# Unit Cell Optimization of Groove Gap Waveguide for High Bandwidth Microwave Applications
## 2 Materials and Methods  
### Design of Unit Cell  
The design of the proposed unit cell structure was simulated in Microwave Studio CST. The goal was to achieve a wide bandwidth of transmission lines using simple periodic structures. To achieve this objective, the bed-of-nails unit cell structure was thoroughly studied and redesigned to improve the bandwidth of the EBG structure. The nail structure was modified in the shape of a mushroom-unit-cell-based EBG design. In the GGW design, dielectric losses do not exist because of the pure metallic structure, and a mushroom-shaped design provides more bandwidth along with greater air gap independence. For a unit cell design, the physical dimensions of the standard conventional rectangular waveguide WR284 EIA standard were taken as a reference. The selection of the WR284 waveguide provided a flexible input option for the testing of the proposed waveguide. To analyze the distinctive performance of the proposed new unit cell, a groove configuration of the gap waveguide was selected. The EBG structure was created on a lower metallic plate, and an upper metal plate was aligned parallel and placed at a right angle to the periodic structure. This model is termed a mushroom-unit-cell-based groove gap waveguide (MGGW). For the modeling of the proposed design, the width and height of the WR284 rectangular waveguide adapter were used as a reference because we needed to practically connect it for testing. Its operating frequency range is specified in Table 1.  
To develop an innovative unit cell for groove gap waveguides, dispersion properties were explored through the eigenmode solver of Microwave Studio CST. The dispersion diagram provides details about the cut-off band [38]. To begin with, the bed-of-nails EBG structure was analyzed using the same physical dimensions (width and height) as WR284, \begin{table}
\begin{tabular}{c c c} \hline
**Cut-Off Frequency of Lowest Mode (GHz)** & **Cut-Off Frequency of Upper Mode (GHz)** & **Recommended Frequency Band (GHz)** \\ \hline
2.078 & 4.156 & 2.6–3.95 \\ \hline \end{tabular}
\end{table} 
Table 1: Standard working parameters and constraints of WR284. Figure 1: Illustration of three types of gap waveguides: (**a**) strip, (**b**) groove, and (**c**) ridge. and its dispersion characteristics were parametrically optimized with a consistent air gap maintained between the upper metal plate and the top surface of the EBG structure. An analysis of the dispersion curve depicts that maximum optimized bandgap is achieved in two instances: first, when the air gap is selected as 4.036 mm between two surfaces, and second, when the air gap is selected \(\leq\)0.05 mm. However, to create such a narrow air gap precisely across the entire design is a challenging task as it requires a costly and expensive fabrication process. The dimensions of the bed-of-nails unit cell periodic EBG structure that were studied and evaluated are shown in Figure 2a. The dispersion characteristics of periodic configuration based on a bed-of-nails unit cell are depicted in Figure 2b. The first two dominant propagating modes, mode 1 and mode 2, experience a bandgap of 2.309 GHz from 1.91533 GHz to 4.2251 GHz. The energy will propagate along the periodic cells for the observed bandgap and will not be dispersed through pins. To increase the bandwidth of a unit cell, the dispersion properties of various configurations were analyzed. In this research, a shape analogous to a mushroom-type EBG structure was employed to redesign a bed-of-nails unit cell for microwave frequency applications. In the literature, mushroom-type structures have been explored for low-frequency applications [39; 40], utilizing printed circuit boards (microstrip) only. We modeled mushroom unit cells purely with metal structures for microwave applications.