## 2 Antenna Design
### Parameter Analysis of the CP Element  
To guide the design of the proposed element, some key parameters are studied. In the analysis, only one parameter is swept, while others are the same as those in Table 1. According to the working principle discussed in Section 2.2, the ME dipole forms a good CP wave in a lower frequency band, while in a higher frequency band, the ME dipole and vertical metal walls work together to form a CP wave. Therefore, some parameters of the ME dipole affect the AR of the element in the whole operating frequency range. First, let's analyze the parameters of electromagnetic dipole, such as the width of the metalized strip used to connect the diagonally located patches P, the interval between adjacent patches G (G = G1 = G2), and the interval between the metalized strip and patch G3. These parameters have a vital impact on AR performance. Figure 5 displays in detail the simulated ARs of the element with various P, G, and G3 values. As observed in Figure 5a, the value of P affects the AR of the whole working frequency band. The AR around 24 GHz and 34 GHz increases with the increase of P, while the AR around 30 GHz decreases with the increase of P. Furthermore, as P decreases, the AR bandwidth becomes wider, but AR fluctuation in the working frequency band increases. Therefore, the value of P is set at 0.93 mm in order to achieve wide bandwidth and maintain the AR's stability simultaneously. Figure 5b shows the effect of the interval between adjacent patches G. It also affects the AR of the whole frequency range, but it has a more obvious influence on the AR in the lower frequency range. When G is raised from 0.27 mm to 0.37 mm, the AR bandwidth increases, while the AR value within 24.5 GHz to 29.5 GHz also increases. In order to achieve better AR performance, the value of G is set at 0.32 mm. Moreover, G3 has a similar influence on the antenna. In addition, the parameters of the vertical metal walls affect the AR performance of the presented element at the upper frequency band. There are six parameters: the spacing between the metal walls and the patches along the \(x\)-axis direction D1, the spacing between the metal walls and the patches along the \(y\)-axis direction D2, the spacing between the metal walls along the \(y\)-axis direction D3, the spacing between the metal walls along the \(y\)-axis direction D4, the length of the metal walls along the \(x\)-axis direction L3, and the length of the metal walls along the \(y\)-axis direction L4. The influence of all the parameters mentioned above on the AR is displayed in Figure 6. As displayed in Figure 6a, the AR in the upper band is significantly altered and mildly shifted to a lower band when D1 is raised from 1.6 mm to 2 mm (other parameters are the same as those in Table 1). Figure 6b illustrates how D2 has an impact on the AR. When D2 increases, the AR in the upper band also shifts to the lower frequency range. Other parameters also have an obvious influence on the AR in the upper band range, which can be explained by the working principle of antenna. It is worth noting that the parameters (D2, D4, L4) of the vertical metal walls placed along the \(y\)-axis direction have less influence on the AR than the parameters (D1, D3, L3) of the vertical metal walls placed along the \(x\)-axis direction because the induced current intensity of the vertical metal walls along the \(x\)-axis direction is greater than that of the vertical metal walls along the \(y\)-axis direction at upper frequency band. Therefore, by properly selecting the parameters of the vertical walls, the AR bandwidth in the higher frequency range can be effectively expanded. Figure 5: Simulated AR for three parameters of the ME dipole: (**a**) width of the metalized strip used to connect the diagonally located patches P; (**b**) interval between adjacent patches G; (**c**) interval between the metalized strip and patch G3.