Tri-band MIMO antenna design based on characteristic modes manipulation

Several MIMO antennas based on the theory of characteristic modes have been proposed, but few of them introduce systematic design methods for microstrip MIMO antenna to obtain three frequency bands and as well as to reduce mutual coupling between the MIMO units. In this paper, a tri-band MIMO antenna is designed under the guidance of characteristic mode analysis. Firstly, the characteristic modes of the basic rectangular microstrip antenna are adjusted by loading rectangular spiral shaped stub and L-shaped slot at its proper positions to form a tri-band antenna unit. Next, based on the analysis of Modal significance and modal current distributions, the mode causing mutual coupling of the initial two-port MIMO antenna is determined, and then the meandering neutralization line connecting two adjacent edges and the rectangular gap in the middle of the common ground plane are designed as decoupling structures, which improve the isolation to more than 18 dB between two units with a distance of only 7.4 mm. Finally, the measured results prove the correctness and feasibility of the proposed systematic design process for the MIMO antenna, in terms of its compact size 32 mm x26 mm, envelope correlation coefficient (ECC) less than 0.35, peak gain greater than 1.26 dBi and radiation efficiency greater than 76% in the three desired frequency bands (2.35-2.5 GHz, 3.25-3.85 GHz, and 4.4-6.2 GHz).