In this work, we studied and compared the performances of a linearly polarized microstrip antenna with another modified structure, which performed as a circularly polarized antenna. A bar which bridged one of the two slots, designed at a certain location and thickness, yielded a circularly polarized structure. Single layer dielectric substrate of 0.8 and 1.6 mm, for FR4, or 15 mm and 30 mm, for air, were utilized in the designs. Simulation and measurement results of the return loss and radiation patterns are shown, for the linearly polarized structure. Detailed simulation results are shown, for the considered circularly polarized structure.