Design of passive UHF RFID circulators/duplexers using directional couplers

The design of passive circulators/duplexers for RFID applications in the UHF band is presented. The design idea is based on the use of simple passive directional couplers, offering simplicity and low cost compared to ferrite devices or active architectures. The Tx/Rx isolation problem is solved exploiting a mismatched load, which creates a cancelling signal on the Rx port. Additional impedance matching is applied at the Tx port, in order to balance the effects of the mismatch load at the input (Tx). Both branch-line and ring couplers are alternatively used, designed mainly for unequal power division between output ports. A variety of design examples is presented, including miniaturized topologies, aiming at the reduction of size and cost. Design frequency is chosen at 900MHz, trying to cover both 860MHz and 910MHz bands.