RFID reader front-end having robust Tx leakage canceller for load variation

A novel front-end structure for ultra-high-frequency RF identification reader is proposed. The proposed front-end consists of a balanced coupler and four-port feed antenna system. The proposed balanced coupler structure has high transmit/receiver isolation feature and good insensitivity to load variations. The proposed four-port feed antenna system is applied for removing the coupling among antennas, which deteriorates the isolation. The proposed system is numerically analyzed and verified experimentally as well. Index Terms--Antenna isolation, directional coupler, load impedance variation, ultra-high-frequency (UHF) RF identification (RFID).