Polarization Converting Metasurface Inspired Circularly-polarized Antenna for OFF Body Communications

This paper presents a novel polarization converting metasurface (PCMS)-inspired circularly polarized monopole antenna (MA) for 2.4 GHz off-body communication. The PCMS reflects a 90°-phase-shifted y-polarised wave upon the incident of an x-polarised wave from the monopole structure, and that combined with the direct wave generates circular polarisation (CP). The PCMS is designed by arranging 2×2 arrays of polarisation-converting unit cells (PCUC) on a metal-backed felt substrate. Initially, a conventional rectangular unit-cell (CRUC) is designed theoretically, and then, using CST Software, a square cut is engraved into the upper right corner of the CRUC to create the PCUC. Such a unit cell is chosen to attain −90° of reflected phase at 2.45 GHz for achieving right-hand circular-polarization (RHCP). Using the orientation of surface-current distribution and radiation patterns, the resulting state of polarisation is determined. In the operating band, an axial ratio (AR) bandwidth of 110 MHz is attained. The proposed metasurface improves the impedance bandwidth to 660 MHz, the peak gain to 6.5 dB, and the radiation efficiency to 68% in free-space (FS). In addition, the on-body (OB) performance is identical to that of free space due to the presence of metasurface, with a very low SAR value.