An Overview of Metamaterial Absorbers and Their Applications on Antennas

Metamaterial absorbers, composed of artificial unit cells, have been widely investigated in the past few decades due to their unique performance of low profile, low cost, lightweight, and customized absorption responses, etc. The principles of the metamaterial absorber to absorb incoming electromagnetic waves lie in the impedance matching between the metamaterial absorber and free space (120$\pi$). This paper reviews the developments of the metamaterial absorbers, from narrow to wide band, single- to dual-polarization, polarization-sensitive to polarization-insensitive, small- to wide-angle absorption. The topologies and solutions to achieve wideband, dual-polarized, polarization-insensitive, wide-angle absorption metamaterial absorbers are clarified and discussed from the antenna reciprocity and equivalent circuit viewpoints. One of the applications of metamaterial absorbers is to reduce the scattering effects of a metal object by coating the metamaterial absorbers on the metal object. With the appearance of the band-notched metamaterial absorber and hybrid architecture, metamaterial absorbers can be readily integrated and co-designed with antennas to improve antennas’ performance. This paper also presents the applications of the metamaterial absorbers on antennas from topological analysis to practical implementations. Two topologies to achieve low scattering antennas are described in detail. One is utilizing the notch band of a band-notched metamaterial absorber as the metal ground for an antenna (planar patch antenna, dipole antenna, or monopole antenna); within the notch band, the antenna can radiate efficiently, while the band-notched metamaterial absorbers can absorb incoming electromagnetic waves to reduce the scattering effects significantly out of the notch band. The other is using hybrid architecture to design reflectarray antennas with gain filtering and low scattering effects by proposing a unit cell with simultaneous phase shift and electromagnetic wave absorption properties. The manipulations of the phase shift and electromagnetic wave absorption are independent of each other to facilitate the design of the reflectarray antennas. The future perspectives on metamaterial absorbers are discussed finally.