A Compact UHF Yagi-Uda Circular-Polarized Antenna

A compact circular-polarized (CP) antenna using multi-section printed square-ring resonators is proposed based on the Yagi-Uda concept. The antenna consists of eight sections separated by air gaps: a reflector, a driven element, and six directors. A local-grounded slotted-patch coupler is integrated inside the square-ring resonator of the driven element to excite two orthogonally linearly polarized fundamental modes of the ring resonator, leading to a CP radiation. To obtain a high directional CP radiation pattern, a meandered square ring printed on the bottom substrate is used as a reflector, and six square rings are placed on the top of the driven element as directors. A prototype antenna operating at 440 MHz is designed, fabricated, and measured, and the measured results show a good agreement with the simulated ones. Owing to the miniaturized slotted-patch coupler and the meandered square-ring reflector, the transverse size of the antenna is only about l0/4*l0/4.