Evaluation of passive carrier-suppression techniques for UHF RFID systems

In this paper we investigate the signal that is returned from an interrogator antenna when communicating with a radio-frequency identification (RFID) transponder at a carrier frequency of 866 MHz. This signal contains the response of the transponder and also a part of the carrier signal that is leaking into the receive path. Depending on the type of interrogator antenna configuration used, both signal components are expected to vary widely in terms of power. We perform measurements with patch antennas that radiate linearly and circularly polarized waves in various configurations. Furthermore, passive methods that minimize the leaking carrier are explored and compared. Results are obtained for transponder positions covering the transition region between near- and farfield. It is found that separated transmitter and receiver antennas offer highest carrier-suppression and thus require the least effort for the receiver.