APassive Circulator forRFIDApplication withHighIsolation using aDirectional Coupler

Inthis paper, anewlyinvented circulator forT/R switch ofUHF radio-frequency identification (RFID) application isproposedto overcomeTX-to-RXleakageproblem.A microstrip coupled-line directional coupler isusedforthis passive circulator andtheisolation characteristic isdrastically improved notbycomplexmethodsuchascompensating phase velocity, butbyconsidering bothdirectional coupler itself and imperfect inputimpedance oftheemployedantenna. The measurement result oftheproposed circulator usingmodified microstrip coupled-line directional couplerwithemployed antennashowsexcellent TX-to-RXleakage suppression inthe 860MHz - 960MHz,morethan45dB enhancement atthe centerfrequency comparedto theconventional coupler. Experimental verification using RFIDsystemisalsoperformed toprovethevalidity oftheproposed circulator. IndexTerms - Coupled line directional coupler, RFID,High Isolation, TX-to-RXleakage.