A single-layer bidirectional antenna that provides circularly polarized (CP) radiation of the same sense in both of the hemispheres has been designed and evaluated. The concept is the combination of two identical endfire CP antenna elements in a back-to-back configuration. The design of each element is based on the superposition of two closely spaced complementary dipoles connected by phase delay line. A single coaxial probe can be used to simultaneously excite the two elements. Taking 5.8-GHz ISM band applications (5.725-5.875 GHz), for example, experimental results show the designed antenna has a usable bandwidth from 5.72 to 5.88 GHz (the common overlapped bandwidth limited by the 3-dB axial ratio and -10-dB |S11 |). The overall antenna size is only 0.53λ 0 × 0.47λ 0 with a low profile of 0.029λ 0 .