Anisotropic Metasurface Inspired Circularly-polarized Monopole Antenna for OFF Body Communications

Abstract

This paper presents a novel approach for achieving circular-polarization (CP) in the 2.4 GHz ISM-band for OFF-body communication. The behaviour is modelled by the incident of $x$-and $y$-polarized waves generated from a 45°-tilted monopole structure to an anisotropic metasurface. The reflected waves from the metasurface are then combined with the direct wave in the far-field, resulting in an equalization of the orthogonal component's amplitude with a 90° phase difference. The monopole structure is designed first at 2.45 GHz, and then the metasurface is designed according to the axial ratio (AR) characteristics. The metasurface is made up of $3\times 3$ ring-type unit cells. The proposed metasurface not only generates CP but also aids in the reduction of backward radiation and provides miniaturization compared to the conventional rectangular unit-cell. The design is accomplished using CST Microwave Studio. The proposed metasurface-based antenna produced an impedance bandwidth of 330 MHz, an AR bandwidth of 74 MHz, a peak gain of 6.6 dBi, and a radiation efficiency of 69 % in free space (FS). On the body, its performance remains the same as in free space and provides a SAR value of 0.09 and 0.04 W/Kg under the 1 g and 10 g standards, respectively.