Filter

A cavity filter, comprising at least first and second dielectric resonator bodies, each incorporating a piece of dielectric material, each piece of dielectric material having a shape such that it can support at least a first resonant mode, at least one excitation device for at least one of establishing an electromagnetic field within at least a first dielectric resonator body or extracting energy from an electromagnetic field located within the at least a first dielectric resonator body, a layer of electrically conductive material in contact with and covering at least a surface of the first dielectric resonator body and a surface of the second dielectric resonator body, at least one aperture in the layer of electrically conductive material for at least one of inputting signals to the second dielectric resonator body and outputting signals from the second dielectric resonator body wherein the excitation device is arranged to directly excite the first resonant mode or directly extract energy from the first resonant mode in the second dielectric resonator via the at least one aperture.