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A new compact quadrature hybrid based upon artificial transmission lines (ATLs) is presented. Miniaturization is achieved through reduced even- and odd-mode phase velocities which are achieved using capacitive loading with interdigital open-circuited stubs. A proof-of-concept microstrip implementation of the quadrature hybrid exhibits performance comparable to a conventional branch-line coupler while occupying only 14% of the area. The compact structure of the hybrid is useful for RF applications and is suitable for scaling to frequencies above 100GHz where the size of shielded-microstrip components must be limited to prevent excitation of high-order modes.