# Unit Cell Optimization of Groove Gap Waveguide for High Bandwidth Microwave Applications
## 1 Introduction  
Waveguides are essential components of a microwave system. They are used for transferring energy to and from the components of a microwave system. To fulfill the demands of high data rates in modern microwave systems, waveguides must be able to handle wider bandwidths and more power [1]. The most common waveguide design that has been extensively studied and investigated is the rectangular waveguide (RWG) because it offers several advantages over conventional transmission channels, including high power, minimal attenuation, a simple structure, and ease of installation. However, the waveguide is unable to provide sufficient bandwidth for modern microwave applications [2, 3]. Therefore, researchers have proposed various designs to improve the bandwidth while addressing the requirements of intended applications, such as wireless communication and satellite communication [3]. Several types of waveguides being investigated for microwave applications include coaxial lines, rectangular waveguides, microstrip structures, and substrate-integrated waveguides. Each design has its own set of benefits and limitations. Among these, coaxial lines provide better performance at low frequencies; however, in the microwave spectrum, due to reduced dimensions, power handling capability is limited, and losses caused by the skin effect in the inner conductor and outer dielectric are significant. Because of their pure metallic construction and greater power handling capabilities, rectangular waveguides are considered to perform better in the microwave range. However, rectangular waveguides' primary drawback is the expensive and challenging task of creating accurate metal junctions for smaller dimensions in the microwave spectrum to prevent leaks. Microstrip lines have a simple construction and are made of a groundplate, a dielectric substrate, and a metal strip which is typically utilized in inexpensive printed circuit boards. Significant losses result as the substrate's dielectric constant decreases and approaches that of air in the microwave spectrum. Moreover, these waveguides are susceptible to crosstalk and have the potential to radiate parasitically [4]. Substrate-integrated waveguides have the combined benefits of both a microstrip and rectangular waveguides. They are compact, easy to integrate, and have more power-handling capability. However, their performance is significantly affected in the microwave spectrum due to dielectric losses [5]. To overcome the challenges faced by various types of transmission waveguides, gap waveguides are considered as one of the most promising candidates in the microwave spectrum. A gap waveguide is an engineered, rectangular structure for the enhancement of the bandwidth [6; 7]. A groove gap waveguide (GGW) confines energy in a rectangular propagation channel using a periodic structure of square metal pins. This structure creates an electromagnetic bandgap effect on both sides of the channel, ensuring efficient wave propagation without the need for metal contact between the lid and the main body. This effect restricts certain frequency components from propagating, effectively enhancing the bandwidth of gap waveguides compared to RWGs. The formation of a waveguide between two parallel metal plates with a small gap between them is known as a gap waveguide phenomenon. A significant advantage of the gap waveguide, compared to traditional waveguides, is its two independent metallic pieces, which can be combined to form a waveguide structure. Therefore, it eliminates metal-to-metal contact issues, leading to better performance and remaining completely invariant to the re-assembling process. Gap waveguides' design eliminates the need for metal contacts between two parallel metal plates, relying solely on sidewalls to support the upper metal plate and maintain it at a sufficient, consistent height above the bottom plate.