## 1 Introduction  
Cooperative Intelligent Transport Systems (C-ITS), centered on real-time traffic, seek to overcome the drawbacks of existing Intelligent Transport Systems, which mainly focus on traffic management [1; 2; 3]. Despite various sensors installed in a car, only limited environmental information is obtained when driving over high hills or in foggy weather. Vehicle-to-Everything or Vehicle-to-X (V2X) is a communication technology that allows you to collect and share real-time traffic information, both visible and invisible, with other vehicles, road infrastructure devices, networks, and pedestrians [4]. The Cellular version of V2X (C-V2X) combines Wireless Access for Vehicle Environment (WAVE) and communication technology, both designed to operate in the dedicated frequency band (5.85 to 5.925 GHz) for V2X applications [5]. Irrespective of using either WAVE or C-V2X, the communication environment must be seamless and reliable. It must efficiently relay information regarding road safety-related situations, such as lane changes, warnings, and collision detection. The antenna is a crucial component of the V2X system, which provides a reliable connection between the vehicle and other networks. In actual road conditions, vehicle locations vary depending on the distance and their driving lanes. Therefore, for stable Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communication, the radiation pattern of the antenna must be 360\({}^{\circ}\) (omnidirectional)and horizontally oriented [6]. Generally, an increase in antenna gain enables long-distance communication, but in this case, the bandwidth must be reduced. However, if a null point occurs in a specific direction, communication gets disconnected in that direction. Therefore, to achieve stable communication without interruption in any situation, the antenna gain of 360 degrees must be at or above a certain value, with omnidirectional radiation beam patterns. That is, a new antenna structure is required to minimize the null points with sufficient gain. Stable communication is possible while driving only when the antenna radiation pattern forms at 90\({}^{\circ}\) in the horizontal direction for both front and rear vehicle communication, 85\({}^{\circ}\) for a vehicle on a hill, and 80\({}^{\circ}\) for a base station at a height of 6 m, respectively [7]. Although the shape of the actual theoretical radiation pattern is omnidirectional, it is difficult to implement this for an antenna attached to the top of a metal vehicle, due to the wide reflector, a nearby GPS antenna, and another antenna, which results in a null area. In [8], for a bidirectional patch antenna, a gain of 6.62 dBi was obtained using three patches, but a null region of \(-10\) dBi or less occurred due to the flat conformal structure. In [9], despite implementing omnidirectionality, it was difficult to install two antennas inside the shark-fin module due to its large size. In [10; 11], for the monopole structure, a significant null region occurs due to the finite ground size. In [12; 13], two compact V2X antenna solutions were proposed, but due to the characteristics of the collinear array antenna structure, the height was approximately 50-60 mm. In order to simultaneously implement the frequency modulation (FM), amplitude modulation (AM), and WAVE antennas in the shark-fin module, the height of the WAVE antenna must be lowered. In this study, we propose an antenna structure with omnidirectional characteristics while lowering the height of the collinear array antenna in the finite ground of the shark-fin module by adding closed-loop devices to the monopole. In particular, the gain can be improved by focusing the flow of the surface current of the ground around the antenna through closed-loop devices. In this study, we investigated the influence of closed-loop devices on beam patterns radiated from existing monopole WAVE antennas. The effects were analyzed by introducing single, dual, and quadruple closed-loop devices into an existing monopole antenna.