# Low-Profile Dual-Polarized Antenna Integrated with Horn and Vivaldi Antenna in Millimeter-Wave Band
## 2 Design of Dual-Polarized Antenna  
The geometry of the dual-polarization antenna is displayed in Figure 1. The antenna is composed of an SIW \(H-\)plane horn antenna, a Vivaldi antenna, and a dielectric lens. Two Arlon AD430 substrates (\(\varepsilon\)r = 4.3, tan\(\delta\) = 0.003) and three copper layers are used to construct the antenna. These structures are assembled by screws. The dual\(-\)polarized antenna was modeled in CST studio 2020. The Vivaldi antenna is etched on both surfaces of the horn antenna and fed through a microstrip line with a fan\(-\)shaped terminal. The fan\(-\)shaped feeding line is located at the top of Substrate 1 as port 1. The horn antenna is located on the lower dielectric substrate and fed by a grounded coplanar waveguide (GCPW) to an SIW transition line as port 2. A lens is loaded at the aperture of the horn antenna for better impedance matching and higher directivity. The structure of the \(H-\)plane horn antenna is shown in Figure 2. To ensure that the horn antenna only transmits the TE\({}_{10}\) mode throughout the operating band, the width \(W_{2}\) and \(W_{6}\) and thickness \(t_{2}\) of the antenna are calculated and designed according to [25; 26]. Since the thickness of the antenna is much smaller than the operating wavelength, the beamwidth of the \(E-\)plane radiation pattern is wide, and the impedance matching is poor. To solve these problems, a dielectric lens is loaded at the aperture of the horn antenna as a dielectric guided structure in the \(E-\)plane. The simulated impedance matching is shown in Figure 3. After adding the dielectric lens, the impedance matching is obviously improved,  
Figure 1: Geometry of the proposed antenna. (**a**) 3D view. (**b**) Top view. (**c**) Side view. which covers the bandwidths of 25.31-26.71 GHz. The comparison of the radiation patterns with and without the lens is plotted in Figure 4. With the loading of an appropriate\(-\)size lens, the radiated beamwidth of the horn antenna is significantly narrowed. Consequently, the directivity of the horn antenna is improved from 5.8 dBi to 9.0 dBi at 26 GHz. In order to realize the dual polarization and maintain wide\(-\)band performance, a typical Vivaldi antenna that provides VP is designed on the basis of the existing \(H-\)plane horn antenna. By adding a Vivaldi antenna, the size of the whole antenna is hardly changed but obtains VP. The Vivaldi antenna is fed by a fan\(-\)shaped microstrip line, as shown in Figure 5, and radiates VP electromagnetic waves. After optimizing the size of the Vivaldi antenna, the parameters of the final antenna design are shown in Table 1. The simulated impedance matching of the Vivaldi antenna is shown in Figure 6, which has a simulated \(-\)10 dB impedance bandwidth of 23.25% (21.10-26.65 GHz). The bandwidth basically  
Figure 4: Comparison of radiation pattern of SIW horn antenna with and without lens at 26 GHz. (**a**) \(E-\)plane. (**b**) \(H-\)plane. Figure 3: Comparison of \(S-\)parameters of SIW horn antenna with and without lens. Figure 2: Geometry of SIW \(H-\)plane horn antenna. (**a**) Top view. (**b**) Side view. changed, the other parameters remain the same. The effects of the radius of the Vivaldi antenna fan\(-\)shaped feed \(R\), the thickness of the upper substrate \(t_{1}\), and the width of the horn antenna \(W_{6}\) are analyzed. These three parameters significantly affect the antenna's impedance matching. As shown in Figure 8, with the increase in fan\(-\)shaped feed radius \(R\), the impedance matching becomes worse at high frequencies. The radius of the fan\(-\)shaped structure is about a quarter wavelength of the center frequency point of the antenna. Therefore, with the increase in fan\(-\)shaped feed radius \(R\), the operating band of the antenna is shifted to the lower frequency, and the impedance matching at high frequency becomes worse. In order to realize the best performance, the final fan\(-\)shaped feed radius \(R\) is determined to be 2.3 mm.