The simultaneous transmission and reception technology plays a crucial role in sensor tag technologies similar to radio-frequency identification (RFID), which have simultaneous wireless information and power transfer (SWIPT). This technology allows for the simultaneous transmission and reception of information on the same frequency, or the simultaneous power supply and reception of messages. On the reader side, the transmitter and receiver must have high isolation to avoid self-interference caused by the transmitter. Sufficient sensitivity is required for a long reading distance. Using a circulator as a duplexer, In-Band Full-Duplex operates. Although the circulator transmits and receives signals simultaneously, it has insufficient isolation. In addition, reflections caused by imperfect antenna matching also interfere with the receiver. Since the reflection signal of the antenna and the leakage signal of the circulator are at a similar power level, we designed an impedance adjustment circuit to adjust the amplitude and phase of the reflection signal at the antenna end to counteract the self-interference signal of the circulator for self-interference elimination. Unlike the traditional pi-type impedance adjustment circuit with three variables, the proposed impedance adjustment circuit has two variables to adjust the impedance of the antenna in the range of voltage standing wave ratio (VSWR) of less than 3. A micro controller is used to automatically find the best result. The experimental results showed that the isolation at 2.45 GHz was 74 dB with which IoT sensor tag readers had power supply and communication in 5 m.
