Remote Antenna Impedance Estimation Using a UHF RFID Chip

This article shows that it is possible to estimate the impedance of a remote antenna connected to a UHF RFID chip. The proposed method relies on the measurement of four backscattered field values and the knowledge of only two load impedance values. The field values can be obtained using the autotuning functionality available in modern UHF RFID chips. The method does not make any assumption about the antenna geometry and can be performed multiple times to obtain the impedance of the antenna over a given bandwidth (or any other parameter). More importantly, the method is not limited by the number of autotune capacitors in the chip. The method allows one to directly transform a UHF RFID tag into a sensor if the relation between the antenna impedance and a physical quantity is known. Thanks to the proposed method, a significant part of the UHF RFID tags already deployed in the field can be transformed into sensors.