Triple-mode resonator and waveguide filter comprising the same

A triple-mode resonator, comprising:

• a main body having a block of a dielectric material and an overlay of conductive material on the block;
• a first set of coupling windows, which are provided in the overlay of conductive material on a first face of the main body to be coupled with a first single-mode resonator, and through which signals can be coupled into the main body;
• and
• a second set of coupling windows, which are provided in the overlay of conductive material on a second face of the main body to be coupled with a second single-mode resonator, and through which signals can be coupled out of the main body,
• wherein the first and second sets of coupling windows each comprise an annular coupling window and a plurality of neighboring coupling windows located adjacent to the annular coupling window,
• when viewed in cross-section, a substantially closed region defined by an inner edge of each annular coupling window is positioned substantially within an electric field-concentrated area respectively associated with the first or second face of the main body, and each annular coupling window is configured such that predetermined settings of parasitic zero point can be obtained by its cooperation with neighboring coupling windows on the first and second faces of the main body.