A Novel Circularly Polarized Antenna With Coin-Shaped Patches and a Ring-Shaped Strip for Worldwide UHF RFID Applications

A circularly polarized radio frequency identification (RFID) reader antenna is presented for worldwide ultra- high frequency (UHF) applications. The antenna is composed of two coin-shaped patches, a ring-shaped strip, and a suspended feeding strip with open-circuited termination. The measurements show that the antenna has an impedance bandwidth (S₁₁ <;-21 dB) of 22.4% (768-962 MHz) and a stable gain with a peak value of 9.8 dBic. Moreover, it is immune to polarization mismatch with a 3-dB axial-ratio (AR) bandwidth of 15.9% (816-957 MHz) and a 3-dB AR beamwidth of more than 66°. The proposed antenna can be a candidate for universal RFID readers at the UHF band of 840-955 MHz. Due to the fact that the square holes in the center of the coin-shaped patches can embed one additional small antenna, the proposed antenna has the potential to be a dual- or tri-band combined RFID reader antenna, spanning both the conventional UHF and the ISM band of 2.45 or 5.8 GHz.