Air and vacuum variable capacitors for comparison: The
air capacitor shown is variable from 34 to 864 pF (25:1 capacitance
range), and has a plate spacing of 1.6 mm giving a voltage rating
of 5 kV peak (3.5 kV RMS). The dimensions of the capacitor frame
(excluding protruding studs and mounting brackets) are: 260 ×
126 × 135 mm. The vacuum capacitor (shown to comparitive
scale) also has a voltage rating of 5 kV peak, but is variable
from 10 to 1000 pF (100:1 range). It is 77.5 mm in diameter at
its widest point, and is 171 mm long excluding the control shaft.
Notwithstanding its advantages in terms of dimensions and variation
range, the vacuum capacitor can be expected to have an ESR considerably
smaller than that of the air capacitor, and being more compact
has a much smaller inductance.