## 3 Measurement Results  
The antenna prototypes were characterized using a 67 GHz vector network analyzer (VNA) to measure their scattering parameters. Both single-element and four-port MIMO configurations were evaluated using precision 1.85 mm end-launch connectors, as illustrated in Figure 8. Figure 8a specifically shows the measurement setup for the single-element prototype connected via an SMA connector to the VNA. The comparison between measured and simulated reflection coefficients, presented in Figure 8b, demonstrates good agreement in both the first and third frequency bands. However, an approximately 1 GHz frequency shift was observed in the middle band. This discrepancy may be attributed to several factors including impedance mismatches, SMA connector effects, environmental influences during measurement, or inherent approximations in the simulation models. Despite this frequency shift, the measured bandwidths  
Figure 8: (**a**) Measurement setup of single-element antenna and (**b**) measured reflection coefficient as compared to the simulated one. across all three bands remained consistent with the simulated results, representing good overall consistency between the fabricated design and its computed performance. As a rapid fix, laser trimming or post-fabrication tuning can experimentally adjust this frequency shift in the middle band without altering the 28/38 GHz bands. The scattering parameters for the MIMO antenna were assessed using the VNA, with the unconnected ports terminated by a 50 \(\Omega\) matching load, as depicted in Figure 9a. This methodology ensures accurate measurement of the inter-port coupling effects while maintaining impedance matching for the unmeasured ports, contributing to a comprehensive assessment of the antenna's performance characteristics. Figure 9b displays the results of the measured reflection coefficients of each MIMO port (S\({}_{11}\), S\({}_{22}\), S\({}_{33}\), and S\({}_{44}\)) as compared to those of the simulated results. A noteworthy consistency is observed between the simulation and experimental measurements, highlighting the reliability of the analysis. Furthermore, a notable increase is observed in the bandwidth of each one of the three frequency bands to 1.1, 2.2, and 3.7 GHz, respectively. The reflection coefficients are accompanied with a substantial reduction in loss for all of the measured antenna ports. This emphasizes the efficacy of the design enhancements in improving overall antenna performance, highlighting the importance of accurate simulations and careful experimental validation in optimizing the antenna characteristics. Figure 10 demonstrates simulated/measured S-parameters for the MIMO configuration. The close correlation between the simulation and measurement results indicates significantly reduced mutual coupling between the antenna ports. The measured S\({}_{21}\) isolation is slightly degraded as compared to the simulation results; this stems from unmodeled practical effects including fabrication tolerances (dielectric variation, etching precision), SMA connector parasitics and imperfect launches, and substrate/ground-coupled surface waves. At mmWave frequencies, sub-100 \(\upmu\)m dimensional errors significantly impact coupling. Improved agreement requires EM models incorporating measured material data and connector S-parameters. Despite this, there is strong agreement between simulation and measurement that still reveals excellent isolation performance across all three operational frequency bands, validating the effectiveness of the incorporated MTM design in refining MIMO isolation. The improved isolation directly contributes to reduced inter-port interference, ultimately improving the overall antenna system performance. These results confirm the successful implementation of the metamaterial approach in achieving superior isolation characteristics for multi-port MIMO antenna systems.