## 1 Introduction  
Millimeter-wave (mm-wave) band attracts increasing interests in 5G and satellite communications due to its merit of wide bandwidth [1; 2]. To compensate for path-loss, a high-gain antenna array is essential [3]. On the other hand, circular polarization is preferred because it has the merits of multipath interference suppression and no polarization alignment loss [4]. It is rewarding to design a mm-wave CP antenna array that has features such as a wide bandwidth, excellent gain, and simple structure. Over the previous decades, many wideband CP elements have been presented for array designs [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Stacked patch [5; 6; 7], ME dipole [8; 9], curl antenna [10; 11; 12], spiral antenna [13; 14], cavity antenna [15; 16; 17], slot antenna [18], and dielectric resonator antenna [19; 20; 21] are the popular elements. For example, in [6], a wideband circularly polarized radiation is generated owing to the combination of a pair of shorted pins with four identical square patches, and the 3-dB AR bandwidth of the stacked patch array achieves 25.4%. In [9], by truncating several corners and adding L-shaped branches into the ME dipole element, 3-dB AR bandwidths of the element and the 4 \(\times\) 4 array are 17.3% and 16%, respectively. In [18], a differentially fed slot antenna contains 20.4% AR bandwidth, whereas the 4 \(\times\) 4 array possesses a bandwidth of 33.6%. However, among the above-mentioned designs, some arrays have a relatively narrow bandwidth, while others require more substrate layers or a high profile. Another way to increase the AR bandwidth is utilizing the sequentially rotated (SR) feeding technique [22; 23; 24; 25; 26; 27; 28; 29]. For instance, a 2 \(\times\) 2 subarray with 17% AR bandwidth is proposed in [22], with each element fed by equal amplitude and phase signals. Through an H-shaped SIW SR feeding network, the four subarrays' excitation signals have a 90\({}^{\circ}\) phase difference from one another, thus expanding the bandwidth of the 4 \(\times\) 4 antenna array to 27.8%. In [25], the element's AR bandwidth is 23.8%, and that of the 2 \(\times\) 2 array is 30% because of the utilization of a microstrip SR feeding structure. However, the SR feeding structure increases the complexity of the antenna. Recently, [PERSON] et al. proposed the method of using vertical metallic walls to improve the AR bandwidth of antennas [30; 31; 32]. The metallic walls are sequentially placed at each edge of the ground planes, working as parasitic radiators. For example, a cross dipole antenna working at 2 GHz is put forward in [30]. The AR bandwidth is extended from 30% to 130% by utilizing four vertical metallic plates. Similarly, a dielectric resonator antenna with vertical metallic plates is studied in [31]. Because the orthogonal current is coupled to the metal plates, an AR bandwidth of 49.5% is obtained. However, these antennas have 3D structures and are designed in low frequency regime. Furthermore, all the designs in [30; 31; 32] only focus on a single antenna prototype. The array prototype with metallic walls is yet to be developed. On the other hand, to minimize the profile of CP array or the number of substrate layers, it is an effective method to choose microstrip lines as excitation structures [6; 9; 26]. A SIW structure can also be used as an antenna excitation. In spite of the fact that the array based on SIW feed technology in [11] only uses two dielectric substrate layers, it suffers from a narrow operating bandwidth. Some CP array designs use multiple substrate layers to enhance the bandwidth of the array [8; 22; 33; 34; 35]. In [33], a SIW feeding network consists of two layers of substrate, and the 4 \(\times\) 4 patch antenna array achieves 23.8% bandwidth. However, the assembly process becomes more complicated and more expensive, which limits the application of the arrays for commercial purposes. In this study, a compact wideband CP ME dipole antenna array with vertical metallic walls is proposed at 28 GHz band.