# Wideband Circularly Polarized Antenna Based on a Non-Uniform Metasurface
## 1 Introduction  
In recent years, many researchers have focused on designing antennas with circularly polarized (CP) radiation [1, 2] rather than linear polarization [3]. The benefits of adopting CP radiation in antennas have been illustrated for multipath interference reduction and polarization mismatch mitigation. Besides, as electronic devices are becoming increasingly small and high-speed communication is also highly desirable, antennas with a simple configuration and low profile as well as wideband operation features have received a great deal of attention from researchers.  
Single-fed patch antennas are considered to be a potential candidate to fulfil the abovementioned requirements. However, an inherently narrow operating bandwidth (BW) caused by a high quality factor is the critical drawback of this antenna type [4]. To date, the task of the BW enhancement of single-fed CP patch antenna while maintaining the low profile feature continues to be very challenging. In the open literature, one of the most common methods is the use of a low dielectric constant substrate in which an air gap is introduced between the ground plane and the radiating element [5, 6, 7, 8, 9, 10, 11, 12]. In fact, this method can significantly increase the BW up to approximately 24% [12]; however, the antenna's profile is typically higher than 0.1\(\lambda_{o}\) at the center operating frequency. Alternatively, wideband performance is also attained by using a meandering probe feed technique [13, 14]. Nonetheless, multiple stackedsubstrates are required in such designs, leading to a high profile configuration as well. To overcome this deficiency, another effective solution is the combination of the fundamental mode and high-order mode of the square patch presented in [15]. Although the resulting antenna has a very low profile of \(0.01A_{o}\), its operating BW is limited by 15.8%.  
In recent years, the metasurface (MS) has been widely applied to obtain BW enhancement for microstrip patch or slot antennas [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. A MS layer is commonly constructed with an array of periodical uniform unit cells of various shapes, such as square, rectangular, H-shape, etc. Then, the MS is arranged surrounding, above or below the primary radiating element. In general, this antenna type is able to exhibit a wide operation BW, which is typically less than 25%, with a low profile configuration of lower than 0.1\(\lambda_{o}\). To the best of our knowledge, the best performance with more than 30% BW was obtained in [19] by using a modified cross-slot coupled with a uniform MS.  
In this paper, a MS-based low profile CP antenna with more than 30% operating BW is proposed. It is noted that, unlike conventional CP MS antennas, in which the MS layer is comprised of multiple uniform unit cells, our antenna employs a different approach; here, the non-uniform MS is utilized and coupled with a truncated corner square patch to achieve a wide operating BW. To make the design procedure straightforward, the BW enhancement using uniform MS is considered first. Next, the antenna is designed with a non-uniform MS.