# Metasurface Source Antenna Gain Improvement Using Simple Side Metal Structure
## 1 Introduction  
Antennas are a fundamental technology in communication systems, playing a crucial role in applications such as satellites, mobile communications, and autonomous driving. Among the various antenna technologies, TAs have recently emerged as a promising candidate to replace traditional high-gain antennas. This increasing interest in TAs stems from their potential to offer enhanced performance while combining advantages in terms of ease of production and cost efficiency. In 2011, metasurfaces capable of controlling transmitted and reflected electromagnetic wavefronts were introduced [1], drawing significant attention and leading to extensive research [2; 3; 4; 5; 6; 7]. Consequently, earlier studies on TAs [8; 9] has evolved, with substantial efforts focused on integrating planar metasurfaces into transmit antennas [10; 11; 12; 13]. These advancements have fostered the exploration of novel antenna designs that dynamically manipulate electromagnetic waves to achieve the desired beam steering and gain characteristics. Moreover, studies on reconfigurable intelligent surfaces (RIS) have gained momentum, as these surfaces enable real-time reconfiguration of electromagnetic behavior, further enhancing the functionality and adaptability of modern communication systems. Considering these developments, transmitarray technology is expected to be widely applied across various fields in the near future. TAs were first proposed in earlier studies and have been evolving towards designs that offer high aperture efficiency and a low profile [14; 15; 16; 17; 18; 19; 20; 21]. To achieve these characteristics, the design of TAs requires a complex structure, incorporating both the array antenna and a power supply structure at the source antenna. Active TAs have been developed to meet these demands and are designed using components such as phase shifters, PIN diodes, and varactor diodes, which are essential for beam steering [22; 23]. However, Active TAsare known to incur losses due to the presence of these active elements [24; 25]. To mitigate these losses, Active TAs can reduce phase quantization loss by employing 2-bit phase control instead of 1-bit, although this approach necessitates a more complex structure. Despite these efforts, the complexity and associated losses present ongoing challenges in the design of Active TAs. In contrast, Passive TAs do not incorporate active elements and instead consist of an array of unit cells, typically composed of patterned structures. This simpler configuration allows Passive TAs to avoid many of the losses associated with active components, but it also limits the beam steering angle [26; 27; 28; 29; 30]. To overcome this limitation, methods such as varying the array structure or adjusting the physical location of the source antenna have been explored [31]. However, these approaches often require the use of a power supply structure or a relatively bulky motor, making them less suitable for the current research trends in metasurface technology. Metasurface feed antennas typically utilize horn antennas and patch antennas. While horn antennas are generally classified as medium-gain antennas, they have limitations when applied to metasurfaces. Specifically, the longer distance required from the feed to the metasurface makes it difficult to achieve a low-profile design. In contrast, patch antennas offer a closer feed distance, making them a more suitable alternative for low-profile applications. However, patch antennas are inherently low-gain, necessitating methods to enhance their gain to ensure adequate performance when used with metasurfaces. Several techniques have been explored to improve the gain of patch antennas. One common approach involves using metallic structures. For example, in study [32], a metal plate was used as a reflector to reduce the side lobes and increase the size of the main lobe, thereby improving the overall gain. In this study, applying two metallic plates to a \(4\times 1\) patch array achieved a gain of 9.8 dBi.