## 2 Antenna Design
### Working Mechanism of the CP Element  
Compared to other kinds of antennas, an ME dipole has low profile, excellent antenna performance, such as a good impedance bandwidth, stable gain, and desirable radiation front-to-back ratio, making it ideal for mm-wave application. Therefore, many CP arrays based on an ME dipole have been proposed in the mm-wave band [8; 9; 22; 36]. In [22], an ME dipole antenna fed by a SIW to coaxial transition structure has \(-10\) dB impedance bandwidth of \(24.2\%\) and a \(3\)-dB AR bandwidth of \(16.5\%\). In addition, an aperture-coupled ME dipole achieves a bandwidth of \(25.9\%\)[36]. In this design, the method of vertical metal walls is applied to the element in [36] to further expand its bandwidth. The design process of the proposed element is illustrated in Figure 2. As seen in Figure 2, Type is a traditional ME dipole fed by SIW. The performance of Type I is described in detail in [36]; the impedance bandwidth is \(28.8\%\), and the AR bandwidth is \(25.9\%\). To minimize the number of substrate layers of the antenna array, the SIW Excitation structure is replaced by a microstrip line in Type II. Next, in Type III, eight vertical metal walls are added around \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(L1\) & 2.06 & G3 & 0.11 & S2 & 1.9 \\ \hline \(L2\) & 2 & \(D1\) & 1.8 & \(P\) & 0.93 \\ \hline \(L3\) & 2.9 & \(D2\) & 1.9 & \(W\!f\) & 0.22 \\ \hline \(L4\) & 1.3 & \(D3\) & 0.7 & \(D\!f\) & 0.49 \\ \hline \(G1\) & 0.32 & \(D4\) & 4.1 & \(W\!s\) & 0.36 \\ \hline \(G2\) & 0.32 & \(S1\) & 2.35 & \(Ls\) & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specified dimensions for the proposed CP element (unit. mm). Figure 1: Geometry of the CP element: **(a)** 3D explosive view; **(b)** top layer; **(c)** bottom layer. Type II for enhancing the bandwidth of the element. To fabricate the antenna through PCB technology, the metallic walls are replaced by vias and metallic strips in Type IV. For comparison, the EM dipole is the same size in all models, as shown in Table 1 in Section 2.1. Figure 3a,b depict the elements' simulated reflection coefficient and AR performance. It is shown that the impedance and AR bandwidth of Type II is 34.9% (24.8 GHz to 35.3 GHz) and 23% (23.8 GHz to 30 GHz), respectively, which is close to the performance of Type I. This indicates that the microstrip line can replace the SIW structure for feeding without affecting the performance of the element. When the vertical metallic walls are introduced, both the impedance bandwidth and AR bandwidth are significantly improved. Additional resonance is observed in the upper frequency band, which is expected to be attributed to the vertical metallic walls. Compared with Type II, the impedance bandwidth of Type III is increased from 34.9% to 44.5% (22.7 to 35.7 GHz) and the AR bandwidth is increased from 23% to 36.5% (23.3 to 33.7 GHz). The performances of Type IV are similar with those of Type III. It means that using vias and metallic strips to replace the metallic walls has little influence on the performances. To further study the working mechanism of the ME dipole surrounded by vertical metallic walls, the currents distributed on the antenna are exhibited at two frequency points, as depicted in Figure 4. At 26 GHz, strong currents are observed on the ME dipole, whereas weak currents are on the vertical metallic walls. It means that the ME dipole is the main  
Figure 3: Simulated performance of the different types of CP antennas: (**a**) simulated S11 of the antennas; (**b**) simulated AR of the antennas. Figure 2: Evolution process of the proposed CP element. radiator at 26 GHz. The directions of currents at t = 0\({}^{\circ}\) and t = 90\({}^{\circ}\) are orthogonal in space, and the amplitudes are close. Therefore, CP fields at 26 GHz are generated by the ME dipole and the vertical metallic walls have negligible effect. At 32 GHz, the currents are strong at both the ME dipole and metallic walls.