Well Tom,

What I recall about the construction of a capacitor is the following:

There are various capacitive plate and dielectric material which can affect
values and voltage ratings.  I don't recall off-hand which ones are for
high-voltage.

Of course, with a tuning capacitor the dielectric is generally air, which is
lower loss than a solid-dielectric variable cap.  In this capacitor, you have
fixed parallel plates called the stator.  Then, as you turn the shaft, the
rotor plates intersperse themselves between each of the fixed stator plates.
As you turn, you have the plates nearly completely covering the fixed plates,
and then as you continue through the rotation, the plates only partially cover
the plates.