Multi-mode filter

1. A multi-mode cavity filter comprising:

a dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material being free of cutouts and holes for coupling probes and having a shape supporting at least a first resonant mode and a second substantially degenerate resonant mode;

a conductive layer substantially covering the dielectric resonator body but having one or more apertures therein allowing access to a surface of the dielectric resonator body; and

a coupling structure arranged in an aperture of the one or more apertures, said coupling structure being on or contacting the surface of the dielectric resonator body, said coupling structure comprising at least one coupling path for at least one of coupling an input signal to the first and second resonant modes and coupling an output signal from the first and second resonant modes, the coupling path having an open-circuit end located adjacent to an edge of the aperture for controlling a strength of electric field generated by the coupling structure,

wherein the open-circuit end is a first open-circuit end, wherein the edge of the aperture is a first edge of the aperture, and wherein the coupling path comprises a second open-circuit end located adjacent to a second edge of the aperture, and

wherein the first open-circuit end is located a first distance from the first edge of the aperture, wherein the second open-circuit end is located a second distance from the second edge of the aperture, and wherein the second distance is greater than the first distance.

2. The multi-mode cavity filter according to claim 1 , wherein the conductive layer comprises a recess surrounding, on two or more sides, the open-circuit end of the coupling path.

3. The multi-mode cavity filter according to claim 1 , wherein a first electric field portion generated at the first open-circuit end has a different magnitude from a second electric field portion generated at the second open-circuit end.

4. The multi-mode cavity filter according to claim 1 , wherein the coupling path is electrically decoupled from the conductive layer.

5. The multi-mode cavity filter according to claim 4 , wherein a first electric field portion generated at the first open-circuit end is in an opposite direction from a second electric field portion generated at the second open-circuit end.

6. The multi-mode cavity filter according to claim 1 , wherein the first and second edges are on opposite sides of the aperture.

7. The multi-mode cavity filter according to claim 1 , wherein the coupling path comprises a conductive track.

8. The multi-mode cavity filter according to claim 1 , wherein the aperture is formed in the conductive layer on a face of the dielectric resonator body, and wherein the aperture has substantially the same shape as the face.

9. A multi-mode cavity filter comprising:

a dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material having a shape supporting at least a first resonant mode and a second substantially degenerate resonant mode;

a conductive layer substantially covering the dielectric resonator body but having one or more apertures therein allowing access to the dielectric resonator body; and

a coupling structure arranged in an aperture of the one or more apertures, comprising at least one coupling path for at least one of coupling an input signal to the first and second resonant modes and coupling an output signal from the first and second resonant modes, the coupling path having an open-circuit end located adjacent to an edge of the aperture for controlling a strength of electric field generated by the coupling structure,

wherein the conductive layer further comprises a protrusion extending across the aperture towards the open-circuit end of the coupling path.

10. A multi-mode cavity filter comprising:

a dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material having a shape supporting at least a first resonant mode and a second substantially degenerate resonant mode;

a conductive layer substantially covering the dielectric resonator body but having one or more apertures therein allowing access to the dielectric resonator body; and

a coupling structure arranged in an aperture of the one or more apertures, comprising at least one coupling path for at least one of coupling an input signal to the first and second resonant modes and coupling an output signal from the first and second resonant modes, the coupling path having an open-circuit end adjacent to an edge of the aperture for controlling a strength of electric field generated by the coupling structure,

wherein the conductive layer comprises a recess in the edge of the aperture, extending away from the open-circuit end of the coupling path.

11. A multi-mode cavity filter comprising:

a dielectric resonator body incorporating a piece of dielectric material, the piece of dielectric material being free of cutouts and holes for coupling probes and having a shape such that it can support at least a first resonant mode and a second substantially degenerate resonant mode;

a conductive layer substantially covering the dielectric resonator body but having one or more apertures therein allowing access to a surface of the dielectric resonator body; and

a coupling structure arranged in an aperture of the one or more apertures, said coupling structure being on or contacting the surface of the dielectric resonator body, said coupling structure comprising at least one coupling path for at least one of coupling an input signal to the first and second resonant modes and coupling an output signal from the first and second resonant modes, the coupling path having an open-circuit end located adjacent to an edge of the aperture for controlling a strength of electric field generated by the coupling structure,

wherein the open-circuit end is a first open-circuit end, wherein the edge of the aperture is a first edge of the aperture, and wherein the coupling path comprises a second open-circuit end located adjacent to a second edge of the aperture,

wherein the coupling path is electrically decoupled from the conductive layer, and

wherein a first electric field portion generated at the first open-circuit end is in an opposite direction from a second electric field portion generated at the second open-circuit end.

12. The multi-mode cavity filter according to claim 11 , wherein the conductive layer comprises a recess surrounding, on two or more sides, the open-circuit end of the coupling path.

13. The multi-mode cavity filter according to claim 11 , wherein the first open-circuit end is located a first distance from the first edge of the aperture, wherein the second open-circuit end is located a second distance from the second edge of the aperture, and wherein the second distance is greater than the first distance.

14. The multi-mode cavity filter according to claim 13 , wherein the first electric field portion generated at the first open-circuit end has a different magnitude from the second electric field portion generated at the second open-circuit end.

15. The multi-mode cavity filter according to claim 11 , wherein the first and second edges are on opposite sides of the aperture.

16. The multi-mode cavity filter according to claim 11 , wherein the coupling path comprises a conductive track.

17. The multi-mode cavity filter according to claim 11 , wherein the aperture is formed in the conductive layer on a face of the dielectric resonator body, and wherein the aperture has substantially the same shape as the face.