Evaluation for RFID tag antennas by antenna clearance based on power reflection coefficient

To evaluate the robustness of various RFID Tag antennas against close to metallic objects or liquid bottles, the concept of the antenna clearance, which shows the minimum distance from an obstacle with stable resonance, will be useful. We propose a new definition of the antenna clearance employing the variation of power reflection coefficient with the characteristic impedance of its feed line. A conducting wire of half wavelength is used as an obstacle. Plural definitions of antenna clearances of both an H-shaped antenna and a dipole antenna are compared respectively in this paper.