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An electrically small spherical UHF (911.25 MHz) radio frequency identification (RFID) tag antenna is proposed. A dipole-typed RFID tag antenna is wound into a spherical shape to achieve small size and quasi-isotropic pattern. The overall diameter of the antenna is 25 mm (0.076¿), and the resulting electrical size is a kr of 0.24. A short stub is employed for good conjugate impedance matching with a RFID chip (Zchip= 14 - j145 ¿). The designed antenna has a -10-dB bandwidth of 7 MHz (0.77%) and the maximum readable distance of 132 cm at 5.2 W EIRP.
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Ryu et al. (2010) studied this question.
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