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Compact symmetric-slit square microstrip-patch antennas are proposed for circularly polarized (CP) radiation. A symmetric slit on a patch radiator can be used for circularly polarized radiation with a compact size. Various slit shapes embedded along the four diagonal directions on the patch radiators are realized for compact circularly polarized symmetric-slit microstrip antennas based on a single feed. Circularly polarized radiation is achieved by slightly varying the slit's circumference along one of the diagonal axes. The performance and sizes of the proposed antennas with various slit shapes are studied and compared. The proposed antennas were designed on a low-loss Rogers RO4003C substrate and on low-cost lossy FR4 substrate. The 3-dB axial ratio (AR) and 10-dB return-loss bandwidths of the antenna on the FR4 substrate were larger compared to those of the antenna on the RO4003C substrate. The gain of the antenna on the RO4003C substrate was higher compared to that of the antenna on the FR4 substrate with a fixed overall antenna size. The measured 10-dB return loss and 3-dB axial-ratio bandwidths were 3.85% and 1.5%, respectively, with a maximum gain of around 0.8 dBic, for the antenna on the FR4 substrate. For the antenna on the RO4003 substrate, the measured 10-dB return loss and 3-dB axial-ratio bandwidths were 1.5% and 0.5%, respectively, with a maximum gain of around 3.4 dBic.