## 3 Results
However, the proposed antenna, a monopole with a closed-loop device, has an omnidirectional beam pattern without a null point. Figure 5: Radiated beam patterns obtained from the monopole-only WAVE antenna on the shark-fin module at the angles of (**a**) 80\({}^{\circ}\), (**b**) 85\({}^{\circ}\), and (**c**) 90\({}^{\circ}\); and (**d**) 3D view. Figure 8 shows the surface current of the antennas when installed on the shark-fin module. For the monopole-only antenna (in Figure 8b), it can be seen that the surface current affects surrounding antennas through the ground. However, for the proposed antenna (in Figure 8a), the surface current is concentrated only around the antenna. This proves that the four devices form a closed-loop path for the surface current. By allowing the surface current to focus only around the antenna, the characteristics of the gain and beam pattern of the antenna can be improved. These characteristics improve isolation performance in the shark-fin module, which requires multiple antennas to be installed on the common ground. Figure 6: Radiated beam patterns obtained from monopole WAVE antenna with quadruple closed-loop devices on the shark-fin module at the angles of (**a**) 80\({}^{\circ}\), (**b**) 85\({}^{\circ}\), and (**c**) 90\({}^{\circ}\); and (**d**) 3D view. Figure 7: _Cont._  
We also implemented monopole WAVE antennas with four closed-loop devices and characterized their properties to assess their practical applicability. The implemented monopole WAVE antenna was mounted on a shark-fin antenna (Figure 1c,d). Figure 9 shows the result of analyzing the beam pattern for smooth communication at 80\({}^{\circ}\), i.e., the communication between a vehicle on the road and a base station installed at a height of 6 m.  
The simulation results of the case where there was only a monopole antenna and when quadruple closed-loop devices were installed in the monopole antenna on the shark-fin module were compared with the experimental results. When only the monopole was installed (Figure 1d), it was confirmed that there was a null point, indicating a very low gain value at 24\({}^{\circ}\). However, when quadruple closed-loop devices are installed in the monopole antenna (Figure 1c), the gain value increases to 24\({}^{\circ}\), and the null point shown in the simulation result disappears. The gain values at this angle were \(-31\) and \(-4.9\) dBi, respectively. The measured results obtained by mounting on the actual shark-fin antenna are shown in blue. The gain value at the same angle is 0.74 dBi, which is a 32.74 and 7.64% increase compared to the simulation result, respectively. The average gains of the WAVE antenna were also compared in addition to the gains at the null point. The average gain of the monopole-only WAVE antenna was \(-2.94\) dBi, which was obtained via HFSS simulation. Conversely, the average gain obtained from the WAVE antenna with closed-loop devices was \(-0.3\) dBi, which was further increased. Furthermore, the average gain obtained from the measurement of the WAVE antenna with closed-loop devices was 1.57 dBi. Therefore,  
Figure 1:0 Surface current when installed on the shark-fin module: (**a**) Monopole with quadruple closed-loop devices, and (**b**) monopole-only wave antenna. Figure 1:1 Two-dimensional null value comparison of monopole-only (red color) and monopole with quadruple closed-loop devices (green color) at the angles of (**a**) 80\({}^{\circ}\); (**b**) 85\({}^{\circ}\); and (**c**) 90\({}^{\circ}\). the average gain increased in the fabricated monopole antenna with quadruple closed-loop devices. It can be concluded that the role of closed-loop devices in reflecting the radiated signals from the monopole antenna coincided with the results of Figure 1:2  
Figure 10 shows the return loss of the monopole antenna with quadruple closed-loop devices. The simulated and measured return losses within the operating frequency were below \(-14\) and \(-17\) dB, respectively.