You recall from the previous topics that sections of transmission lines can work as inductors. They can act as capacitors, too. If a section of transmission line is less than 1⁄ 4 wavelength long, and is left open at the far end (rather than shorted out), it behaves as a capacitor. The capacitance increases with length.
The most common transmission-line capacitor uses two telescoping sections of metal tubing. This is called a coaxial capacitor. It works because there is a certain effective surface area between the inner and the outer tubing sections. A sleeve of plastic dielectric is placed between the sections of tubing, as shown in following Figure. This allows the capacitance to be adjusted by sliding the inner section in or out of the outer section.
A simplified drawing of a coaxial variable capacitor.
Coaxial capacitors are used in RF applications, particularly in antenna systems. Their values are generally from a few picofarads up to about 100 pF.