An indirect non-invasive method for measuring input impedance and connection effects of an RFID tag antenna

This paper presents a non-destructive, non-invasive method for measuring the input impedance of a Radio Frequency Identification (RFID) transponder (tag) antenna at the operating frequency The derivation of the theoretical basis for determining the antenna impedance is presented along with the experimental measurement method using a Vector Network Analyzer (VNA) in conjunction with a Giga-Hertz Transverse Electromagnetic (GTEM) cell where the tag is located. By providing three different terminations to the antenna, the frequency dependent parameters can be measured in order to define a two-port network and determine the input impedance of the antenna. All measurements were performed using the actual RFID tag that operates in UHF (915 MHz) band. The resulting input impedance of the antenna was validated with the electromagnetic simulation software by constructing a model of the testing system and applying the proposed measurement method. The simulated results were found to be in agreement with the experimental measurements accounting for any discrepancies.