A Miniaturized Yagi-Uda-Oriented Double-Ring Antenna With Circular Polarization and Directional Pattern

A compact circular polarized (CP) double-ring antenna with directional pattern is proposed based on the Yagi-Uda concept. After integrating a local-grounded coupler inside the square ring, a circular polarized wave can be effectively radiated by exciting two orthogonally linearly polarized fundamental modes of the ring resonator, leading to a smaller size than using other high-order modes of the ring. Since the side length of miniaturized patch hybrid coupler is much smaller than the quarter-wavelength (
$\lambda _{0} /4$
), the whole coupler plus feeders can be fully embedded inside the ring radiator. To obtain a directional radiation pattern, the second meandered ring is then mounted under the square ring, which can be considered as an inductive Yagi-Uda reflector. Since the physical area of the reflector is almost the same with the square-ring radiator, the total size of the antenna is only about
$\lambda _{0} /4 \times \lambda _{0} /4 \times \lambda _{0} /16$
, which can be a good candidate of the antennas for handled mobile terminals. Finally, a prototype of the proposed antenna is designed and fabricated. The measured results show a good agreement with the simulated results.