An antenna system for a radio frequency identification system comprising: (a) a base; (b) a first patch antenna disposed on the base and having a circular polarization and adapted to convert between electrical signals and electromagnetic fields; and (c) a second patch antenna disposed on the base and having a circular polarization and adapted to convert between electrical signals and electromagnetic fields, the second antenna being disposed at a distance from the first antenna to define a region between the first and the antenna, the second patch antenna being positioned relative to the first patch antenna such that the electromagnetic fields of the two antennas are substantially opposing in the region.
1. An antenna system comprising:
a base;
a first patch antenna disposed on said base and having a circular polarization and adapted to convert between electrical signals and electromagnetic fields; and
a second patch antenna disposed on said base and having a circular polarization and adapted to convert between electrical signals and electromagnetic fields, said second antenna being disposed at a distance from said first antenna to define a region between said first antenna and said second antenna, said second patch antenna being oriented relative to said first patch antenna such that electric and magnetic fields of the two antennas are substantially opposing in said region.
2. The antenna system of claim 1 , wherein said system has an axis that runs through the center of said first and second antennas, and wherein each antenna has a slot to impart circular polarization to said electric and magnetic field, the slot of said first antenna being orthogonal to the slot of said second antenna, and wherein said first antenna has a first feed point at a first position and said second antenna has a second feed point at a second position, said first and second positions being located on different sides of said axis.
3. The antenna system of claim 2 , wherein said slots are at about a 45-degree angle to said axis, and said feeds are aligned with the center of their respective antenna along an imaginary line perpendicular to said axis.
4. The antenna system of claim 2 , wherein the circular polarization of one of said first and second antennas is clockwise, and the other of said first and second antennas is counterclockwise.
5. The antenna system of claim 2 , wherein said antennas are essentially the same size and shape.
6. The antenna system of claim 2 , wherein said first antenna has a diameter, and wherein the ratio of said distance to said diameter of is no greater than about 3:1.
7. The antenna system of claim 1 , wherein there are no partitions in said region.
8. The antenna system of claim 1 , wherein said antenna system has an S21 isolation of less than −30 db.
9. The antenna system of claim 1 , wherein the electric field in said region compared to that at the center of the first antenna is less than −3 db, and the magnetic field in said region compared to that at the center of the first antenna is less than −1.5 db.
10. An antenna system comprising:
a base
a first patch antenna mounted on said base and comprising a first circular surface defining a first slot and being capable of transmitting a circularly polarized signal, said first patch antenna comprising a first feed point at a first position within said first circular surface; and
a second patch antenna mounted on said base comprising a second circular surface defining a second slot orthogonal to said first slot, and being capable of receiving a return signal from a tag, said second circular surface being about the same size as said first circular surface, said second patch antenna comprising a second feed point in a second position on said second circular surface;
wherein said system has an axis that runs through the center of said first and second antennas, and wherein said first and second positions are located on different sides of the said axis.
11. The antenna system of claim 10 , wherein said first and second antennas define a region therebetween and said region is free of partitions.
12. The antenna system of claim 10 , wherein said base is elongated and has x and y axes with said x axis running along the length of said base and through the center of said first and second patch antennas, said axis being said x axis, the center of said first patch antenna being at position −x′, y′ and the center of said second patch being at position x′, y′, where y′=0, said first and second feed points being located in one of a first or second position scheme, in a first position scheme, said first feed point being at position −x″, y″, and said second feed point being at position x″, −y″ within a tolerance window, and, in said second position scheme, said first feed point being at position −x″, −y″, and said second feed point being at position x″, y″ within said tolerance window, said tolerance window being no greater than 2% of the diameter of said first antenna.
13. The antenna system of claim 12 , wherein one of said first and second antennas has a slot with a slope of x′″/y′″, and the other of said first and second antennas has a slot with a slope of about −x′″/y′″.
14. The antenna system of claim 10 , wherein said first and second patch antennas define a distance between their centers which is no greater than about 3 times the diameter of said first patch antenna.
15. The antenna system of claim 10 , wherein said antennas are about the same size and shape except for the position of their respective slots and the position of their feed point.
16. The antenna system of claim 10 , wherein either said first or second patch antenna is a transmit antenna and the other is a receive antenna.
17. A radio frequency identification (RFID) system comprising:
a controller;
an interrogator communicatively linked to said controller and comprising at least a transmitter, a receiver, and an antenna system; and
said antenna system comprising at least:
a base;
a first patch antenna mounted on said base and connected to said transmitter, said first patch antenna comprising a first circular surface defining a first slot and being capable of transmitting a circularly polarized signal to a tag, and a feed point in a first position within said first circular surface; and
a second patch antenna mounted on said base and connected to said receiver, said second patch antenna comprising a second circular surface defining a second slot orthogonal to said first slot, and being capable of receiving a return signal from a tag, said second circular surface being about the same size as said first circular surface, said second patch antenna comprising a feed point in a second position within said second circular surface;
wherein said system has an axis that runs through the center of said first and second antennas, and wherein said first and second positions are located on different sides of the said axis.
18. The RFID system of claim 17 , wherein said first and second antennas define a region therebetween and said region is free of partitions.
19. The RFID system of claim 17 , wherein said first and second patch antennas define a distance between their centers which is no greater than about 3 times the diameter of said first patch antenna.