A re-entrant resonant cavity 12 includes a first metallized molded plastic component 18, which comprises a re-entrant stub 17, an end wall 14 and a cylindrical side wall 13. The component 18 is surface mount soldered to a metallized PCB substrate 19. A rostrum 24 is located facing the end face 21 of the stub 17 to define a capacitive gap 22 with it. The end face 21 of the stub 17 and the rostrum 24 are configured such that relative rotation between them changes the profile of the gap 22 and hence the gap capacitance. By suitably locating the two parts during manufacture, a particular capacitance may be chosen to give a desired resonance frequency from a selection available depending on the relative angular position of the stub 17 and rostrum 24. In another cavity, the rostrum is replaced by an etched metallization layer of a printed circuit board.