## 2 Electromagnetic Radiation Testing Method
### Analyzing Connected Vehicle Conditions in Urban Environments  
When evaluating wireless terminal communication performance in real scenarios, one effective method is road testing technology, which involves manually conducting actual communication performance evaluations in high-rise buildings or suburban vegetation areas. In practice, road testing technology has shown limitations in meeting all the wireless measurement requirements, mainly due to the following characteristics. First, the electromagnetic radiation in the surrounding environment during road tests is unknown and uncontrollable, such as during rainy or sunny days when the electromagnetic parameters of the propagation environment fluctuate; thus, measurement results often lack high repeatability and comparability. Second, the cost and time of measurements are high, with road test evaluations often requiring several days or even weeks of repeated measurements to arrive at converging conclusions.  
First, we established a two-dimensional model of the city using WinProp 2023 software, which we then converted into a three-dimensional visual model [7]. To achieve a more accurate simulation, we placed an omnidirectional antenna on the rooftop of a high-rise building to simulate the base station's functionality while simulating a 4G receiving antenna in a car on another street. This setup allowed us to comprehensively analyze and assess the wireless signals propagated by the base station. The simulation results displayed the omnidirectional radiation gain status of the base station relative to the vehicle, as depicted in Figure 5. Further observation of its radiation path revealed that the radiation path from the base station was essentially consistent with the signal propagation path to the antenna. This finding enabled us to confirm the signal propagation characteristics between the base station and the vehicle, allowing us to analyze antenna radiation properties to more accurately describe the signal path.  
Further analysis of channel transmission conditions for connected vehicles in urban environments revealed that the vehicle, while in motion, receives signals transmitted from different directions. As the complexity of the surrounding environment increases, multi-path effects become more pronounced, and the number of NLOS paths also increases, as  
Figure 4: Near—and far—field radiation patterns of the vehicle—level antenna system.  
Figure 5: Simulation of the radiation path from the base station to the vehicle terminal in an urban environment.  
illustrated in Figure 6. These environmental factors significantly influence signal propagation; therefore, special attention must be paid to spatial correlation coefficients when designing an antenna communication testing system for the complete vehicle.  
Through this analysis, we recognize that onboard antennas in urban environments are influenced by multiple factors. These factors specifically include the angle distribution of multipath effects, the spacing between antennas, the coupling characteristics of antenna gain, and the radiation directions. The combined effects of these factors not only affect the quality and stability of the signal, but also place higher demands on the vehicle's communication performance in dynamic environments. Thus, considering the interactions of these parameters will be crucial in the design and layout of connected vehicle antenna systems.