A Passive Circulator for RFID Application with High Isolation using a Directional Coupler

In this paper, a newly invented circulator for T/R switch of UHF radio-frequency identification (RFID) application is proposed to overcome TX-to-RX leakage problem. A microstrip coupled-line directional coupler is used for this passive circulator and the isolation characteristic is drastically improved not by complex method such as compensating phase velocity, but by considering both directional coupler itself and imperfect input impedance of the employed antenna. The measurement result of the proposed circulator using modified microstrip coupled-line directional coupler with employed antenna shows excellent TX-to-RX leakage suppression in the 860 MHz - 960 MHz, more than 45 dB enhancement at the center frequency compared to the conventional coupler. Experimental verification using RFID system is also performed to prove the validity of the proposed circulator.