# A Low-Profile Wideband Linear-to-Circular Polarization Conversion Slot Antenna Using Metasurface
## 1 Introduction  
With the rapid development of wireless communication technology, there is a demand for the utilization of efficient antennas [1]. Recently, circularly polarized (CP) antennas have become an attractive choice for many communication systems, such as satellite communications and global positioning systems. This is mainly because CP antennas have numerous benefits over linearly polarized (LP) antennas, such as reducing the physical losses, providing anti-jamming, receiving an arbitrary polarization wave, and so on [2; 3]. Microstrip patch antennas have been widely used in CP antennas due to their advantages in regard to their low cost, small size, simple structure, and easy integration [4]. Usually, a single-layer microstrip antenna can be circularly polarized by single-point feeding and multi-point feeding. The single-point feeding CP antenna has a simple structure but a narrow 3 dB axial ratio bandwidth (ARBW). The multi-point feeding CP antenna has the advantages of low cross-polarization, wide impedance bandwidth, and wide ARBW. However, it requires complicated feeding networks and increases the profile height of the antenna. Therefore, widening the circular polarization bandwidth while maintaining the antenna's low profile is still a hot topic in current research [5]. As an artificial structural material, metasurfaces have received extensive research attention due to their novel electromagnetic properties that have not been found in nature [6]. New methods todesign low-profile wideband CP microstrip antennas using metasurfaces have been demonstrated in the past few years, with resulting benefits over conventional CP microstrip antennas, such as reconfigurable polarization, miniaturization, wide bandwidth, and high gain [7]. Left- and right-hand circular polarization could be achieved by rotating the metasurface relative to the microstrip-fed slot antenna [8]. A metasurface with a unit cell consisting of five horizontal strips and a single diagonal strip realized the conversion from LP waves to CP waves [9], while a metasurface with square rings and splits achieved circular polarization and high gain [10]. By putting a double-stacked metasurface layer over a CP microstrip antenna, the ARBW was increased by 3% [11]. However, the sizes of the above designs were large, and their S\({}_{11}\) bandwidth and ARBW were narrow, which limited the use of these antennas. Therefore, some metasurface CP microstrip antennas were designed to solve this issue of large size. In [5], a low-profile CP antenna was proposed by using a metasurface composed of square units with truncated corners. In [7], a miniaturized antenna loading capacitive strips on the corner-truncated patch was investigated. In [12], by introducing a triangular interdigitated capacitor and diagonal slot loading to the metasurface, an advanced metasurface CP antenna with a compact footprint and improved 3 dB ARBW was constructed. In [13], by directly placing a circularly shaped metasurface atop a slot antenna, a compact, low-profile, polarization-reconfigurable antenna was presented. In [14], the metasurface of a \(3\times 4\) rectangular patch array was utilized to design a low-profile broadband circularly polarized antenna for C-band satellite communication. These designs reduced the overall size slightly at the cost of reduced S\({}_{11}\) bandwidth and ARBW. In addition, some other metasurface CP microstrip antennas were introduced to broaden the ARBW. A corner-truncated square metasurface [15] and a \(7\times 7\) smaller patch array metasurface [16] were used to construct wideband CP slot antennas. These designs realize a wide S\({}_{11}\) bandwidth and ARBW but still have a large-sized structure. This paper designs a new low-profile wideband linear-to-circular polarization conversion antenna for satellite communications by placing a \"parasitic square cross gap\" metasurface on a slot microstrip antenna. In Section 2, the geometry of the proposed antenna is presented, followed by polarization reconfigurability.