I Introduction
In communication and detection systems, antennas are used as a transition from circuits to free space, and antennas with various properties have been proposed at millimeter and microwave frequencies. Low sidelobes in radiation patterns are desired to mitigate signal interference [1], [2], and the main beam can also be steered in tracking and sensing applications [3]-[5]. Apart from modification of radiation patterns, the type of polarization in radiation also plays an important role in practice. Compared with linear polarization (LP), circular polarization (CP) could resist multipath distortion and improve polarization matching in wireless communications [6]. Therefore, it is worthwhile to investigate methods of LP-to-CP transformation.