# Unit Cell Optimization of Groove Gap Waveguide for High Bandwidth Microwave Applications
## 3 Results and Discussion
The proposed design of the unit cell is specifically designed to ensure a large air gap is maintained between the upper metal plate and the top surface of the unit cell for achieving improved performance as well as quick and easy manufacturing of products. Evaluation of Fabricated Design Modeled at 3 GHz SpectrumFor the fabrication of an MGGW for the S band, a straight MGGW was modeled at the center frequency of 3 GHz, and the optimized MGGW model was successfully fabricated. Following this, the prototype device was tested using a Keysight Vector Network Analyzer (VNA), model E5063A. The waveguide adapters utilized were WR284 EIA standard rectangular plane adapters. These adapters were calibrated prior to the prototype waveguide testing. The calibration of adapters was carried out independently by using an S-band rectangular waveguide termination of 100 watts and collectively by connecting adapters directly with each other. During the calibration, the RF cables were first calibrated as one port, and a two-port calibration procedure was adopted using the electronic calibration (ECAL) kit of the VNA. In the second step, waveguide adapters were connected to the 100-watt WR284 rectangular waveguide termination individually, and their response was observed. Waveguide adapters were then connected with each other directly, and their response was again verified as satisfactory. The prototype MGGW was finally connected between waveguide adapters and RF cables to observe a response using the VNA. A pictorial representation of the test setup is displayed in Figure 9, which shows the prototype device connected to a calibrated VNA utilizing proper radio frequency cables equipped with an N-type RF connector. The measured results of the prototype MGGW were analyzed  
Figure 8: Comparison of simulated S11 and S21 results for **(a)** GGW with double right-angle 90\({}^{\circ}\) bend having bed-of-nails structure; **(b)** MGGW with double 90\({}^{\circ}\) bend and proposed mushroom unit cell EBG structure. for a frequency range from 50 KHz to 8 GHz. These results are very promising in the band of interest and are in good agreement with the simulated results, as their comparison in Figure 10 shows. The achieved reflection coefficient is less than \(-\)20 dB for the entire band of interest. From the results, it can be concluded that even though fabrication was performed with a 50-micron milling tolerance, the entire fabricated MGGW's length had a good surface finish, which produced the intended measured results because there is enough of an air gap (4.036 mm) between pin structure and top metal plate. As we discussed earlier in Section 2, an air gap of \(\leq\)0.05 mm is very difficult to achieve through conventional fabrication, and for the complete length of the design, the air gap may be reduced to zero,  
Figure 10: Simulated vs. measured result comparison of S11 and S21 parameters for an MGGW design having a proposed EBG unit cell structure. Figure 9: Test setup for S11 measurement of GGW prototype with a mushroom-type EBG unit cell structure. which will cause a disturbance in the wave propagation through the waveguide channel. The measured bandwidth of the fabricated model is 1.576 GHz for a passband ripple of 1 dB with a reflection coefficient of \(-\)20 dB. The results achieved are in good agreement with the simulated model. As we have used WR284 rectangular waveguide adapters for validation, the measured results match the simulated results in the WR284 recommended frequency band of 2.5-3.95 GHz. We have also observed that due to the WR284 rectangular waveguide adapters, the measured response beyond 3.95 GHz starts to degrade with a higher passband ripple (>1 dB) even though the reflection coefficient is still below the \(-\)20 dB threshold. Therefore, we have presented the recommended useable bandwidth of the prototype design model of MGGW, considering only up to 1 dB passband ripple, which extends up to 3.824 GHz.