## 3 Results and Discussions
The tuning waveform is visible for the change in the inner ring diameter. The first band shows good reflectance and isolation for the 6 mm inner ring diameter, so the optimized value for the inner ring diameter is kept at 6 mm. The outer ring of the complementary split ring resonator is varied, and the variation in the results is observed in Figure 6. The variation in \(\mathrm{S}\)-parameter results is observed from 8 and 9 mm. Similarly, the tuning is a waveform visible for the change in the outer ring diameter. The 8 mm outer ring radius shows better reflectance and isolation, as is visible from the blue line curve and yellow isolation curve. The red curve for the 9 mm outer  
Figure 2:0 Directivity plot for both designs. (**a**) Complementary SRR design. (**b**) Circular disk design. The complementary split ring resonator design has higher directivity of 5.95 dB. ring shows less reflectance and only one band in the X band, while the 8 mm outer ring diameter gives two bands and good isolation for both bands. The gain polar plot and directivity results were analyzed for both antenna designs, and their results are shown in Figures 7 and 8. The 5.38 dB of gain results are presented in Figure 7 for the CSRR metasurface MIMO antenna. The circular disk design has the highest gain of 4.35 dB. The gain polar plot is oriented towards the positive z direction, which shows it has directional radiation. The directivity results of both antenna designs are presented in Figure 2:1 The directivity was analyzed for \(-180\) to \(180^{\circ}\) theta values. The phi values were used with four degrees, and those were 0, 90, 180, and 270\({}^{\circ}\). Two directivity peaks around \(0^{\circ}\) theta values are visible in the CSRR metasurface design. The circular disk design has a single peak at \(0^{\circ}\) theta value. Therefore, the directivity values are varied for the different phi values of the design. The S-parameters of both MIMO antenna designs were checked using a vector network analyzer. The results are presented in Figure 2:2 The agreements among S\({}_{11}\) and S\({}_{21}\) effects in the measured and simulated results were observed. The CSRR MIMO results are presented in Figure 9a, and the circular disk is shown in Figure 9b. The fabricated prototypes are presented as an inset in the figures. Figure 2:3 S-parameter comparison for the measured and simulated results. (**a**) Complementary split ring resonator design. (**b**) Circular disk MIMO antenna. The charge distribution of the presented design is represented in the figure. The shape variation alters the surface charge distribution. Figure 10a shows the Efield of the circular disk-based patch is 2.56 \(\times\) 10\({}^{1}\) V/cm. Figure 10b shows the Efield of the CSRR-based structure is 3.062 \(\times\) 10\({}^{1}\) V/cm. A better surface charge was attained in the presented approach. The presented design's simulated copolar and cross-polar responses are shown in Figure 2:4 The copolar is used when the sender and receiver antenna elements are oriented in the same direction. The cross-polar is used when the sender and receiver antenna elements are orthogonal. The copolar and cross-polar plots for the circular disk-shaped and CSRR-shaped patch structures are represented in Figure 11a,b. The broader directivity is also observed for both design structures. Figure 2:5 The surface charge distribution of the presented design. (**a**) The circular disk represents Efield of 2.56 \(\times\) 10\({}^{1}\) V/cm. (**b**) CSRR-based structure means an Efield of 3.062 \(\times\) 10\({}^{1}\) V/cm. The proposed work's simulated and measured antenna gain is represented in Figure 2:6 There is a good similarity observed between both results. The minor variation is regarded because the dimension variation, calibration issue, and fabrication glitches lead to slight variation among both results. The broader gain is attained in the CSRR metamaterial-loaded MIMO antenna structure. The performance of the antenna was analyzed based on the antenna efficiency.