Simple and Accurate Radiation Pattern Measurement of UHF RFID Transponders

In this contribution we describe a simple measurement method to characterize the antenna of radiofrequency identification (RFID) transponders at the ultra-high frequency (UHF) band. The transponder that consists of an antenna and a transponder chip is fixed in a rotation apparatus and exposed to a field produced by a calibrated reference antenna. By monitoring the presence of the transponders response and simultaneously adjusting the transmit power, the transponders minimum operating field can be found. A series of such measurements versus azimuth and polar angle as well as transmit antenna polarization allows to determine the radiation pattern. With this method it is possible to analyze the influence of materials placed in the proximity of the transponder. Measurement results for a dipole-like transponder and an omnidirectional transponder that is subject to a proximity effect are presented.