A low-cost impedance measurement method for UHF band RFID near field antenna

In this paper, an impedance measurement method for UHF band RFID near field reader antenna (NFRA) by using coaxial line is proposed. The coaxial line is directly connected to the balanced output port of the NFRA, and then the S parameter is obtained including NFRA and the coaxial line. The impedance can be derived by using deembedding technique which removes the effect of cascading coaxial line. Therefore, the input impedance of the NFRA can be accurately obtained. Based on the measured input impedance, the matching network can be well designed to match NFRA to 50 ohm input port. This method avoid the inaccuracy of the simulated impedance caused by dramatic variation with frequency, and final measured and simulated results agree well and therefore verify the efficiency of the method shown in this paper