# Unit Cell Optimization of Groove Gap Waveguide for High Bandwidth Microwave Applications
## 1 Introduction
Consequently, the manufacturing of gap waveguides is regarded as more beneficial compared to the manufacturing of traditional waveguides in any frequency spectrum for desired applications. Additionally, the employment of solely metallic plates and patterned surfaces makes gap waveguides superior compared to microstrip or coplanar transmission lines (CPWs), which require the use of dielectric materials. The electromagnetic bandgap (EBG) effect, modular fabrication, and absence of metal contact make gap waveguides an attractive choice for achieving a wider bandwidth compared to traditional RWGs. Currently, gap waveguides are frequently researched for commercial utilization for a wide range of products, including transducers, the packaging of components, transitions, filters, couplers, antennas, and transmission waveguides [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. Figure 1 shows a sketch of three different gap waveguide setups to illustrate the gap waveguide idea. In the literature, various periodic configurations have been reported to develop EBG structures for gap waveguides, e.g., mushroom EBG configurations for microstrip lines, spring-like arrays, patterned zigzag conductors, periodically arranged conical pins, and structures with an inverted pyramid geometry [27; 28; 29; 30; 31; 32; 33]. Among these EBG configurations, the bed-of-nails structure is the most common and widely researched structure due to its ease of fabrication and its wider bandwidth applications. The periodic structure is augmented with a complete design of the desired model, including the antenna, feed network, filter, divider, etc. For a specific user application, the bandwidth of a completed structure is reduced significantly when multiple passive components like the feed network, filters, splitters or dividers, and antenna are designed simultaneously for a completed structure [21; 24; 25; 32; 34; 35]. Unit cells with wider bandwidths are always considered effective for the design of any passive component or complete structure. To attain a wider bandwidth, it is important that the air gap separating the top metallic plate from the EBG structure is very small [36]. In conventional fabrication methods, the entire length of the design demands accurate surface finishing of both the upper metallic plate and the EBG pattern, which is an expensive and time-consuming operation. Additionally, the design also requires a customized feed to achieve a wide bandwidth [21; 25; 35]. In this research, we propose a new mushroom-type GGW configuration that significantly improves the bandwidth of the gap waveguide design for microwave applications. The design of the GGW was simulated in the eigenmode solver of Microwave Studio CST. The results from the simulations indicate that the bandwidth was improved by 8% compared to traditional unit cells in the microwave spectrum. To validate our proposed method for the physical dimensions of the unit cell structure, we practically developed an MGGW structure for the S band which shows similar trends to those of the simulations. The measured results are promising, with a reflection coefficient of less than \(-20\) dB attained over the whole range of the Rs-261-B [37] standard waveguide adopter. The proposed MGGW structure will provide new opportunities for researchers to develop ultra-wide bandwidth microwave applications, i.e., filters, transmission lines, resonators, attenuators, etc. The research presented in the subsequent paragraphs will follow the following sequence: the design of the new unit cell, MGGW design for the microwave spectrum, an evaluation of existing designs based on the bed-of-nails unit cell concept, the fabrication of the design, and an analysis of the MGGW prototype's results.