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We devise and experimentally validate a systematic method for designing frequency selective surface (FSS)-based multiresonant ultrabroadband microwave absorbing structures. Crucially, our method offers the capability to engineer the frequency separation between consecutive harmonics of an FSS-based sub-wavelength cavity, thereby engendering an ultrabroadband performance (atleast 90% absorption between 5.7 and 46.2 GHz) in an extremely low-profile design. Furthermore, the materialization and manipulation of a zeroth-order absorption resonance in the structure not only helps us limit the cavity thickness to merely 0.134 , but also exhibits a frequency pinned angular performance. Our formulation starts by realizing that the incorporation of a phase discontinuity in ground plane scattering response can effectuate simultaneous and tuneable emergence of zeroth and first-order resonances in a single …