A single-layer circularly polarized (CP) antenna with omnidirectional radiation in the azimuth plane has been designed and evaluated. The concept is the combination of multiple identical endfire CP antenna elements in a circular array configuration. The design of each element is based on the superposition of two closely spaced complementary current sources, which are, respectively, realized by a sectorial parallel-plate cavity and arc-shaped strip arms. A meandered double-sided parallel strip line is introduced to adjust the phase relationship between two orthogonal linear polarized components. All antenna elements can be simultaneously excited by adopting a single coaxial probe in the center of the overall structure. Take 5.8-GHz Industrial Scientific Medical band applications (5.725-5.875 GHz), for example, a prototype with a diameter of 0.62λ 0 and a low profile of 0.029λ 0 was fabricated. Experimental results show that the designed antenna has a usable bandwidth from 5.68 to 5.91 GHz (the common overlapped bandwidth limited by the -10 dB |S 11 | and the 3-dB axial ratio). Within the operation band, good omnidirectivity and a wide coverage can be obtained.