Design of a U-shaped RFID tag antenna with an isotropic radiation characteristic

In this paper, we propose a simple U-shaped radio-frequency identification (RFID) tag antenna with an isotropic radiation characteristic for stable operation of an RFID system. The antenna is composed of a U-shaped half-wavelength dipole and a rectangular-shaped feed. The feed is connected at the bottom of the dipole for conjugate impedance matching with a commercial tag chip. The gain deviation of the antenna can be further improved by inserting a rectangular slit in the lower central portion of the antenna body. For VSWR ≤ 2, both designs comply with the Korea ultra-high frequency RFID bandwidth. Within the operational frequency range, the gain deviation of the tag with a slit was less than 0.71 dB, while the gain deviation of the tag without a slit was less than 1.62 dB.