Broadband Circularly Polarized Helical Antenna Decoupling for Massive MIMO Applications

In this letter, a circularly polarized (CP) massive multiple-input–multiple-output (MIMO) antenna with wide impedance/axial ratio (AR) bandwidths and reduced size is proposed. The antenna element is based on a monofilar helical antenna (MHA) where an impedance transformer is adopted to improve the impedance and AR bandwidths, simultaneously. The mutual coupling of closely-packed MHAs is studied, and a decoupling method is proposed by using a self-rotation method. A four-element helical MIMO antenna with enhanced impedance/AR bandwidths and radiation characteristics is firstly designed by employing the self-rotation and the sequential rotation techniques. Based on it, an 8 × 8 CP massive helical MIMO antenna is studied. The four-element and the 64-element helical MIMO antennas are prototyped and verified. The measured results demonstrate an overlapped bandwidth from 5.2 GHz to 7.1 GHz, providing a fractional bandwidth (FBW) of 30% for the four-element helical MIMO antenna, and a bandwidth from 5.7 GHz to 6.6 GHz (FBW = 15%) for the 64-element massive helical MIMO antenna, respectively.