Lumped directional coupler with a varactor tuned reflector for FRID applications

In this paper, a lumped directional coupler with a tunable reflector is proposed for an UHF passive RFID front-end system in which the sensitivity is limited by the isolation between transmitter and receiver. The proposed directional coupler can tunably eliminate TX-to-RX leakage caused by both the imperfect isolation of a directional coupler and the mismatch of antenna impedance which may be changed due to near-field environment. The measured result shows excellent TX-to-RX isolation above 70 dB at 910 MHz and 55 dB over Korean UHF-RFID band.