A wideband circularly polarized (CP) stacked square-patch antenna using single-feed is proposed for ultra-high
frequency (UHF) radio frequency identification (RFID) readers. The proposed wideband CP antenna comprises of a main radiating patch, a parasitic patch, and a probe. A cross-shaped slot and a square-ring-shaped slot are cut into the radiating square-patch and the parasitic square-patch, respectively. The CP radiation is generated by the embedded cross-shaped slot inradiating square-patch along the diagonal line. The area and location of the embedded slots as well as the separation between the patches are optimized for a wideband CP radiation. The measured impedance (greater than 10 dB return loss) and CP radiation (3 dB axial ratio) bandwidths of an antenna prototype are 18.0%, or 165 MHz (0.840 GHz – 1.005 GHz) and 11.6%, or 105 MHz (0.855 GHz – 0.960 GHz), respectively. The measured gain is greater than 7 dBic across the 3 dB axial ratio frequency range, while the antenna dimension is 0.54λo×0.54λo×0.087λo (λo is free-space wavelength at 0.9 GHz).