# A Highly Integrated Millimeter-Wave Circularly Polarized Wide-Angle Scanning Antenna Unit
## 1 Introduction  
The rapid advancement of wireless communication technology has placed greater demands on data transmission rates, quality, and spatial coverage in communication systems. Millimeter-wave communication, known for its narrow beamwidth, robust interference resistance, and high capacity, has increasingly attracted attention [1; 2; 3; 4; 5]. [PERSON], [PERSON] et al. pointed out that, in future 5G (5 th Generation Mobile Communication Technology) systems, the millimeter wave band will be used to support a large capacity for current mobile broadband [4]. As a result, there has been extensive research into high-performance millimeter-wave active phased array antennas that offer a wide-angle scanning capability, low loss, and strong interference resistance [6; 7; 8; 9; 10; 11; 12; 13]. For example, [PERSON], [PERSON] et al. developed a 64-unit modular millimeter wave phased array antenna to meet the needs of civil military and civilian dual purpose millimeter wave communication [8]. [PERSON], [PERSON] et al. proposed a broadband high-solation and low-cost millimeter-wave antenna array unit, which is suitable for engineering applications. Circularly polarized antennas are widely utilized in satellite communication and other applications due to their ability to mitigate polarization mismatches and their superior resistance to effects like multipath interference and rain attenuation [14; 15; 16; 17; 18]. Currently, extensive research has focused on circularly polarized antennas [19; 20; 21]. [PERSON] et al. pioneered a circularly polarized antenna using half-mode substrate integrated waveguide (HMSIW) technology, achieving size reduction [32]. However, it can only work in one polarization mode and has a narrow bandwidth. [PERSON] et al. proposed a low-profile single-feed circularly polarized (CP) patch antenna using a metasurface for broadband operation [19]. The antenna comprises a truncated corner square patch sandwiched between a lattice of \(4\times 4\) periodic metal plates and the ground plane. [PERSON] et al. introduced a 28 GHz circularly polarized antenna employing four-layer dielectric substrates, producing left- or right-handed circular polarization through interlayer gapsand apertures [33]. [PERSON] et al. applied traveling wave antenna technology to extend the working bandwidth of circularly polarized antennas [34]. These studies have enhanced the benefits of circularly polarized antennas in terms of size, polarization mode, and bandwidth. Based on these studies, we give full play to their advantages and integrate them into an antenna unit, and use a special arrangement method to obtain the highly integrated millimeter-wave circularly polarized wide-angle scanning antenna unit. This paper presents a novel, compact, highly integrated circularly polarized wide-angle scanning antenna based on substrate-integrated waveguide (SIW) technology. The antenna unit is based on SIW, integrating multiple devices such as circular polarizer, filter, matching load, etc., and giving full play to the advantages of these devices. Different polarization characteristics can be obtained using the circular polarizer and selecting different flat SIW feeding, and free switching between left-hand circular polarization and right-hand circular polarization can be realized; by connecting the filter after the feeding port, good out-of-band suppression performance can be achieved; by integrating the matching load at the end of the flat SIW without feeding, cross polarization caused by re-radiation of reflected waves can be avoided. Operating in the millimeter-wave frequency range with a center frequency of \(f_{0}\), a bandwidth of 1 GHz, and a scanning angle of 120\({}^{\circ}\), the antenna offers strong integration, effective out-of-band suppression, and the capability to adjust its polarization mode to meet engineering requirements, supporting both left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP).