Filter

1. A cavity filter comprising:

a dielectric resonator structure including a piece of dielectric material having a shape supporting a first resonant mode and a second resonant mode, said second resonant mode being substantially degenerate with the first resonant mode;

and

a coupling structure for exciting at least one of the first resonant mode and the second resonant mode within the piece of dielectric material or for extracting energy from at least one of the first resonant mode and the second resonant mode within the piece of dielectric material,

wherein the coupling structure includes a single patch element in contact with a surface of the piece of dielectric material, the patch element being a substantially flat conductive element arranged to couple directly to said first and second resonant modes simultaneously,

the cavity filter further including first and second feedpoints connected to the patch element for feeding input signals to the patch element or for receiving output signals from the patch element,

wherein the first and second feedpoints are connected to the substantially flat conductive element so as to generate a circularly polarized field.

2. A cavity filter comprising:

a dielectric resonator structure including a piece of dielectric material having a shape supporting a first resonant mode and a second resonant mode, said second resonant mode being substantially degenerate with the first resonant mode; and

a coupling structure for exciting at least one of the first resonant mode and the second resonant mode within the piece of dielectric material or for extracting energy from at least one of the first resonant mode and the second resonant mode within the piece of dielectric material,

wherein the coupling structure includes a single patch element in contact with a surface of the piece of dielectric material, the patch element being a substantially flat conductive element arranged to couple directly to said first and second resonant modes simultaneously,

the cavity filter further including first and second feedpoints connected to the patch element for feeding input signals to the patch element or for receiving output signals from the patch element,

wherein the first and second feedpoints are connected to the substantially flat conductive element so as to generate a first linearly polarized field and a second linearly polarized field.

3. A cavity filter comprising:

a dielectric resonator structure including a piece of dielectric material having a shape supporting a first resonant mode and a second resonant mode, said second resonant mode being substantially degenerate with the first resonant mode; and

a coupling structure for exciting at least one of the first resonant mode and the second resonant mode within the piece of dielectric material or for extracting energy from at least one of the first resonant mode and the second resonant mode within the piece of dielectric material,

wherein the coupling structure includes 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,

the cavity filter further including a feedpoint connected to the patch element for feeding input signals to the patch element or for receiving output signals from the patch element, wherein the feedpoint is connected to the patch element at a location displaced from an electrical center of the patch element,

wherein the patch element is located on a face of the piece of dielectric material, and wherein the feedpoint is displaced from the electrical center of the patch element in a direction oblique relative to sides of the face.

4. The cavity filter according to claim 3 , wherein the substantially flat conductive element is adapted for generating an electric field at least partially for exciting the first resonant mode and a magnetic field at least partially for exciting the second resonant mode, or at least partially for extracting energy from an electric field of the first resonant mode and a magnetic field of the second resonant mode.

5. The cavity filter according to claim 3 , wherein the patch element is located at a position which is offset from a center of the face.

6. The cavity filter according to claim 5 , wherein the patch element is offset from a center of the face in a direction oblique relative to sides of the face.

7. The cavity filter according to claim 3 , wherein the shape of the piece of dielectric material is such that excitation of the first resonant mode causes excitation of the second resonant mode.

8. The cavity filter according to claim 3 , wherein the shape of the piece of dielectric material is such that the first resonant mode and the second resonant mode are capable of being simultaneously independently excited.

9. The cavity filter according to claim 3 , wherein the piece of dielectric material is covered by a layer of conductive material, the layer of conductive material having an aperture in which the coupling structure is located.

10. The cavity filter according to claim 3 , wherein the coupling structure is a first coupling structure for exciting a resonant mode within the piece of dielectric material or for extracting energy from a resonant mode within the piece of dielectric material, the cavity filter further comprising a second coupling structure for exciting a resonant mode within the piece of dielectric material or for extracting energy from a resonant mode within the piece of dielectric material.

11. The cavity filter according to claim 3 , wherein the single patch element has a shape arranged to couple to the first and second resonant mode via at least one of E-field excitation and H-field excitation.

12. The cavity filter according to claim 3 , wherein the piece of dielectric material is cuboid.