Measurement of UHF RFID tag antenna impedance

A methodology to measure UHF RFID tag antenna impedance using network approach is presented. The balanced UHF RFID tag antenna is considered as a two-port network and the impedance of the antenna is characterized by network parameters. In the measurement, the balanced antenna is connected to the two ports of a two-port vector network analyzer (VNA) through a test fixture. The influence of the test fixture is de-embedded by using a portextension technique and then the antenna impedance is extracted directly from the measured S-parameters. The proposed method is validated by comparing the measured and the simulated impedance of an asymmetrical dipole antenna and a symmetrical meander line dipole antenna both for UHF RFID tags.