# A Versatile Shared-Aperture Antenna for Vehicle Communications
## 4 Discussion  
Nowadays, the vehicle antenna is constantly developing towards miniaturization and integration, and shared-aperture technology is one of the important means to promote the integration of vehicle antennas. This article mainly proposes a compact multi-function shared-aperture antenna for vehicle communication, which consists of a GPS microstrip antenna, an S-band circularly polarized dipole antenna, and a V2V pattern reconfigurable  
Figure 17: Radiation pattern of cross dipoles at (**a**) 2 GHz, (**b**) 3 GHz, and (**c**) 4 GHz. Figure 18: Simulated and measured radiation pattern of pattern reconfigurable antenna. (**a**) state 1. (**b**) state 2. (**c**) state 3. (**d**) state 4.  
antenna. According to the application scenario of GPS, the antenna at 1.575 GHz usually uses circular polarization to reduce multipath effects, rain attenuation, and other issues, and there is no difficulty in aligning linear polarization. The S-band satellite communication and GPS application scenarios are similar, both requiring circular polarization. However, the V2V scenario requires communication between vehicles and surrounding vehicles. Conventional omnidirectional monopoles have low gain and do not have scanning capabilities, so an antenna with scanning capabilities is needed for more secure communication. When considering the common means of designing a shared-aperture array in a multi-frequency scenario, there are three main design difficulties: array distribution, decoupling of the same frequency unit, and decoupling of the different frequency unit. The array distribution determines the array factor of the shared-aperture array, while the decoupling of the same frequency elements determines the scanning performance of the antenna array. The decoupling of the different frequency elements determines the isolation between the high-frequency and low-frequency channels in the antenna array. In addition, antennas for in-vehicle communications applications need to be designed using circular polarization, which has better anti-interference than linear polarization. At the same time, it also increases the wiring difficulty for the design of shared aperture. In this design, we also use optimized array distribution to ensure that the array factor does not exhibit grating lobes and maintains the same frequency element distance at around half a wavelength without affecting scanning, and we use filter antennas to improve the isolation between antennas operating at different frequencies. To be specific, the distribution of antenna elements is very important to ensure that the antenna does not have grating lobes. Therefore, two antenna distributions are designed and simulated, as shown in Figure 19. Both antennas can achieve the characteristic of three-frequency shared aperture, but there is a significant difference in the array spacing. The spacing between the S-band antenna elements and the 5.9 GHz antenna elements in Array 1 exceeds the full wavelength, which is prone to grating lobes. However, the antenna distribution in Array 2 fully considers the array spacing, and the unit spacing is less than half a wavelength, so that the small array can achieve the effect of not scanning out the grating lobe. Another issue worth discussing is how to prove that the design can be used for automotive in-vehicle applications. In order to simulate a real vehicle situation, we also simulated the scene of the antenna being installed on the car, especially the pattern reconfigurable antenna operating at 5.9 GHz. Therefore, we simulated the antenna pattern on the top of the car based on the SBR+ algorithm, and the outer surface of the car was set to the SBR+ boundary. Because the patterns of other antennas are broadside, the metal surface of the car almost has no impact. From Figure 20, it can be seen that the metal on the top of the car makes the pattern of the reconfigurable antenna biased upward, but it still has  
Figure 19: Comparison of the distribution and array spacing of two shared-aperture arrays.