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A novel broadband and ultra-low-loss electric metamaterial EM isolation slab is proposed to improve the performance of a microstrip array. The metamaterial slab is formed by periodically grounded edge-coupled split-ring resonators (PGE-SRRs). Outstanding improvements - including over -50 dB peak isolation, 15% fractional bandwidth (-10 dB isolation) and almost lossless operation - are obtained. The metamaterial slab is inserted halfway between the adjacent E-coupled elements in the microstrip array to suppress mutual coupling. A strong mutual-coupling suppression of -16.8 dB was exhibited experimentally in a two-element microstrip array with an element spacing of three-quarters of the operating wavelength. Theoretical and numerical studies were done to improve the performance of microstrip phased arrays using the proposed metamaterial slab. The analysis indicated that the scan blindness in an infinite phased array is well eliminated, the wide-angle impedance matching is remarkably improved, and the scanning range is extended from [-13°, 13°] to [-28°, 28°]. A 7×3 microstrip array was simulated to study the influence of the metamaterial slab on the array's performance. The results indicated that the metamaterial slab can also enhance the radiation characteristics, extend the scanning range and suppressing grating lobes in microstrip phased arrays.