## 2 Design of the Metamaterial Unit Cell
The outer two SRRs and the inner double C-shape are interconnected with a metallic stripe; as a result, we obtain tremendous results with a strong lower frequency in this compact size structure. The new design is adopted with the frequencies shifted towards the lower frequencies where the mutual coupling is introduced and magnitudes are changed. The shifted transmission coefficients are 2.4 GHz, 5.6 GHz, 8.9 GHz, and 11.5 GHz with a magnitude of \(-32.77\), \(38.28\), \(-26\), and \(-44.7\) dB, respectively, which are shown in Figure 3a; the reflection coefficients are 2.8 GHz, 6.14 GHz, 9.1 GHz, and 12.6 GHz with a magnitude of \(-35.9\),  
The metallic part of the design was tweaked until we obtained good results in the computer simulation technology (CST) studio suite (2019). In this study, different substrates were utilized with different thicknesses, dielectric constants, and loss tangents. The design began with a single SRR on the FR-4 substrate and ended with two SRR bounded double C-shaped metallic copper coatings on the Rogers RT5880 substrate. The outer two SRRs are interconnected and there is a connection with the inner design of the double C-shaped metallic part. The inner metallic parts relate to each other at the top and bottom area, where they appear as a bold C shape. The width and length of the double C shape are w3 and l3, respectively. The width of the outer C is denoted by c1, and the length of the outer C is denoted by c2. The inner C shape is denoted by c3, c4, which are the width and length of the inner C shape, respectively. The free space between the head and tail of the C shape is identified by g4. The whole double C shape is attached to an SRR where l3 is denoted for the length. The SRR has two split gaps at the top and bottom area; the length is 0.02 mm for each gap denoted by g2 and g3. The single SRR-attached design is also bounded with another SRR, and they are interconnected with two metallic parts; the metallic part is denoted by m. The height and width of those connectors are identified as m = 0.2 and g4 = 0.5, respectively. The gap length between the two SRRs is 0.5 mm. The length and width of the most outer SRR are denoted by l and w. Finally, the whole copper coating structure is placed over a Roger RT5880 substrate, which has a length and weight that are denoted by a and b. The transmission coefficient (S21) and reflection coefficient (S11) of the designs are shown in Figure 3 based on the design evaluation. The results of the polarization angle and the incident angle of the incident wave are shown in Figure 4. Figure 4a displays the S21 comparison for distinct incident wave polarization, indicating that the suggested metamaterial design has a similar response in different circumstances. This polarization angle effect is analyzed by varying the polarization angle, \(\Phi\), from 0\({}^{\circ}\) to 90\({}^{\circ}\), with four identical steps. The S21 response was also observed from different angles of the incident wave, which is represented in Figure 4b. The effect of this oblique incidence is analyzed by varying the incident angle, \(\theta\), from 0\({}^{\circ}\) to 90\({}^{\circ}\). The transmission coefficient is unaffected by changes in polarization and incident angle, demonstrating that the suggested design structure has a consistent response for every angle of incidence. As a result, the suggested metamaterial structure is insensitive to changes in polarization and oblique incidence angle.