I. Introduction
With the rapid development of millimeter-wave (mmW) components and fabrication process, antennas for mm-W systems are becoming one of the most critical components [1]. W-band, ranging from 75 to 110 GHz, attracted more and more attention due to the low atmosphere absorption, short wavelength, and wide available frequency bandwidth properties [2]. Antenna arrays with low cost and large operating bandwidth are necessary for W-band applications, such as long-range high-resolution radar systems, high-precision sensing systems, and high-definition media transfer systems [3]. In these transmit-receive systems, circularly polarized (CP) antennas are more attractive and equipped with desirable advantages, including flexibility, mobility, weather penetration, and reduction in multipath reflections.