A Compact Circularly Polarized Microstrip Antenna with Stable Radiation Patterns

A compact microstrip antenna with stable radiation performance is proposed in this paper. The antenna is designed for OBU(On-Board Unit) in ETC(Electronic-Tolling Collection) applications. It operates in the frequency band of 5.8GHz. A group of microstrip structures is designed for microstrip to coaxial transition, improving radiated stability of the antenna. Furthermore, parasitic patches are introduced for enhanced gain. The antenna achieves a maximum gain of 6.4dBi in dimensions of $0.68 \lambda_{0} * 0.6 \lambda_{0} * 0.17 \lambda_{0}$, with a stable E-plane HPBW of 63.3°±1° over different coaxial pigtail lengths. The direction of the beam peak in E-plane varies within ±2°.