Circularly polarized patch antenna with high Tx / Rx-separation

In this contribution an antenna is described that allows simultaneous transmission and reception in the same frequency band. Circular polarization is employed for a most reliable communication with radio frequency identification (RFID) transponders that typically have linearly polarized antennas. To achieve high separation between transmitted and received signals, a square patch antenna originally transmitting horizontally and vertically polarized radiation is combined with a 3 dB-hybrid circuit. With this hybrid circuit the antenna can simultaneously radiate a right-hand circularly polarized wave and receive a left-hand circularly polarized wave. Furthermore, the transmit signal that is unintentionally leaking into the receive path can be compensated by tuning the hybrid circuit with two variable capacitance diodes. At 866 MHz a maximum Tx/Rx-separation of 65 dB was achieved in a static scenario. In a time variant indoor scenario with a metal object moving on a conveyor belt a Tx/Rx-separation of more than 52 dB was achieved by continuously tuning the hybrid circuit with a minimum-search algorithm.