###### Abstract  
In this paper, we report a sequential-phase fed broadband circularly polarized array antenna loaded with an artificial magnetic conductor-reflecting surface. Our proposed antenna is a two-part group. The upper structure is a sequential-phase fed circular wide-slot antenna. The lower part is an artificial magnetic conductor structured reflective surface. The overall antenna size is 106 \(\times\) 106 \(\times\) 14.9 mm\({}^{3}\), both adopting 1.6 mm thick FR4 material. The thickness of the air layer sandwiched between the antenna and the artificial magnetic conductor reflective surface is 14 mm. The antenna consists of four circular wide-slot antenna units with a sequential rotation technique. To broaden the axial ratio bandwidth, three L-shaped branches and four metal plates are attached to the circular wide-slot antenna unit and around the artificial magnetic conductor-reflecting surface, respectively. To verify the accuracy of the simulation, we fabricated the sample and then tested it in an anechoic chamber. The measured results demonstrate that the proposed broadband circularly polarized array antenna realizes an impedance bandwidth of 77.67% (2.74-6.22 GHz) and an axial ratio bandwidth of 65.16% (3.00-5.90 GHz) with a peak gain of 11.1 dBi. The design can be used in space-constrained environments, such as indoor and dense building areas. array antenna; circular polarization; sequential phase; broadband +
Footnote †  
radiation. The same backward radiation is produced in a number of studies [13; 14; 15; 16], and this problem can be well solved by using the back cavity structure [17; 18]. [PERSON] et al. [19] use an L-shaped antenna and couple it with a vertically fed probe to the SP-fed network, achieving 55% ARBW, but the vertically fed design is more complex [20]. The SP feed with non-uniformly bent transmission lines in [PERSON]'s work [21] minimizes the overall antenna size, but the bandwidth is only 15.5%. In recent years, artificial magnetic conductor (AMC), electromagnetic bandgap structure (EBG), and frequency selective surface (FSS) based on the metasurface structure have been poured into the field of array antenna design owing to their unique electromagnetic characters [22; 23]. The aspects of metasurface performance enhancement mainly include improving impedance bandwidth [24], enhancing gain [25], increasing AR bandwidth [26; 27], suppressing surface waves [28], reducing radar Cross Section (RCS) [29], and polarization conversion [30]. In two studies [24; 25], the operating bandwidth of the CP antennas loaded with the metasurface structure can reach 41.67% and 23.16%, respectively. In another study [26], two SP-fed networks are designed to achieve RHCP and LHCP polarization forms, but the bandwidth still does not meet the operating requirements. The dipole antenna and wide slot antenna are relatively easy to achieve broadband performance, but the above antennas produce bidirectional radiation and low gain. When the communication equipment is in a restricted environment, it is evident that the backward radiation is unnecessary, and the antenna loaded with the AMC structure [31; 32] can effectively reduce the backward radiation and profile compared with the antenna [33; 34; 35; 36] using a PEC reflector plate. The combination of metasurface and antenna also makes design optimization more complex and, in recent years, characteristic modal analysis (CMA) has been increasingly used in antenna design, making the overall design optimization process more controllable [37; 38; 39; 40]. In this paper, an SP-fed broadband circular slot array antenna loaded with an AMC is proposed. The circular slot serves as an antenna radiation unit, and the L-shaped branches are introduced into it to improve the bandwidth. The antenna itself has broadband performance and comes with bi-directional radiation.