Antenna apparatus and method

1. An antenna comprising:

at least two antenna elements arranged in a recess of a ground conductor,

a wall of the recess being arranged so that the recess tapers outward from a narrow base inside the recess to a broader mouth to provide an open inverted frustum,

the wall being configured to provide a ground plane for the at least two antenna elements, and

the at least two antenna elements each comprise a conductive plate arranged perpendicular to the mouth of the recess and to the wall to provide slots between the wall of the recess and the edges of each of the at least two antenna elements.

2. The antenna of claim 1 wherein the antenna further comprises at least two signal couplings each arranged to couple to a corresponding one of the at least two antenna elements to drive said corresponding one of the at least two antenna elements with respect to the ground conductor.

3. The antenna of claim 2 wherein the antenna

a) is arranged to provide one receive channel per antenna element, and/or

b) is arranged to provide one transmit channel per antenna element, and/or

c) is adapted to transmit and/or receive a plurality of independent signals, and/or

d) comprises at least four antenna elements to provide a 4×4 MIMO antenna.

4. The antenna of claim 3 wherein the at least two antenna elements are each arranged to provide different bandwidths and/or a different center frequency.

5. The antenna of claim 4 wherein the antenna elements each comprise a part-circular sector, wherein the radius of the part-circular sector of a particular antenna element is chosen to select the bandwidth of the particular antenna element.

6. The antenna of claim 4 wherein the antenna elements each comprise a feed-point for coupling the antenna element to a signal cable, and a part-circular sector, wherein the positioning of the feed-point with respect to the part circular sector is chosen to select the center frequency of each antenna element.

7. The antenna of claim 5 wherein

a) the center frequencies of at least two of the antenna elements are different, and/or

b) the bandwidths of at least two of the antenna elements are at least partially overlap, and/or

c) the bandwidths of at least two of the antenna elements are at least partially different.

8. The antenna of claim 1 comprising at least one half-Vivaldi antenna element arranged in a recess, wherein the recess is configured to provide a ground plane for the at least one half-Vivaldi antenna element.

9. The antenna of claim 8 wherein the at least one half-Vivaldi antenna element is arranged to provide a slot between the edge of the at least one half-Vivaldi antenna element and a wall of the recess, wherein the at least one half-Vivaldi antenna element comprises a conductive plate arranged perpendicular to a mouth of the recess, wherein the slot is directed towards the mouth of the recess.

10. The antenna of claim 8 comprising at least one antenna element arranged in a recess, wherein the recess is configured to provide a ground plane for the at least one antenna element and the antenna element comprises a conductive plate arranged perpendicular to the mouth of the recess and being spaced from a wall of the recess to provide a slot between an edge of the conductive plate and the wall of the recess.

11. The antenna of claim 1 wherein the slot broadens out towards the mouth of the recess so that the shape of the slot comprises at least one of an exponential curve, a linear taper, a stepped profile, and at least one change in the angle of the slot.

12. The antenna of claim 1 comprising at least two of the antenna elements in the recess, wherein the slots of the at least two antenna elements are directed away from each other, wherein the slots are directed in different azimuthal directions, wherein being directed away from each other comprises being directed in orientations which differ by at least 90 degrees.

13. The antenna of claim 1 wherein the wall of the recess comprises a planar face and the at least one antenna element is arranged perpendicular to the planar face, wherein the planar face is tapered from a narrow apex within the recess to a broader base at the mouth of the recess, so that the wall of the recess provides an open polyhedral form.

14. The antenna of claim 1 wherein the wall of the recess comprises a curved face and the at least one antenna element is arranged perpendicular to the curved face.

15. The antenna of claim 14 wherein the curved face has a negative curvature so the sloping of the face is less towards the perimeter of the recess.

16. The antenna of claim 1 wherein the recess is one of: (i) open; and (ii) enclosed by a non-conductive material such as a dielectric radome.