## 3 Results  
Figure 2 shows the surface current distribution when there are one, two, and four closed-loop devices with a general monopole antenna. The surface current is wider in the direction where there is a closed-loop, as illustrated in Figure 2. Therefore, it can be expected that the beam pattern will become stronger in the opposite direction of the closed-loop devices. As the number of closed-loop devices increases, the surface current gets centered around the monopole antenna, and its density also increases. Figure 3 shows the surface current and magnetic field characteristics of a closed-loop device. Because of the role of closed-loop devices as a reflector of the beams emitted from the monopole, the density of the magnetic field is high around the monopole, and there is a slight fluctuation according to the difference in gain value. The magnetic field distribution shown in Figure 3b is strongly distributed around the monopole and the closed-loop device, and the distribution is stronger around the monopole. If the dimension of the closed  
Figure 1: (**a**) Schematic of a closed-loop device; (**b**) Image of the designed monopole WAVE antenna with quadruple closed-loop device; (**c**) Image of the fabricated monopole WAVE antenna with quadruple closed-loop devices installed on the shark-fin module; and (**d**) Monopole-only WAVE antenna, which was used for characterization. loop device is adjusted, the strength of the magnetic field in a specific direction can be strengthened, thereby enabling smoother communication in that direction. This is one way to increase the gain value of the beam pattern between 80\({}^{\circ}\) and 90\({}^{\circ}\) in a specific direction for facilitating smooth communication between vehicles on the road, vehicles traveling on hills, and base stations. The beam patterns radiated from the monopole WAVE antenna on the 400 mm ground were analyzed by changing the number of closed-loop devices. As in the surface current distribution characteristics shown in Figure 2, the effects of the number of closed-loop devices on the radiated beam patterns from the monopole WAVE antenna appear similar. Even when there is only a monopole antenna without closed-loop devices, omnidirectional radiation characteristics are exhibited to some extent. However, for a specific angle, a decrease in gain value can be seen, as presented in Figure 4a. As shown in Figure 4b, when one closed-loop device is installed, the gain value of the beam pattern increases in the opposite direction, and is denoted as a red circle. That is, when the closed-loop device is installed at around 120\({}^{\circ}\) to 150\({}^{\circ}\), the density of the beam pattern increases in the area between 270\({}^{\circ}\) and 360\({}^{\circ}\). It was verified that the role of the closed-loop device is to reflect the signal radiated from the monopole antenna in the opposite direction of its installed location. Figure 3: (**a**) Surface current; and (**b**) Magnetic field distribution obtained from the monopole WAVE antenna with quadruple closed-loop device. Figure 2: Surface current distribution obtained from (**a**) Monopole-only WAVE antenna; (**b**) Single; (**c**) Dual; and (**d**) Quadruple closed-loop devices with monopole WAVE antenna. Figure 4c shows the results of the beam pattern analysis when two closed-loop devices were installed in a diagonal direction. Similar to Figure 4b, a closed-loop device was additionally installed between 30\({}^{\circ}\) and 60\({}^{\circ}\) and another between 210\({}^{\circ}\) and 240\({}^{\circ}\). The gain value of the beam pattern increased in the opposite direction (marked by red circles) in the results shown in Figure 4c. Compared to the result shown in Figure 4b, which is of a single installed closed-loop device, the density of the beam pattern increased even in the region between 120\({}^{\circ}\) and 150\({}^{\circ}\), indicating that the two closed-loop devices worked independently.