Compact quad-band polarization independent metamaterial absorber using circular/square metallic ring resonator

In the paper, a compact quad-band polarization-independent metamaterial absorber has been presented for different microwave frequency bands. The proposed absorber unit cell comprises of two square and two concentric circular rings and it exhibits the absorption peaks greater than 90% at 3.1 GHz, 5.5 GHz, 7.6 GHz and 10.98 GHz respectively under the normal incidence. The full width at half maximum (FWHM) bandwidth is 150 MHz (3.03-3.18 GHz), 180 MHz (5.38-5.56 GHz), 330 MHz (7.44-7.77 GHz), and 560 MHz (10.70-11.26 GHz) at the four frequency bands. The proposed absorber is polarization and incidence angle insensitive due its symmetrical. The proposed structure also shows more than 90% absorptivity up to 60 degrees incidence angle. The absorber unit cell is compact in configuration with size of 0.12 lambda(o) x 0.12 lambda(o) and ultrathin thickness of 0.016 lambda(o), where lambda o is the wavelength at lowest frequency absorption peak. To recognize the absorption phenomenon of the absorber, input impedance, E-field and surface current distribution characteristics have been demonstrated. The structure has been fabricated and it shows good agreement between simulated and measured results.