A Compact Dual-Port Circularly Polarized MIMO Antenna for On-/Off-Body WBAN Applications

In this letter, a novel compact dual port circularly polarized multiple-input–multiple-output (MIMO) antenna for on-/off-body wireless body area network (WBAN) applications is proposed, radiating unidirectional and omnidirectional radiation patterns by two ports, respectively. By adding a circular arrangement of multiple shorting pins, the frequency points of the TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub>/TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> hybrid mode and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode of the circular patch are adjusted to the same frequency band, which are used as the resonant mode of two ports, respectively. In order to achieve circular polarization at the unidirectional port, a rectangular slot is etched in the center of the patch by bypassing the omnidirectional radiation port, and two rectangular slots are etched at the edge of the patch. The omnidirectional bandwidth is 70 MHz [(3.45 to 3.52) GHz], and the unidirectional bandwidth is 190 MHz [(3.41 to 3.6) GHz]. By integrating a decoupling method based on characteristic mode analysis with circularized polarization, not only high port isolation was achieved but also communication degrees of freedom for the unidirectional mode were enhanced, making it suitable for a wider range of applications.