A metamaterial photonic bandgap (MTM-PBG) periodic structure is used as a decoupling frame to improve the isolation between transmit–receive (T/R) sections of densely packed array antenna in synthetic aperture radar (SAR) and multiple-input multiple-output (MIMO) systems. With this technique the MTM-PBG structure is shown to effectively suppress surface wave propagations between the T/R array antennas by an average of 12dB. MTM-PBG layer comprises a periodic arrangement of dielectric circles etched in the cross-shaped microstrip frame that is inserted between the radiating elements. Unlike other recently reported methods, the advantages of the proposed technique are:(i) simplicity; (ii) cost effectiveness as there is no need for short-circuited via-holes or 3D metal walls; and (iii) can be retrofitted in existing array antennas. The proposed T/R array antennas were designed to operate over an arbitrary frequency range (9.25-11GHz) with a fractional bandwidth (FBW) of 17.28%. With this technique (i) the side-lobes are reduced; (ii) there is minimal effect on the gain performance; and (iii) the minimum edge-to-edge gap between adjacent radiating elements can be reduced to 0.15at 9.25GHz.