## 3 Results and Discussion
### Measurement Results  
A vector network analyzer (VNA) operating to 67 GHz was used to measure the scattering parameters of the antennas fabricated. The antennas under test were connected to the VNA through 1.85 mm end-launch connectors (Southwest Microwave Inc., Tempe, AZ, USA) as illustrated in Figure 10a. Quad-port MIMO antennas were prototyped and experimentally evaluated. Figure 10b shows setup for measuring individual reflection coefficient of each port (i.e., S\({}_{11}\), S\({}_{22}\), S\({}_{33}\), and S\({}_{44}\)). Figure 10c displays the setup for measuring mutual coupling between each pair of antennas. Figure 11 shows a comparison between measured and simulated reflection coefficients as the fabrication process requires a lot of specifications that should be taken into consideration to give the best results. There are some challenges facing the implementation of the proposed design that is listed below. The fabrication complexity is firstly concerned, as metamaterial unit cells often require high-precision manufacturing techniques such as photolithography or multilayer PCB fabrication, especially in the higher frequency spectrum at 28 GHz, which can introduce alignment errors and increase production costs. Additionally, fabrication tolerances leading to variation in the electrical properties of the proposed antenna and the MTM unit cell will vary in terms of the permittivity and permeability and will shift the operating frequency. Integration challenges also arise when implementing metamaterial structures in compact devices, as the MTM unit cell insertion must not affect the antenna radiation efficiency. Finally, the validation of the proposed design and accurate measurement require controlled testing environments, such as anechoic chambers, to ensure reliable mutual coupling reduction. Discrepancies between simulated and fabricated designs must be carefully analyzed to ensure consistency across MIMO designs. Addressing these challenges is essential for the successful realization of the proposed antenna structure in practical communication systems. The measured and simulated results exhibit generally good agreement within the specified frequency bands. However, a discrepancy of approximately 0.8 GHz is observed in the resonance frequency of the central band. This frequency shift could be attributed to several factors, including impedance mismatch, the effect of the SMA interface connector, environmental conditions such as temperature or humidity, and potential assumptions made during the modeling process that may not fully capture real-world conditions. Additionally, fabrication tolerances, such  
Figure 10: (**a**) Measurement setup of reflection coefficient of individual antenna port, (**b**) measured reflection coefficient of two ports, and (**c**) setup of measuring mutual coupling between each port pair. as slight variations in material properties or dimensional inaccuracies, may contribute to this deviation. Despite this offset, the measured bandwidths remain in close agreement with the simulated values, further indicating that the antenna's performance in terms of bandwidth is consistent across the three frequency bands. This suggests that while the resonance frequency is slightly altered, the overall functionality and design objectives of the antenna are still achieved in the measured frequency range, confirming the robustness of the antenna's performance. Measurement results show the reflection coefficients for each MIMO port (S\({}_{11}\), S\({}_{22}\), S\({}_{33}\), and S\({}_{44}\)) against the predictions from simulation. The experimental results correlate well with theoretical predictions and thus prove that the design analysis is robust. It can also be seen from the measured results that there is a significant increase in the overall bandwidth across the investigated band, as well as a considerable decrease in the measured reflection coefficients, thus validating the effectiveness of these design improvements in general antenna performance.