High voltage variable capacitor design
I made some loops with the objective of as close to zero cost as possible, for use with modest powers.
The loop was thick bamboo tied with bicycle tube rubber, tension spokes to an axle rod (think penny farthing bike wheel) and wrapped with aluminium foil.

I tried several capacitors.
Two sleeving PET drink bottles with foil glued on them as a trombone capacitor was one.
A tubular butterfly capacitor was another (again foil on PET bottles). 

A more expensive but much better cap was a (rotary) single vane butterfly capacitor using two sheets of FR4 PCB material with the butterfly copper pattern and Kapton film (used for 3D printer beds) as the dielectric. For my loop it was about 5" diameter from memory, and I put a small amount of dishing preload on it. (push rotor disc into the stator disc) (silicon spray to improve slip) (push-pull dyneema dial cords to rotate it)

The point about butterfly capacitors is that you do not have a moving electrical connection to the rotor. 

As long as the area is symmetrical between the two halves (true for rotary , false for linear), then the voltage is split between acoss the two halves of the butterfly, through two sheets of dielectric for double voltage.

Theoretically this is over 20kV for 2x 50um. 

Who knows what cheap 3D printer Kapton achieves in practice. If you have two pinholes in a single sheet, and each is on different halves of the butterfly then you have dielectric failure. If you have two sheets of Kapton (one on rotor pcb, one on stator pcb) then pinholes are unlikely to overlap, and you can assume you have 1 sheet of dielectric strength (x2 for equal butterfly)

At a rough calculation, a 200mm diameter single butterfly has approx 5000mm^2 area per half, and with 2x50um Kapton film, gives you roughly 600pF