Cavity Filter

Microwave filters are used to eliminate unwanted frequency components in the output from microwave transmitters. They are typically inserted between a power amplifier and an antenna. The amplifiers are nonlinear and produce harmonics that must be eliminated with filters that have a rather narrow passband.
Due to high power loads but also possibly from harsh environmental conditions (such as a cellular base station sitting in the sunshine in Phoenix), it is necessary to estimate the drift of the passband frequency due to thermal expansion.
It is easy to demonstrate that by using steel for the cylinder, a temperature-driven adjustment of the distance between the cylinder end and the brass box (where the adjustment screw can automatically compensate for most of the thermal drift.