The paper presents a technique to enhance the isolation between adjacent radiating elements that is common in densely packed antenna arrays. Such antennas provide frequency beam‐scanning capability needed in multiple‐input multiple‐output (MIMO) systems and synthetic aperture radars. The method proposed here uses a metamaterial decoupling slab (MTM‐DS), which is located between radiating elements, to suppress mutual coupling between the elements that would otherwise degrade the antenna efficiency and performance in both the transmit and receive mode. The proposed MTM‐DS consists of mirror imaged E‐shaped slits engraved on a microstrip patch with inductive stub. Measured results confirm over 9–11 GHz with no MTM‐DS the average isolation (S12) is −27 dB; however, with MTM‐DS the average isolation improves to −38 dB. With this technique the separation between the radiating element can be reduced to 0.66λ0, where λ0 is free space wavelength at 10 GHz. In addition, with this technique there is 15% improvement in operating bandwidth. At frequencies of high impedance match of 9.95 and 10.63 GHz the gain is 4.52 and 5.40 dBi, respectively. Furthermore, the technique eliminates poor front‐to‐back ratio encountered in other decoupling methods. MTM‐DS is also relatively simple to implement. Assuming adequate space is available between adjacent radiators the MTM‐DS can be fixed retrospectively on existing antenna arrays, which makes the proposed method versatile. Key Points A technique is presented to enhance isolation between adjacent radiating elements that are found in densely packed antenna arrays The proposed method uses a metamaterial decoupling slab located between radiating elements to suppress mutual coupling between the elements Over the antenna's operating frequency (9‐11 GHz) without MTM‐DS maximum isolation is ‐37 dB so with MTM‐DS it improves to ‐57 dB (35% growth)