Vacuum variable capacitor

This allows for a higher voltage rating than an air dielectric[1] using a smaller total volume.

The most common form is inter-meshed concentric cylinders, which are contained within a glass or ceramic vacuum envelope, similar to an electron tube.

A metal bellows is used to maintain a vacuum seal while allowing positional control for the moving parts of the capacitor.

They are used in equipment such as high-powered broadcast transmitters, amateur radio RF amplifiers and large antenna tuners.

[1] The main applications today are RF plasmas of 2 to 160 MHz where the vacuum capacitor is used as the impedance variation part in an automatic matching network in the fabrication of chips and flat panel displays.