## 2 Design of the Six-Element MIMO Antenna Structure  
The schematic of the six-element, split-ring resonator-shaped MIMO antenna is depicted in Figure 1. Figure 1a presents a 3D viewpoint of the proposed MIMO antenna, complete with input port labeling. On the dual-layer FR4 substrate material, copper is chosen as the material choice for the bottom as well as the top layer of the proposed structure. A schematic view of other two- and six-element antennas is shown in Figure 1b-e. Table 1 gives a complete idea of antenna dimensions. The overall antenna is in size of 74 \(\times\) 100 mm\({}^{2}\). We have performed numerical analysis on five distinct antenna designs that contain the radiation patch elements either on a split ring resonator or in a circular patch shape of the structure. In all the antenna designs, the ground is either continuous for all patches in the shape of a rectangle, or individual rectangular copper patches are placed over the individual MIMO elements, which can be seen in Table 2. The antennas are classified into five types as depicted in Table 2. Table 2 illustrates the simulated antenna design along with the fabricated antenna images and its structure description. The proposed structure is simulated using the commercially available software HFSS. The proposed antennas are designed and built with a variety of criteria in mind, including their reflectance coefficient, gain, directivity, and radiation pattern for the 1 to 25 GHz frequency range. A vector network analyzer (VNA) N9918A (30 kHz to 26 GHz) with a purpose-built antenna and electromagnetic compatibility (EMC) test anechoic chamber has been used to experimentally verify the suggested antennas. The mathematical equations and computational facility have been involved as per equations suggested in the upcoming results and discussion section of this manuscript. We have calculated the reflectance response of all five types of antennas by applying the excitation at all antenna ports. The reflectance response achieved through the numerical study is depicted in Figure 2. Figure 2a illustrates the reflectance response for the two-element split ring resonator-based antenna structure. The dotted line in Figure 2 for all the responses is considered for the identification of the values of the reflectance response below the level of \(<\)\(-\)10 dB. This range of the reflectance value helps us to identify the values of the resonating bands. We have calculated the overall resonating conditions where Sij \(<-\)10 dB, where i, j = 1, 2, 3, 4, 5, 6. The reflection conditions Sij \(<-\)10 dB allow us to choose the resonating band where co-port (port to port isolation) and cross-port reflectance (inter element isolation) are below the \(<\)10% for the effective antenna radiation of a specific frequency. Figure 2b shows how the S parameters change when the circuit is excited through three ports. Similarly, Figure 2c-e shows the variation in the co-port reflectance parameters for the six-port excitation in designs 3, 4, and 5 of the antenna structure, respectively. For Design 5 of the MIMO structure based on a split ring resonator, we have also included the computed S parameters for the multi-port excitation configurations. The results obtained for these port excitation conditions are shown in Figure 3. We can observe the values of the S parameters where the values of the reflection coefficients of the input signals for each of the six antenna elements, i.e., Sij, where i and j refer to 1, 2, 3, 4, 5, and 6, are \(<\)\(-\)10 dB and other S parameters S12, S13,... are also \(<\)\(-\)10 dB. These conditions are satisfied for the identification of the overall resonating band where the co-port and cross-port reflectance values are \(<\)10% for the effective radiation.