Broadband extremely close-spaced 5G MIMO antenna with mutual coupling reduction using metamaterial-inspired superstrate.

A metamaterial structure, which has positive and negative permeability over a wide microwave frequency band, has a proposed structure that can be employed as a superstrate for reducing the mutual coupling of a MIMO antenna system. This MIMO antenna system consists of two extremely close-spaced antenna elements. The proposed structure's decoupling mechanism is verified by both the full-wave electromagnetic simulations and experiments, and the simulated and measured results agree very well with each other. The two-element MIMO antenna system, when loaded with the metamaterial-inspired superstrate, shows a high isolation (S21<-15 dB) within a broad matching band of 22.3% from 4.2 to 5.25 GHz covering the 5G frequency band of 4.8-5 GHz with an extremely close edge-to-edge space of just 1 mm (corresponding to 0.017λ at 4.9 GHz). The MIMO antenna system's measured largest isolation with the metamaterial-inspired superstrate is 29 dB. This isolation is characterized by a maximum improvement of 23 dB, compared to the original case. Furthermore, after loading the superstrate, the measured gain is enhanced by more than 0.5 dB in the whole matching band as well, with a 3.2 dB maximum gain improvement.