Bandwidth-Enhanced Circularly Polarized All-Textile Wearable Antenna Using an LP-to-CP Metasurface

In this paper, a bandwidth-enhanced circularly polarized (CP) all-textile antenna integrated with a metasurface (MS) is presented for 5.85-GHz wireless body area network (WBAN) applications. The broadband axial ratio bandwidth (ARBW) can be implemented using a 4\times 4 MS layer on top of the fundamental radiator that is configured with a semi-circular slot antenna and fed by a 50- \Omega microstrip line. Initially, the semi-circular slot antenna yields a broadband impedance bandwidth (IBW) but a narrow ARBW. The metasurface excites additional surface wave resonances, which significantly enhances the IBW. The MS also works as a polarization converter, which converts the incident wave into orthogonal direction with an equivalent magnitude and a 90° phase different producing broadband CP radiation. The proposed antenna consists of a felt substrate and an adhesive conductive textile, which is comfortable for use in contact with the human body. The fabricated antenna is verified by measurement in a free space and a human body environment. The measured results of IBW and ARBW are 83.64% and 49.6%, respectively. The peak measured gain and radiation efficiency are achieved of 9 dBic and 70%, respectively. The simulated specific absorption rates under US and EU standards are below the safe level.