Experimental characterization of the impedance of balanced UHF RFID tag antennas

An experimental procedure for the accurate characterization of the impedance of balanced tag antennas for RFID applications is reintroduced. The balanced devices that are characterized are modeledas 2-port networks and their impedance is derived from network parameters, through a rarely used equation that is preferred because it accounts for the losses of non-reciprocal structures such as imperfect test fixtures or baluns. The measurements are conducted using a test fixture for the proper excitation of the balanced devices, and its effects are removed by shifting the measurement plane to the plane of the device under test. The accuracy of the proposed method is demonstrated through the measurement of the impedance of a balanced dipole antenna, and is complemented by the measurement of the RFID ASIC chip. The experimental characterization of the impedance of an RFID tag antenna constitutes a secure way to achieve conjugate matching between the 2 devices (ie, the antenna and the RFID chip), and finally enhance the reading range of the RFID tags. Also, since RFID tags are designed to be placed directly on different surfaces, the proposed procedure is further evaluated by mounting the antenna onto 2 more surfaces.