Impedance Measurement of a Chip-Based Passive UHF RFID Sensor Transponder

Passive UHF RFID transponders are usually employed with balanced antennas optimized to match the complex conjugate of the chip impedance. Since the degree of impedance matching between the antenna and the chip determines the system performance, it is essential to validate the performance of the transponder through prototype measurements. In this paper, the design of a chip-based passive UHF RFID sensor transponder antenna and the measurement of its impedance is presented. Simulations were carried out in ANSYS High Frequency Structure Simulator (HFSS) and the input impedance of the antenna was matched with the conjugate impedance of the commercial RFID chip, Farsens Rockyl00. A prototype of the antenna was fabricated and its input impedance was measured using image theory. The simulated and measured results are in good agreement, thus validating the antenna's performance. The S
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≤ - 10 dB bandwidth of the antenna covers the entire UHF frequency band. Accordingly, the proposed antenna can be operated in any region within the aforementioned frequency band for RFID applications.