Abstract: A re-entrant resonant cavity 12 includes three parts 18, 19 and 20 which may be manufactured as metallized plastic components. The three parts 18, 19 and 20 are soldered to a multilayer PCB 23 using surface mount technology. The re-entrant stub 16 is in two portions with dielectric material provided by the PCB 23 between them. The cylindrical wall 13 surrounding the stub 16 is also divided into two sections 21 and 22 by the PCB 23. Vias 24 and 25 electrically connect the parts separated by the PCB 23. The pattern of the vias 24 and 25 determines the inductance of the cavity, and hence its resonance frequency. This enables cavities with the same geometry to be operated at different resonance frequencies by using different configurations of through connects. One of the sets of vias may be omitted in some cavities.