# Unit Cell Optimization of Groove Gap Waveguide for High Bandwidth Microwave Applications
## 3 Results and Discussion  
In this section, three optimized MGGW designs (straight waveguide and with transitions) in the S band are analyzed based on simulations performed in CST Microwave Studio. Subsequently, a comparison between the measured and simulation results of the proposed straight MGGW is presented. The results are studied based on the S parameters (S11 and S21) of these designs. First, the simulation results for the proposed MGGW with a mushroom-unit-cell-based periodic structure are presented in comparison with the GGW with a bed-of-nails-based periodic structure. The two waveguides were designed and optimized with similar dimensions and turns (single 90\({}^{\circ}\) and double 90\({}^{\circ}\) bends). The bandwidth analysis of all three designs was carried out for MGGW and GGW. The bandwidth results achieved via CST simulation are tabulated in Table 4. The simulation results depict that the MGGW has a wider bandwidth as compared to the GGW with a straight-pin-based periodic structure. The bandwidth of designs is compared at a \(-\)20 dB reflection coefficient value. Figures 6-8 graphically represent an S-parameter (S11 and S21)-based comparison of the simulation results of the straight, single right-angle bend, and double right-angle (90\({}^{\circ}\)) bend GGW with the proposed MGGW. The results of the proposed unit cell are promising for the designed MGGW. In the case of straight waveguide design, the proposed MGGW provided a bandwidth enhancement of 14.5% compared to bed-of-nails GGW. Similarly, the bandwidth achieved for the proposed MGGW is 16.9% and 32.5% greater than that for the bed-of-nails design for the single right-angle 90\({}^{\circ}\) bend and double right-angle 90\({}^{\circ}\) bend GGW designs, respectively. For practical evaluations, these results have been obtained for an MGGW design with actual waveguide face dimensions that match the EIA standard WR284 waveguide adapter dimensions. The examination of the simulated results suggested that the newly designed unit-cell-based MGGW exhibited broader bandwidth when compared to the conventional bed-of-nails unit cell presented in the literature. The proposed design has also been studied for two transitions, and the observed bandwidth is greater than that of GGW. Since a bed-of-nails structure has been used in the literature for a variety of design applications with customized feed networks, the outcomes of these simulations cannot be compared to any existing study findings. Figure 6: Comparison of S11 and S21 simulation results. (**a**) GGW without any bends with bed-of-nails structure; (**b**) proposed MGGW without any bends with mushroom unit cell periodic structure. Figure 7: Comparison of S11 and S21 simulation results. (**a**) GGW with single 90\({}^{\circ}\) bend with bed-of-nails structure; (**b**) MGGW with single 90\({}^{\circ}\) bend and proposed mushroom unit cell EBG structure. It is important to mention that the proposed work in this study is evaluated by maintaining a fixed air gap between the upper metal plate and the top surface of the unit cell. This consistent fixed air gap is essential to facilitate the propagation of energy through the MGGW channel. To improve the bandwidth capabilities of the MGGW, further optimization of the bed-of-nails structure can be performed. However, this enhancement will lead to a reduction in the size of the air gap between the upper metal plate and the top surface of the unit cell. As the waveguide structure is comparatively large for application within the microwave spectrum, it is either very challenging to attain a refined surface finish through established fabrication techniques like conventional milling or contemporary 3D printing methods such as direct metal laser sintering (DMLS), or this accuracy and precision involve a high fabrication cost.