A Compact Low-Profile Ring Antenna With Dual Circular Polarization and Unidirectional Radiation for Use in RFID Readers

A compact low-profile square ring antenna with unidirectional radiation and dual circular polarization (CP) is proposed for uses in radio frequency identification (RFID) readers. A small size conducting plane are slotted and placed underneath the ring antenna as a reflector to achieve low profile and unidirectional radiation. A branch line coupler is meandered inside the ring radiator to save space and generate dual circular polarization. Four inverted L-shaped strips are introduced to overcome the negative effects of tight coupling between the radiator and reflector. The measured -10 dB impedance bandwidth of vertical bar S-11 vertical bar, 3 dB axial ratio (AR) bandwidth and maximum gain are of 850-1060 MHz, 876-1084 MHz and 5.62dBi, respectively. The lowest measured gain values in RFID band of 900-930 MHz is 4.54dBi and the overall size is 0.31 lambda(0) x 0.31 lambda(0) x 0.034 lambda(0) (lambda(0) is denoted the wavelength at center frequency). The proposed antenna has not only the lowest profile but also the best overall performances. It is an excellent antenna that can be fitted where there is a limited space in RFID system.