It's a Question of Aspect Ratios
I made three single layer air core solenoids, all wound with 1 build insulation 30AWG on PETG 3D printed coil forms, with three different winding aspect ratios (winding length / diameter) of 0.5, 1.0, and 1.88:
The 1.88 aspect ratio coil is the standard kit pitch coil. For the two larger diameter coils I printed some end caps, which really help to stiffen the otherwise too flexible 4 layer walls. For larger coils one could also print extra internal stiffeners and perhaps glue them in. The tallest coil is covered in heatshrink, the other two were instead coated in clear nail polish to secure the windings.
I tested the coils in my test jig, which lowers the drive impedance of the signal generator to approx. 2.4 Ohms, and presents approx. 13.4pF load to the other end of the coil. Q is directly read as the output / input voltage ratio:
Observations:
- DCR increases with aspect ratio due to the reduction in bulk coupling, which requires more wire to reach the target inductance.
- The self resonance frequency is a measure of the self capacitance of the coil (and its environment). Lower aspect ratios place the ends of the coil closer together, which increases the self capacitance.
- The 1:1 aspect ratio coil has slightly higher Q than the other coils, but a bit lower self resonance that the 1:1.88 coil. So 1:1 (or thereabouts) has optimal bulk coupling for resonance with the jig capacitance, but at the expense of an increase in self capacitance.
- Any of these coils would function well in a digital Theremin because the influences of the aspect ratio are broad minima / maxima; ideally one would pick 1:1 or somewhat higher.
- Q is more important than self resonance because it directly increases both the antenna voltage swing and the frequency selectivity.
- Increased antenna voltage improves external noise rejection.
- Increased frequency selectivity improves both oscillator stability and external noise rejection.
