A cavity filter, including a dielectric resonator structure comprising a piece of dielectric material having a shape such that it can support a first resonant mode and a second resonant mode which is substantially degenerate with the first resonant mode; and a coupling structure for exciting a resonant mode within the piece of dielectric material or extracting energy from a resonant mode within the piece of dielectric material; wherein the coupling structure consists of a single patch element in contact with a surface of the piece of dielectric material, the patch element being arranged to couple directly to said first and second resonant modes simultaneously.