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This paper reports the design and the realization of a variable-sized MEMS-based Faraday cage, used to tune the resonant frequency of a silicon micromachined cavity resonator. The Faraday cage is defined by a 2D-array of electrostatically controllable metal cantilevers. The cantilever array effectively shields an enclosed (air) volume (forming the Faraday cage) located inside the micromachined cavity resonator. The Faraday cage perturbs the electromagnetic field distribution inside the cavity resonator and provides a means to electrically tune the resonant frequency of the resonator. In this realization, the cavity resonator was micromachined using bulk KOH etching in silicon and assembled in a low temperature die-to-wafer soldering process. The electrical feeding was realized in thin-film metallizations and the resonator is formed as a two-chip assembly. We measured a maximal frequency tuning of 1.2 GHz at 60 GHz with a tuning voltage of 16 VDC.