Mutual Coupling Reduction in CP MIMO Crossed-Dipole Antenna Array

In this letter, a circularly polarized (CP) 1×2 multiple-input–multiple-output (MIMO) crossed-dipole antenna array is investigated. The antenna element consists of a pair of orthogonal linear dipoles, which are connected by two quarter phase-delay lines, generating CP fields. It is found that within the operating CP band, both the excited and coupled crossed-dipole antennas are of circular operation and the mutual coupling is rather strong, but at higher frequency there exists a self-decoupled band (transmission stopband with low |S21|) in which the excited antenna is of elliptical polarization while the coupled antenna is of linear polarization. In order to reduce the mutual coupling level in the operating band and make it applicable, a single metallic post is added in the middle of the MIMO array, shifting the self-decoupled band downwards to the desired CP band. As a result, the port isolation is enhanced by 13 dB and becomes higher than 20 dB within the entire CP bandwidth of 13.2%.