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A dual-band (DB) microstrip branch-line coupler with quadrature phase difference based on the interdigital transmission line (TL) is presented. Interdigital TLs are realized by cascading interdigital capacitors (cells of the structure) in series. The design procedure to reach the dual-band interdigital transmission lines at two arbitrary frequencies is presented. Dual-band operation for branch-line coupler is achieved by implementing the interdigital TLs with 3 cells in each branch to provide a phase of -90° at the lower operation frequency band (2GHz) and -270° phase shift at the upper frequency band (3GHz). Branches length is approximately 0.8 λ _g and as compared to the conventional DB composite right/left handed (CRLH) branch-line couplers, the proposed structure achieves smaller dimension. Also, simulation results show that the dual-band proposed coupler exhibits an amplitude balance of ±2 dB from 1.9 GHz …