(A) study on WAVE antenna with omnidirectional beam patterns for V2X communication

Abstract

Traditional sensor-based recognition technologies used in automobiles are limited in obtaining environmental information through sensors when automobiles are climbing a high hill or in foggy weather. In order to overcome these limitations, researches on cooperative intelligent transportation systems (C-ITS) that can collect and share traffic conditions even with invisible places through vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications are being actively conducted. The most important advantage of C-ITS is that it can prevent accidents and reduce congestion. There are two communication systems supporting V2V and V2I communications, such as wireless access in vehicular environment (WAVE) and cellular vehicle-to-everything (C-V2X). Although C-V2X technology includes the long-term evolution (LTE) band, it uses WAVE band 5.85~5.925 GHz. The WAVE antenna used in automobiles is usually located on the roof of the vehicle and is placed in a small module along with several other antennas used for different purposes. Therefore, the small antenna module installed on the roof of a vehicle should contain GPS (Global Positioning System), LTE, WiFi, Frequency Modulation (FM) / Amplitude Modulation (AM), and two WAVE antennas. The most important characteristics of the WAVE antenna is that it must be small enough in order to minimize interference with other antennas. And since communication must be seamless no matter where the vehicle is located, it must have a high gain while forming an omnidirectional beam pattern without null areas. In general, the smaller the size of the antenna is and the lower the height is, the more null area in which the gain of the antenna is rapidly reduced occurs. Previous studies tried to improve performance such as a beam pattern and gain by increasing the electrical length of current with changing the radiator of an antenna, such as a modified dipole structure or collinear structure. However, since the height of the WAVE antenna is still high, it is difficult to simultaneously implement the FM/AM antenna and two WAVE antennas in the vehicle antenna module. In this dissertation, the gain change of the antenna is examined when the EBG structure is inserted into the existing micro-strip array antenna. This is because if the gain of the antenna is high, the radius of V2X communication can be expanded. In order to increase the gain of the antenna, a 1×4 linear array structure was used as a basic structure. In order to form an omnidirectional beam pattern through one microstrip array antenna, the ground under the patch was removed and the ground was designed to be located only between the patches. To suppress the radiation of surface waves from the ground plane and vehicle, EBG cells were inserted between micro-strip patch arrays. Several simulations were performed to determine the optimum EBG cell structure located above the ground plane in a conformal linear micro-strip patch array antenna. The characteristics such as return loss, peak gain, and radiation patterns obtained using the fabricated EBG cellembedded antenna were superior compared to those obtained without the EBG cells. A return loss of 35.14 dB, peak gain of 10.15 dBi at 80°, and improvement of 2.037 dB maximum for the field of view in the radiation beam patterns were obtained using the proposed WAVE antenna. In addition, the performance change of the antenna is examined when the surface current density is increased by forming a closed loop in the surface current flowing on the ground plane of the monopole antenna. This is because a small and low-profile WAVE antenna can be implemented if the performance can be improved by changing the image current part of the monopole antenna, unlike the previous methods in which the performance has been improved by changing the radiator part of the antenna. Through this method, it is possible to simultaneously implement FM/AM and two WAVE antennas in the vehicle antenna module, thereby reducing manufacturing cost. In the vehicle antenna module environment where the size of the ground plane is small, by forming a closed loop on a monopole antenna, we tried to improve the characteristics of the antenna, such as gain, radiation beam pattern, and return loss. Single, dual, and quadruple closed-loop devices were introduced into the monopole antenna, and their surface current density and radiation beam patterns were analyzed by using the high-frequency structure simulator (HFSS) and computer simulation technology (CST) programs. As the closed-loop devices reflected the signal radiated from the antenna, the distribution of the surface current was concentrated around the monopole due to the creation of a closed-loop surface current path, which increased the gain value. The average gain was considerably increased by introducing closed-loop devices. The proposed antenna has an average gain of 1.57 dBi and a peak gain of 6.29 dBi at the operating frequency. Furthermore, omnidirectional beam patterns with a beam width of 359° were obtained by introducing four closed-loop devices into the monopole antenna, which eliminated nearly all null points in the frequency range of 5.85-5.925 GHz.