Abstract: A conventional microstrip directional coupler has an inherent drawback of poor isolation due to an unequal phase velocity between even and odd mode. To solve this problem, the technology based power canceller with the dual-stub is presented. Mismatch is induced by the dual-stub added at the Match Port of the microstrip directional coupler. Parameters adjustment of the dual-stub can change the magnitude and phase of the reflected signal from the Match Port at the RX Port, which can effectively cancel the leaked signal from the TX Port at the RX Port. A microstrip directional coupler for the UHF RFID reader with the operating frequency band of the range of 902~928MHz is proposed. The proposed directional coupler can obtain excellent isolation of -65dB and coupling of -10dB at 920MHz, effectively improve the isolation between TX and RX and decrease the size by 20％. It is successfully used in a practical UHF RFID reader.

Key words: radio frequency identification, directional coupler, isolation, directivity, microstrip line