A metasurface-based circularly polarized (CP) microstrip patch antenna with high performance in terms of the global bandwidth and gain is presented. The antenna configuration was a square-modified microstrip patch that was sandwiched between a metasurface and the ground plane. The metasurface comprised a $4 × 4$ array of square patches, while the single-feed radiating microstrip patch had axial ratio (AR) tuning stubs and a crossed slot. Under excitation, the microstrip patch and crossed slot, in conjunction with the metasurface, generated multiple resonances and AR minimum points that combined to produce large global bandwidths with regard to the impedance, AR, and 3 dB gain. Using a stacked layout, a low-profile design with the dimensions of 54 mm × 54 mm × 3.1 mm (1.2 λ o × 1.2 λ o× 0.07 λ o at 6.8 GHz) was obtained, which generated a highly stable broadside gain above 10 dBic throughout the entire operating AR bandwidth. This antenna had a measured-10 dB impedance bandwidth of 5.62-11.04 GHz (65.06%) and a 3 dB AR bandwidth of 5.64-7.89 GHz (33.25%). In addition, it had a measured 3 dB gain bandwidth of 5.17-8.2 GHz (45.32%), with a measured peak gain of 12.17 dBic.
