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We investigate an innovative decoupling network for dual-band two-element closely spaced arrays in MIMO applications. The new architecture, combining the reactive element decoupling technique and the eigenmode feed network, provides dual-band decoupling with good radiation characteristics and power balance between the modes. The core block of the decoupling network is a dual-mode 180-degree hybrid coupler; it functions as a conventional coupler in one frequency band but can be equivalent to a pair of isolated direct-thru transmission lines in the other band. This unusual dual-mode coupler is realized by synthesized microstrip lines (cells), consisting of line inductors, parallel-plate capacitors, and series LC tanks. A set of design equations, providing a systematic synthesis procedure of the unit cells, is discussed. The scheme of the dual-band decoupling network is illustrated first, followed by the details of the building circuit blocks; the experimental results and discussion are given at the end of the paper.