A microwave resonator comprising a hollow tube comprising fan electrically conductive tube wall which defines a tube bore, the tube extending along a length axis from a first end to a second end; a first electrically conductive closing plate closing the first end of the tube; a second electrically conductive closing plate closing the second end of the tube; a plurality of dielectric resonant pucks, each puck comprising first and second end faces and a side wall extending therebetween, each puck being dimensioned such that its dominant mode is a doubly degenerate mode; the pucks being arranged within the tube bore spaced apart from each other and the closing plates, each puck being arranged with its end faces normal to the length axis and centered on the length axis and its side wall abutting the tube wall such that there is no air gap between the puck and tube wall which extends from one end face to the other of the puck, the puck adjacent to the first closing plate being termed the input puck; each puck being separated from the adjacent puck in the tube bore by a coupling gap, each coupling gap having ah electrically conductive iris plate arranged therein, each iris plate being arranged normal to the length axis, each iris plate comprising at least one coupling slot extending therethrough; an input microwave coupler adapted to receive a microwave signal and provide it to the input puck; each puck comprising a symmetry breaking structure for modifying the frequency of one of the degenerate modes relative to the other and the coupling between the two modes.