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In this paper, a new compact broadband uniplanar 180° phase switch, based on an air-bridged coplanar-waveguide (CPW) cross loaded with two capacitive-contact microelectromechancial systems (MEMS) switches in opposed (on/off) states, is presented. The two phase-switch states (0°/180°) are defined by actuating the MEMS switches from on/off to off/on. The asymmetry in the states of the MEMS switches results in a complex multimodal interaction between the two fundamental even and odd CPW modes at the air-bridged cross. Using the multimodal theory, the phase switch is analyzed, its frequency-in dependent 180°-phase-shift properties are proven, and a set of design equations for perfect port matching are derived. A multi modal circuit model for the phase switch is then presented, and design equations and conditions for compact phase switches are derived. Finally, a very compact phase switch is designed and fabricated using an eight-mask surface micromachining process, featuring a measured phase shift of 180° ± 1.8° in a very wide frequency range (1-30 GHz) and an insertion loss better than 2.1 dB in the design band (10-20 GHz). Experimental results are in very good agreement with electromagnetic and multimodal circuit simulations, thus validating the proposed approach and design procedure.