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A right-hand circularly polarized (RHCP) square patch microstrip antenna for the on board unit (OBU) of electronic toll collection (ETC) system is designed, according to the national standard of GB/T 20851.1-2007. Wide bandwidth is achieved by choosing two suitable coupling slots in the ground plane of the stripline structure. The simulated return loss, radiation patterns, and the influence of via holes in the substrate are presented. The measured results show that a wide 2:1 VSWR bandwidth of 1.2 GHz (or about 21% with center frequency at 5.8 GHz) is obtained, and a RHCP gain of 6.6 dB is reached.