## 3 Results
Moreover, it was verified once again that the role of the closed-loop device is to reflect the signals emitted from the monopole. For the final step, four closed-loop devices were installed on the monopole WAVE antenna to obtain omnidirectional radiation beam patterns with an enhanced gain value. Quadruple closed-loop devices were installed for a 360\({}^{\circ}\) coverage. Compared to instances where one and two closed-loop devices were installed, the gain value of the beam pattern increased evenly over the entire 360\({}^{\circ}\) direction, as shown in Figure 4d (see the different scale in Figure 4). If there was interference between the closed-loop devices and the beam pattern, the gain value of the signal radiated from the monopole would not have increased evenly. This proved that quadruple closed-loop devices work independently without affecting each other. Uniformly increasing the overall gain by reducing the loss results in the signal radiated from the monopole antenna being reflected through the closed-loop device and radiated to the outside. The currently used monopole antenna without a closed-loop device is at a disadvantage, wherein the height of the monopole must be relatively high to minimize the loss of the radiated signal [13; 14]. However, when the closed-loop device is mounted on the monopole antenna, the height can be reduced because the closed-loop device can minimize the density of the signal radiated from the monopole by reflecting the signal in the opposite direction. Therefore, despite reducing the height of the existing monopole antenna from 50 to 60 mm to 9.97 mm, the average gain value was able to maintain the omnidirectional beam pattern characteristics while sustaining the increased gain. It can be  
Figure 4: Radiated beam patterns obtained from (**a**) Monopole-only WAVE antenna; (**b**) Single; (**c**) Dual; and (**d**) Quadruple closed-loop devices with monopole WAVE antenna on the 400 mm ground. Red circles denote the region where gain was improved by introducing closed-loop devices. concluded that the role of the closed-loop device is to increase the intensity of the beam, i.e., the gain value, by reflecting the beams radiated from the monopole antenna back around the monopole, as shown in Figure 4.  
The radiation pattern of the signal at a specific angle is also very important for facilitating the smooth inter-vehicle communication of autonomous vehicles and communication with the base station. The radiation pattern of the antenna is important at 90\({}^{\circ}\) for communication between vehicles, at 80\({}^{\circ}\) for communication with base stations located 6 m high, and at 85\({}^{\circ}\) when driving on a hill [7]. At these angles, it should have an omnidirectional radiation pattern and a gain value large enough for stable communications. Figure 5 shows the two-dimensional (2D) and three-dimensional (3D) radiation beam patterns of the WAVE antenna installed only with a monopole antenna and no closed-loop device on the shark-fin module (in Figure 1d). The characteristics of the radiation pattern were analyzed to verify whether smooth communication was achieved at angles of 80\({}^{\circ}\), 85\({}^{\circ}\), and 90\({}^{\circ}\), respectively; a 3D view is presented in Figure 5d. Conversely, the 2D- and 3D-radiated beam patterns from the monopole WAVE antenna with quadruple closed-loop devices on the shark-fin module (in Figure 1c) are shown in Figure 6. The 2D null value comparison of monopole-only (red color) and monopole with quadruple closed-loop devices (green color) at angles of 80\({}^{\circ}\), 85\({}^{\circ}\), and 90\({}^{\circ}\), respectively, is shown in Figure 7a-c. At 80\({}^{\circ}\), the null value is enhanced from \(-\)30.8 to \(-\)4.7 dBi. At 85\({}^{\circ}\), the null value is enhanced from \(-\)26 to \(-\)5.5 dBi, and at 90\({}^{\circ}\), the null value is enhanced from \(-\)21 to \(-\)6 dBi. From these simulation results, the monopole-only antenna has a serious null point when installed on the shark-fin module.