Coaxial  capacitors

Recall from that the sections of transmission lines can work as inductors. They can behave as capacitors too.

Figure--   A trimmer capacitor.

If a section of transmission line is below 1/4 wavelength long, and is left open at the far end (instead of shorted out), it will behave as a capacitor. The capacitance will increase with the length.

The common transmission line capacitor uses 2 telescoping sections of tubing. This is known as coaxial capacitor and works as there is a certain effective surface area between inner and outer tubing sections. A sleeve of the plastic dielectric can be placed between the sections of tubing, as shown in the Figure given below. This allows capacitance to be adjusted by sliding inner section in or out of outer section.

Figure--   A coaxial variable capacitor.

Coaxial capacitors are taken in use in radio-frequency applications, specifically  in antenna systems. Their values are usually from a few picofarads up to about 100 pF.