Very interesting. I think it's difficult though to make high Q variable capacitors because of dielectric and other losses from things around the capacitor. This means that large mechanical structures near the capacitor plates may lead to loss, as well as suboptimal positioning of or shapes of the plates (which can distort the current flow over the surface as it flows from the non-overlapping part of the plates toward the overlapping portion of the plates). I spent some (too much) time modeling current flow over capacitor plates in OpenEMS (FDTD EM field simulator software) some years ago, and it's an interesting topic because, as I'm sure you know, current flows in weird ways over flat surfaces (concentrated at the edges), and so for optimal design you need to consider where the unwanted current concentrations will be in a multi-plate structure as it is adjusted from minimum to maximum capacitance.
So for maximum Q, I think the tried and true butterfly capacitor and concentric-cylinder vacuum variable capacitor are close to optimal due to the inherent symmetry.
If maximum Q isn't so important in a particular application, I think that even 3D-printed plastic capacitor plates, covered with conductive copper foil, could work reasonably well. Depending on the frequency, it might even have acceptable Q.
With enough patience and copper tape, and careful piece-by-piece and layer-by-layer construction, it might even be possible to make a 3D-printed capacitor consisting of a static stator assembly of multiple concentric cylinders, that is then meshed with a corresponding movable plunger that also consists of multiple concentric cylinders -- exactly the same as in a vacuum variable capacitor, but necessarily built much larger to allow for the coarser manufacturing tolerances with 3D printing and hand assembly.
The diving-board mechanism you linked to above might also be able to be constructed with 3D printing.
Very interesting. I think it's difficult though to make high Q variable capacitors because of dielectric and other losses from things around the capacitor. This means that large mechanical structures near the capacitor plates may lead to loss, as well as suboptimal positioning of or shapes of the plates (which can distort the current flow over the surface as it flows from the non-overlapping part of the plates toward the overlapping portion of the plates). I spent some (too much) time modeling current flow over capacitor plates in OpenEMS (FDTD EM field simulator software) some years ago, and it's an interesting topic because, as I'm sure you know, current flows in weird ways over flat surfaces (concentrated at the edges), and so for optimal design you need to consider where the unwanted current concentrations will be in a multi-plate structure as it is adjusted from minimum to maximum capacitance.
So for maximum Q, I think the tried and true butterfly capacitor and concentric-cylinder vacuum variable capacitor are close to optimal due to the inherent symmetry.
If maximum Q isn't so important in a particular application, I think that even 3D-printed plastic capacitor plates, covered with conductive copper foil, could work reasonably well. Depending on the frequency, it might even have acceptable Q.
With enough patience and copper tape, and careful piece-by-piece and layer-by-layer construction, it might even be possible to make a 3D-printed capacitor consisting of a static stator assembly of multiple concentric cylinders, that is then meshed with a corresponding movable plunger that also consists of multiple concentric cylinders -- exactly the same as in a vacuum variable capacitor, but necessarily built much larger to allow for the coarser manufacturing tolerances with 3D printing and hand assembly.
The diving-board mechanism you linked to above might also be able to be constructed with 3D printing.