Hybrid Isolator for Mutual-Coupling Reduction in Millimeter-Wave MIMO Antenna Systems

A novel millimeter-wave (MMW) hybrid isolator is presented to reduce the mutual-coupling (MC) between two closely-spaced dielectric resonators (DR) antennas at 60 GHz. The proposed hybrid isolator consists of a combination of a new uni-planar compact electromagnetic band-gap (EBG) structure and an MMW choke absorber. The design of the proposed EBG unit-cell is based on the stepped-impedance resonator (SIR) technique. The results show that the proposed EBG structure provides a wide frequency bandgap in the 60 GHz band with miniaturization factors of 0.79 and 0.66 compared to conventional uni-planar EBG and uni-planar compact (UC-EBG) structures, respectively. The proposed EBG is then placed between two Multiple-Input Multiple-Output (MIMO) DR antennas to reduce the MC level. As a result, an average of 7 dB level reduction is obtained. To further reduce the MC level, a thin MMW choke absorber wall is mounted vertically between the two DR antennas and above the EBG structure. An average of 22 dB MC reduction is achieved over the suggested bandwidth while maintaining good radiation characteristics. The measured isolation of the prototype antenna varies from -29 to -49 dB in the frequency range from 59.3 to 64.8 GHz. In fact, the proposed hybrid isolator outperforms other hybrid isolation techniques reported in the literature.