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A new design of a broadband circularly polarized square slot antenna fed by a single coplanar waveguide (CPW) is proposed and experimentally studied. The proposed broadband circular polarization (CP) operation is achieved by protruding a T-shaped metallic strip from the ground plane toward the slot center and feeding the square slot antenna using a 50-Ω CPW with a protruded signal strip at 90° to the T-shaped strip. Experimental results of constructed prototypes show that the obtained 3-dB and 1-dB axial-ratio bandwidths can reach as large as 18% and 13%, respectively. The proposed antenna also has a peak antenna gain of about 4 dBi, with gain variations less than 1 dBi for frequencies within the CP bandwidth.