Compact circular polarized antenna

1. A circular polarized antenna, comprising:

an electrically conductive element having:

a generally annular outer portion, the outer portion having an outer periphery and an inner periphery; and

first and second elongate inner members coupled to the outer portion;

a ground shield spaced from the element, the ground shield providing an effective ground plane, the effective ground plane having a maximum width in a direction parallel to a plane of the element of less than about one-half of an operating wavelength; and

a dielectric material positioned between the element and at least a portion of the ground shield.

2. An antenna as recited in claim 1 , wherein the first and second elongate inner members lie along a common plane with the outer portion.

3. An antenna as recited in claim 1 , wherein the first and second elongate inner members cross each other, each of the elongate inner members having two points of contact with the outer portion.

4. An antenna as recited in claim 1 , wherein the outer periphery of the outer portion is generally rectangular shaped.

5. An antenna as recited in claim 1 , wherein the inner and outer peripheries of the outer portion are square shaped.

6. An antenna as recited in claim 5 , wherein the inner periphery of the outer portion includes corners and straight edges, wherein the first and second elongate inner members are perpendicular to each other and extend between straight edges of the inner periphery of the outer portion.

7. An antenna as recited in claim 6 , wherein the first and second elongate inner members lie along a common plane with the outer portion, wherein the first and second elongate inner members are continuous with the outer portion.

8. An antenna as recited in claim 1 , wherein the first and second elongate inner members are folded.

9. An antenna as recited in claim 1 , wherein a length to width ratio of each elongate inner member is at least about 10 to 1.

10. An antenna as recited in claim 1 , further comprising a first feeding pin coupled to the first elongate inner member and a second feeding pin coupled to the second elongate inner member, wherein signals sent to the feeding pins have different phases relative to each other.

11. An antenna as recited in claim 1 , wherein a portion of the ground shield lies on a same plane as the element.

12. An antenna as recited in claim 11 , wherein the ground shield is positioned closest to the element in an area where the ground shield is in the same plane as the element.

13. An antenna as recited in claim 1 , wherein the dielectric material has a dielectric constant less than about 2 at 0° C.

14. An antenna as recited in claim 1 , wherein a gain of the antenna is greater than about 2 dBi.

15. An antenna as recited in claim 1 , wherein a width of the outer portion as measured between the inner and outer peripheries thereof is at least twice a width of each of the elongate inner members as measured in a direction perpendicular to an axis thereof and parallel to a plane of the element.

16. A Radio Frequency Identification (RFID) system, comprising:

a plurality of RFID tags; and

an RFID interrogator in communication with the RFID tags, the RFID interrogator being coupled to the antenna of claim 1 .

17. A circular polarized antenna, comprising:

a substantially square-shaped electrically conductive element having a plurality of voids defined therein, edges of the element along the voids defining an outer portion of the element and at least two elongate inner members of the element;

a ground shield spaced from the element and having a substantially square-shaped outer periphery, the ground shield providing an effective ground plane, the effective ground plane having widths in a direction parallel to a plane of the element and perpendicular to each other and perpendicular to straight sections of the outer periphery of less than about one-third of an operating wavelength; and

a dielectric material positioned between the element and at least a portion of the ground shield.

18. An antenna as recited in claim 17 , wherein the elongate inner members lie along a common plane with the outer portion.

19. An antenna as recited in claim 17 , wherein the elongate inner members cross each other perpendicularly, each of the elongate inner members having two points of contact with the outer portion.

20. An antenna as recited in claim 17 , wherein the elongate inner members are folded.

21. An antenna as recited in claim 17 , wherein the inner periphery of the outer portion includes corners and straight edges, wherein the elongate inner members are perpendicular to each other and extend between straight edges of the inner periphery of the outer portion.

22. An antenna as recited in claim 17 , further comprising a first feeding pin coupled to one of the elongate inner members and a second feeding pin coupled to another of the elongate inner members, wherein signals sent to the feeding pins have different phases relative to each other.

23. A Radio Frequency Identification (RFID) system, comprising:

a plurality of RFID tags; and

an REID interrogator in communication with the REID tags, the RFID interrogator being coupled to the antenna of claim 17 .

24. A circular polarized antenna, comprising:

a substantially square-shaped electrically conductive element having a plurality of voids defined therein, edges of the element along the voids defining an outer portion of the element and at least two elongate inner members of the element;

a ground shield spaced from the element and having a substantially square-shaped outer periphery, the ground shield providing an effective ground plane, the effective ground plane having widths in a direction parallel to a plane of the element and perpendicular to each other and perpendicular to straight sections of the outer periphery of less than about one-third of an operating wavelength, wherein a portion of the ground shield lies on a same plane as the element; and

a dielectric material positioned between the element and at least a portion of the ground shield.

25. An antenna as recited in claim 24 , wherein the ground shield is positioned closest to the element in an area where the ground shield is in the same plane as the element.

26. A circular polarized antenna, comprising:

an electrically conductive element having a generally annular outer portion and inner members extending from an inner periphery of the outer portion and lying in about the same plane as the outer portion;

a ground shield spaced from the element, the ground shield providing an effective ground plane, the effective ground plane having a maximum width in a direction parallel to a plane of the element of less than about one-half of an operating wavelength; and

a dielectric material positioned between the element and at least a portion of the ground shield, the dielectric material having a dielectric constant less than about 2 at 0° C.

27. An antenna as recited in claim 26 , wherein a portion of the ground shield lies on the same plane as the element.

28. An antenna as recited in claim 26 , further comprising a first feeding pin coupled to a first of the inner members and a second feeding pin coupled to a second of the inner members, wherein signals sent to the feeding pins have different phases relative to each other.

29. An antenna as recited in claim 26 , wherein a length to width ratio of each inner member is at least about 10 to 1.

30. A Radio Frequency Identification (RFID) system, comprising:

a plurality of RFID tags; and

an RFID interrogator in communication with the RFID tags, the RFID interrogator being coupled to the antenna of claim 26 .