A novel broadband circularly polarized patch antenna is presented. The antenna has a thick air-layer substrate; however, it can be easily excited by using a short probe pin. CP operation is obtained by selecting an optimal size of the truncated corners, and good impedance matching over a wide bandwidth is achieved by using a ground plane with an elevated portion. For a prototype built for WLAN operation in the 2.4 GHz band (2.4-2.484 GHz), the obtained CP bandwidth (3-dB axial ratio) reaches about 3.9%-7.5% and the measured antenna gain is about 9.3-7.4 dBic across the CP bandwidth for the air-layer substrate thickness varying from 8 mm to 14 mm. Details of the antenna design and experimental results are presented.