A high isolated coupled-line passive circulator for UHF RFID reader

A coupled-line passive circulator is proposed for improving the TX/RX isolation of the UHF radio frequency identification (RFID) reader. The increased isolation is realized by generating and adding the canceling signal to the leakage. The input return loss of the reader antenna, which can degrade TX-to-RX isolation, as well as the inherently imperfect isolation characteristic of a coupled-line directional coupler, are included in the design of the coupled-line passive circulator. The proposed coupler exhibits up to 60-dB isolation characteristic with the mismatch of the employed reader antenna. The performance comparison between the proposed and the conventional coupled-line directional couplers shows a significant improvement in the leakage-to-signal ratio by up to 35 dB.