Antenna clusters for active device reduction in phased arrays with restricted scan

1. An apparatus for microwave imaging, comprising:

a plurality of antenna clusters forming an antenna array, each antenna cluster comprising: at least two antenna elements; and an active device controlling the at least two antenna elements to direct microwave radiation to and from an object to capture a microwave image of the object, the plurality of antenna clusters further comprising: a first antenna cluster type with a first radiation pattern and a second antenna cluster type with a second radiation pattern having a different angle of tilt than the first radiation pattern.

2. An apparatus as in claim 1 , wherein the first antenna cluster type has a broadside radiation pattern, and the second antenna cluster type has an off-axis radiation pattern.

3. An apparatus as in claim 2 , wherein the first and second antenna cluster types are arranged symmetrically about a plane passing through the antenna array.

4. An apparatus as in claim 3 , wherein the first antenna cluster type is located closer to the plane than the second antenna cluster type, and wherein the off-axis radiation pattern of the second antenna cluster type is tilted towards the object.

5. An apparatus as in claim 2 , wherein the first antenna cluster type further comprises: a first antenna element; a second antenna element excited in-phase with the first antenna element; and a first active device coupled to the first antenna element.

6. An apparatus as in claim 5 , wherein the second antenna element is coupled to the first antenna element by a first transmission line, the first transmission line having a length that inserts a 180° delay between the first and second antenna elements.

7. An apparatus as in claim 6 , wherein the first active device controls the first antenna element by varying the impedance presented to the first antenna element.

8. An apparatus as in claim 2 , wherein the second antenna cluster type further comprises: a master antenna element; a slave antenna element excited out-of-phase with the master antenna element; and a second active device coupled to the master antenna element.

9. An apparatus as in claim 8 , wherein the slave antenna element is parasitically coupled to the master antenna element.

10. An apparatus as in claim 9 , wherein the second active device controls the master antenna element by varying the impedance presented to the master antenna element.

11. An apparatus as in claim 1 , wherein the antenna array scans a solid angle less than 2π steradians.

12. An apparatus as in claim 1 , wherein the plurality of antenna clusters are formed on a planar surface.

13. An apparatus as in claim 1 , wherein the at least two antenna elements are selected from the group consisting of planar patch antennas, dipoles, monopoles, slot antennas, loop antennas, open waveguides, and horns.

14. An apparatus as in claim 1 , further comprising a feedhorn that transmits and receives microwave radiation, wherein the active device controls the at least two antenna elements to reflect the microwave radiation to and from the object by varying the reflection phase of the two antenna elements.

15. An apparatus as in claim 1 , wherein the active device is selected from the group consisting of transistors, diodes, micro-electro-mechanical systems (MEMS), and variable capacitors.

16. An apparatus for microwave imaging, comprising:

a plurality of antenna clusters forming an antenna array, each antenna cluster comprising: at least two antenna elements operated in unison to direct microwave radiation to and from an object to capture a microwave image of the object, wherein the plurality of antenna clusters further comprises a first antenna cluster type with a first radiation pattern and a second antenna cluster type with a second radiation pattern having a different angle of tilt than the first radiation pattern.