Simple Cylindrical Cavity Wavemeter:

Resonant cavities find extensive use as wavemeter at microwave frequencies. They enjoy high accuracy, mechanical simplicity, and a large physical size in proportion to the wavelength being measured. Practical cavity wavemeter are always in the form of cylinders which are tunable by means of a piston that varies the length of the cavity, as in Fig. 16.2.

Coupling to a wavemeter is accomplished by a loop or an orifice. A loop is used when co-axial systems are used to excite a cavity or carry power from it, while an orifice is normally used in a waveguides system.

The accuracy with which frequency can be determined using a well made cavity wavemeter is quite high. Precision of the order of 1 part in 1000 is easy to obtain, and 1 part in 100,000 can be obtained by careful mechanical design combined with temperature compensation and correction