Abstract: A broadband circularly polarized microstrip antenna is proposed at S band in this paper. A truncated corner patch is adopted to realize circular polarized radiation. The high order mode is suppressed by the differential feed to improve the axial ratio. The bandwidth is further broadened by inserting an air layer between two substrates and etching double cross slots on the truncated corner patch. The size of rectangular patch is 38 mm×38 mm and the antenna height is 14.7 mm.Measured results show that the proposed antenna has good broadband characteristic, along with desirable radiation performance during the bandwidth. The effective bandwidth of the reflection coefficient S11 ≤-10 dB and axial ratio AR≤6 dB is 22.3%.