Tuneable frequency selective surface

1. An electronically tuneable surface, comprising:

a conductive sheet comprising at least one opening; and

a biasing circuit comprising:

first and second conductors, separated from the conductive sheet by a dielectric, and arranged at mutually opposing sides of the opening such that each conductor is capacitively coupled to the conductive sheet at the respective side of the opening, with a gap defined between the conductors corresponding to the opening; and

an electrical control element bridging the gap, connected to the first and second conductors, wherein when the electrical control element is in a first state, the electronically tuneable surface has a first frequency transmission characteristic with respect to incident electromagnetic radiation, and when the electrical control element is in a second state, the electronically tuneable surface has a second frequency transmission characteristic, different from the first frequency transmission characteristic.

2. The surface of claim 1 , wherein the first and second conductors are electrically isolated from the conductive sheet.

3. The surface of claim 1 , wherein the state of the electrical control element is controlled by a bias voltage applied between the first and second conductors.

4. The surface of claim 1 , wherein the biasing circuit comprises a plurality of electrical control elements bridging the gap, distributed along a length of the opening.

5. The surface of claim 4 wherein elements among the plurality of elements are independently controllable, so as to provide more than two states, each having a different frequency transmission characteristic.

6. The surface of claim 1 , wherein the opening comprises an elongate portion in an orientation corresponding to a predetermined electromagnetic polarisation.

7. The surface of claim 6 , wherein the opening comprises elongate portions in at least two linearly independent orientations, corresponding to predetermined different electromagnetic polarisations.

8. The surface of claim 7 , wherein the portions of the gap corresponding to the at least two elongate portions of the opening are each bridged by a different electrical control element, these control elements being independently controllable.

9. The surface of claim 1 , wherein the conductive sheet comprises a plurality of openings with associated biasing circuits.

10. The surface of claim 9 , wherein electrical control elements of the biasing circuits for the plurality of openings are independently controllable.

11. The surface of claim 1 , wherein the bias circuit comprises two or more layers of conductors.

12. The surface of claim 11 , wherein two of the layers of conductors of the biasing circuit are at opposing sides of the conductive sheet.

13. The surface of claim 1 , wherein the electrical control element comprises at least one of: a diode; varactor diode; and a MEMS switch.

14. A shield for selectively permitting or denying access by an RFID reader to an RFID tag, comprising the electronically tuneable surface of claim 1 , wherein the first frequency transmission characteristic is such that the RF band used by the RFID reader is blocked by the tuneable surface; and

the second frequency transmission characteristic is such that the surface is transparent to said RF band.

15. A system comprising an RF transmitter and an RF receiver, separated by an electronically tuneable surface according to claim 1 .