have an internal adjustment which is invalidated when one puts the unit back together! I advise doing whatever is needed (hole in the box or whatever) to facilitate this access
Fred.
*The only time I have used screened cable for the antenna was with double screened cable, with the inner screen driven by a shielding signal (which adds no capacitance) and the outer screen grounded.. But its a LOT of difficult design and effort, and not worth doing IMO.