A Compact Circularly Polarized Antenna With Directional Pattern for Wearable Off-Body Communications

This letter presents a geometrically simple and compact circularly polarized antenna with unidirectional radiation characteristics for off-body communications. The proposed antenna is based on a microstrip-line monopole extension from a coplanar waveguide and a protruded stub from one side of the coplanar ground plane along the length of the monopole. The orthogonal components of equal amplitudes required for circular polarization are induced using the vertical stub and the horizontal edge of the coplanar ground plane. To control the radiation of the antenna towards the human body, a flexible, high-permittivity laminated substrate is used as a reflector. The topological simplicity of the antenna permits a small number of adjustable variables and therefore a reduced computational cost of the parameter tuning process. The total footprint of the proposed antenna is only $0.23\lambda _{o}\,\times\, 0.24\lambda _{o}$ = 0.05 $\boldsymbol{\lambda }_{\bf{o}}^2$ at the lowest operating frequency. The antenna is numerically and experimentally characterized in the free space and on a real human volunteer. The simulated and measured reflection coefficient indicates that the operating range of the antenna extends from 5.16 to 6.13 GHz range with 100% bandwidth overlap between the impedance bandwidth and the axial-ratio bandwidth. The peak gain of the antenna is 6.22 dBic with a stable radiation pattern in the + z -direction, high efficiency of 90%, and 0.131 W/g specific absorption rate (SAR). The antenna can be used for Unlicensed National Information Infrastructure (U-NII) worldwide band and Industrial, Scientific, and Medical (ISM) 5.8 GHz band.