Coupled ring split ring resonator (CR-SRR) based epsilon negative metamaterial for multiband wireless communications with high effective medium ratio

In this paper, coupled ring split-ring resonator (CR-SRR) based epsilon negative (ENG) metamaterial with high effective medium ratio (EMR) is designed and investigated for microwave applications. The metamaterial unit cell is a modification of the conventional split-ring resonator, which consists of three major outer split rings with two face to face horizontal E segments placed at the center. All the rings are intercoupled with each other to increase the electrical length. The coupling effect is clearly observable as it provides multi-frequency resonances and a higher value of the effective medium ratio. The electrical length of the proposed unit cell is 0.064λ × 0.064λ, where λ is the wavelength calculated at the lowest resonance frequency (2.24 GHz). The designed unit cell is fabricated on a FR-4 substrate, having a thickness of 1.5 mm. Negative permittivity is observed at frequencies ranging from 2.15 to 2.3 GHz, 4.4–4.95 GHz, 5.7–6.1 GHz, 8.46–9.2 GHz, and 10.6–10.98 GHz. It also exhibits near-zero refractive indexes in the vicinity of these frequency ranges. The equivalent circuit is also designed for the proposed unit cell. The values of the lumped components are optimized by using Advanced Design Software (ADS). This circuit also demonstrates similar multi-band resonances. A high effective medium ratio (EMR) of 16.74 at 2.24 GHz indicates the compactness of the proposed unit cell. Array performance is also investigated by using 1 × 2, 2 × 2, and 16 × 16 array of unit cells. Multiband resonances of the transmission coefficient, |S21| are also noticed from these arrays. For high EMR and small size, the proposed metamaterial is suitable for S, C, and X-bands applications in wireless communications.