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In this letter, a new technique for designing tunable frequency selective surfaces (FSSs) with the second-order band-pass response is presented and experimentally verified. The technique is applied to an FSS utilizing nonresonant, subwavelength, constituting unit cells. Frequency tuning is achieved by adjusting the location of a liquid metal slug within capillary tubes embedded between the FSS's metallic layers. A wideband flexible frequency tuning covering nearly the entire X band is tested in a waveguide environment and excellent agreement between the measured and simulated results is demonstrated.