Impedance measurement for balanced UHF RFID tag antennas

Dipole antennas in UHF RFID tags are driven by differential signals; it is difficult to measure the antenna input impedance accurately using single-ended probes. Furthermore, the testing fixture used in the antenna measurement process also induces additional parasitic effects; accurate measurement cannot be achieved without taking the fixture impact into consideration. In this paper, we propose to characterize the dipole antenna's input impedance using mixed mode S-parameters, the testing fixture impact can also be quantified with port extension and de-embedding techniques. Experiments demonstrate that the proposed measurement method achieves very consistent results over UHF frequency band with very high accuracy.