Enhanced-Performance Circularly Polarized MIMO Antenna With Polarization/Pattern Diversity

Design of a compact wideband circularly polarized (CP) multiple-input multiple-output (MIMO) antenna with polarization diversity is proposed and characterized for off-body communication. The antenna is based on a simple coplanar waveguide (CPW)-fed monopole extension of the microstrip line. The orthogonal field components required by CP are induced using a simply modified right/left side ground plane. In particular, a stub extending from the ground plane along the length of the microstrip line generates the vertical component, whereas the current along the width of the ground plane contributes to the horizontal components. To obtain a unidirectional radiation pattern in the off-body direction and to reduce the sensitivity to the human body loading effects, a flat reflector printed on a high permittivity flexible substrate is applied. The simple topology of the antenna can be described by a few adjustable parameters, which facilitates its EM design closure. Prior to the experimental validation in the free space and on the body, the antenna is optimized at the full-wave level of description for all major performance figures. The overall footprint of the antenna radiator is only <inline-formula> <tex-math notation="LaTeX">$L_{s}\,\,\times $ </tex-math></inline-formula> <inline-formula> <tex-math notation="LaTeX">$W_{s} =0.24\lambda _{0} \times 0.64\,\,\lambda _{0} = 0.15\,\,\lambda _{0}^{2}$ </tex-math></inline-formula>. The proposed MIMO antenna features <inline-formula> <tex-math notation="LaTeX">$\vert S_{11}\vert \le -10$ </tex-math></inline-formula> dB, average isolation <inline-formula> <tex-math notation="LaTeX">$\vert S_{21}\vert \le -22$ </tex-math></inline-formula> dB, and axial ratio (AR) ≤3 dB from 5.2 GHz to 6.3 GHz with 100% bandwidth overlap between the impedance and axial ratio bandwidths. The envelope correlation coefficient (ECC) is less than 0.004 with the maximum diversity gain (DG) of approximately 9.99 dB. Moreover, the antenna maintains a high efficiency of up to 90% when loaded on the body, and a low specific absorption rate (SAR).