High-Gain Normal-Mode Omnidirectional Circularly Polarized Antenna

This letter presents a high-gain normal-mode omnidirectional circularly polarized (CP) antenna with a simple feed structure for the WLAN application. The proposed omnidirectional CP antenna is based on a coaxial line. Helix-shaped slot etched on an outer conductor is used to excite the CP wave. Besides, the rectangular slots cut on an outer conductor are introduced to improve the axial ratio (AR). By adjusting the dimensions of the rectangular slots, the required circular polarization is obtained over the operating band. Additionally, the stepped impedance transformer structure and a core-shaped inner conductor are employed to achieve a good impedance matching. Finally, the measured impedance bandwidth with VSWR <; 2 covers the required frequency band. In the azimuth plane at 2.4 GHz band, the average measured left-handed CP gain and the AR of an antenna are 10.4 and 2.5 dBic, respectively. Thus, the proposed antenna acquires a good CP omnidirectional performance without a complex feeding network, while keeping the stable radiation patterns for the WLAN application.