This two‐part article presents a review of different techniques of mutual coupling (MC) reduction. MC is a major issue when an array of antennas is densely packed. When the separation between the antennas is <λ₀/2, i.e., half of the free‐space wavelength, the antenna parameters suffer from the surface wave, and the space wave coupling effect between the antenna elements. To reduce the MC influence, several techniques have been discussed in the scientific literature. This part of the review paper will discuss strategies: antenna placement and orientation approach, Defected Ground Structures (DGSs), slots/slits‐etching approach, protruding ground stub structures, and parasitic elements/structures. This article concludes with feasibility, applications, and comparison of the techniques mentioned above. Simultaneously, the applications and possibilities of the remaining methods are discussed in part two of the article.

In this paper, a compact 4×4 multiple-input-multiple-output (MIMO) patch antenna has been presented for triple band operation. Out of the 4 antennas, two are tuned to cover DCS (Digital Cellular System) downlink and two are tuned to cover DCS uplink frequency separately along with two common operating frequency centered around 2.45 GHz and 875 MHz. Four complementary split-ring resonators (CSRRs) have been etched in the ground plane exactly below the four patch antennas for attaining compactness from 0.36λ₀² to 0.13λ₀². In addition to this, by optimizing CSRRs outer radius, one lower common band operation of around 875 MHz (0.031λ₀²) has been obtained. Two, square split-ring resonators (SRRs) are placed between patch antennas to increase isolation by almost 7 dB around DCS band. The proposed MIMO antenna has been fabricated on an FR4 substrate (60×60×0.8) mm³ with dielectric constant, ε_r=4.3 having an antenna area of 0.13λ₀². The Proposed MIMO antenna has two common operating bands, 850–900 MHz, 2410–2466 MHz and one individual band from 1725–1770 MHz for one group of two antennas of dimensions 17×22 mm² and another individual band from 1800–1845 MHz for another group of two antennas of dimensions 17×23 mm², where reflection coefficient is less than −10 dB with a minimum isolation of 17 dB. Diversity performance of the proposed MIMO antenna has been verified experimentally on all three bands. However, there is a slight deviation in the DCS operating band due to two different groups of antennas.

This two‐part article presents a review of different techniques of mutual coupling (MC) reduction. MC reduction is a primary concern while designing a compact multiple‐input‐multiple‐output (MIMO) antenna where the separation between the antennas is less than λ₀/2, that is, half of the free‐space wavelength. The negative permittivity and permeability of artificially created materials/structures (Metamaterials) significantly help reduce MC among narrow‐band compact MIMO antenna design elements. In this part two of the review paper, we will discuss techniques: Metamaterials; Split‐Ring‐Resonator; Complementary‐Split‐Ring‐Resonator; Frequency Selective Surface, Metasurface, Electromagnetic Band Gap structure, Decoupling and Matching network, Neutralization line, Cloaking Structures, Shorting vias and pins and few more.