A microwave resonator comprising a hollow tube comprising an 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 (10, 11), each puck (10, 11) comprising first and second end faces (12, 13) and a side wall extending therebetween, each puck (10, 11) being dimensioned such that its dominant mode is a doubly degenerate mode; the pucks (10, 11) being arranged within the tube bore spaced apart from each other and the closing plates (8, 9), each puck (10, 11) being arranged with its end faces (12, 13) 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 which extends from one end face to the other of the puck (10, 11) between the puck (10, 11) and tube wall, the puck adjacent to the first closing plate being termed the input puck; each puck (10, 11) being separated from the adjacent puck (10, 11) in the tube bore by a coupling gap, each coupling gap having an electrically conductive iris plate arranged therein spaced apart from the end faces (12, 13) of the pucks (10, 11), 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 (10, 11) 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, the symmetry breaking structure for that puck (10, 11) comprising any of a) a first electrically conductive adjustment strip arranged on a face of the puck (10, 11), the adjustment strip extending along a first adjustment strip axis passing through the center of the puck (10,11); b) at least one aperture extending through the puck (10, 11) from one end face to the other parallel to but spaced apart from the length axis; c) at least one slot in the puck (10, 11) arranged in a plane normal to the length axis and part way between the first and second end faces (12, 13) of the puck (10, 11); and d) at least one aperture extending from the side wall of the puck (10,11) normal to the length axis. a hollow tube comprising an 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 (10, 11), each puck (10, 11) comprising first and second end faces (12, 13) and a side wall extending therebetween, each puck (10, 11) being dimensioned such that its dominant mode is a doubly degenerate mode; the pucks (10, 11) being arranged within the tube bore spaced apart from each other and the closing plates (8, 9), each puck (10, 11) being arranged with its end faces (12, 13) 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 which extends from one end face to the other of the puck (10, 11) between the puck (10, 11) and tube wall, the puck adjacent to the first closing plate being termed the input puck; each puck (10, 11) being separated from the adjacent puck (10, 11) in the tube bore by a coupling gap, each coupling gap having an electrically conductive iris plate arranged therein spaced apart from the end faces (12, 13) of the pucks (10, 11), 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 (10, 11) 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, the symmetry breaking structure for that puck (10, 11) comprising any of a) a first electrically conductive adjustment strip arranged on a face of the puck (10, 11), the adjustment strip extending along a first adjustment strip axis passing through the center of the puck (10,11); b) at least one aperture extending through the puck (10, 11) from one end face to the other parallel to but spaced apart from the length axis; c) at least one slot in the puck (10, 11) arranged in a plane normal to the length axis and part way between the first and second end faces (12, 13) of the puck (10, 11); and d) at least one aperture extending from the side wall of the puck (10,11) normal to the length axis. a) a first electrically conductive adjustment strip arranged on a face of the puck (10, 11), the adjustment strip extending along a first adjustment strip axis passing through the center of the puck (10,11); b) at least one aperture extending through the puck (10, 11) from one end face to the other parallel to but spaced apart from the length axis; c) at least one slot in the puck (10, 11) arranged in a plane normal to the length axis and part way between the first and second end faces (12, 13) of the puck (10, 11); and d) at least one aperture extending from the side wall of the puck (10,11) normal to the length axis.