# A Versatile Shared-Aperture Antenna for Vehicle Communications
## 1 Introduction  
In recent years, with the rapid development of Internet of Things (IoT) technology, vehicular communication has become a hot topic. As shown in Figure 1, due to the increasing number of functions in vehicles, today's vehicular communication scenarios have become more complex, including GPS communication, satellite communication, vehicle-to-vehicle communication, and more. These communication protocols correspond to different frequencies, and as a result, vehicles need to incorporate more communication links. In the face of such a multi-frequency communication environment, designing antennas that are cost-effective and compact while meeting communication scenario requirements is of utmost importance. Shared-aperture technology is a technology designed for multi-frequency applications. Compared with traditional antenna solutions, shared-aperture technology has a more compact structure, higher aperture utilization, uses a smaller area to achieve more functions, and has lower cost. It involves placing multiple antennas for different frequencies together while ensuring that they do not interfere with each other. Additionally, this technology can reduce the antenna's footprint, thus lowering antenna costs [1; 2; 3; 4; 5; 6; 7]. At present, the structure of the shared-aperture antenna array can be mainly divided into two types. The first type involves distributing different antennas in different layers [1; 8; 9]. In [1], a shared-aperture array of the metasurface antenna array and reflect array was designed, and the two antennas had almost no effect on each other; the array had a 45% operating bandwidth in the X-band and an 11.4% operating bandwidth in the Ka-band. In [9], portions of the low-frequency radiation elements that did not affect radiation performance were removed, while exposing sufficient surface area for the radiation of high-frequency antennas. The second type involves distributing different antennas on the same layer. In [10], placing two types of folded dipoles in the same layer achieved working bandwidths of 8.9% and 17% in the S-band and X-band, respectively. The interleaved structure is the most commonly used structure for shared-aperture antenna arrays in type two. This involves the interleaved arrangement of high-frequency and low-frequency antennas, and this structure is comparatively more effective in reducing mutual interference between antennas than other structures [4; 5]. For modern automobiles, the global positioning system (GPS) is of utmost importance as it allows precise vehicle location determination and provides navigation to destinations [11; 12; 13]. Currently, there are numerous antenna designs proposed for GPS, with circular polarization being the predominant choice. These antenna types include shorting probe-loaded patch antennas [14], cross-dipole antennas [15], slanted patch antennas [16], and microstrip antennas with sequential feeding networks [17], among others. Furthermore, these antennas effectively cover the L1 frequency band of GPS, thereby offering reliable navigation services for vehicles. S-band communication is increasingly being employed in automobiles, including applications such as satellite communication [18] and 5G communication [19]. To avoid issues related to linearly polarized antennas causing signal degradation, antennas are commonly designed in circular polarization. In [20], a 3 dB coupler bridge was utilized in the design, achieving circular polarization at 2.5 GHz. In [21], a cross-dipole antenna with a 90-degree phase shifter was proposed to support circular polarization communication in the 2025-2090 MHz range. With the continuous increase in the number of automobiles in recent years, the frequency of traffic accidents has also been on the rise. To prevent traffic accidents, V2V technology, visual algorithms, and IoT technology have been widely applied in vehicles [22; 23; 24]. Among these, the working frequency band for V2V has been allocated by the Federal Communications Commission within the range of 5.85 GHz to 5.925 GHz [22].