A single-layer single-fed endfire antenna with circularly polarized radiation has been designed and evaluated. The concept is the superposition of two closely spaced complementary dipoles. In terms of implementations, a shorted parallel-plate cavity serves as the magnetic dipole, while double-side-printed conductor arms act as the electric dipole. To satisfy the required magnitude/phase relationship between two dipoles, a T-shaped double side slot-coupled line and a meandered double-side parallel strip line are introduced. With analyses of equivalent models, the operation mechanism and design guidelines can be obtained. Taking 5.8-GHz ISM band (5.725-5.875 GHz) for example, the measured results show that the designed antenna has an impedance bandwidth (|S11| <; -10 dB) from 5.7 to 5.9 GHz (3.5%) and a 3-dB axial-ratio (AR) bandwidth from 5.65 to 5.9 GHz (4.3%). The effective beamwidth in the azimuth/elevation plane is 85°/168° at 5.8 GHz.