Harmonic Engineering in Sub-Wavelength Cavities & Design of Ultrabroadband Microwave Absorbers by Reflection Phase Manipulation

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
$\lambda _{L}$
, 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 sub-wavelength cavity. With this conception, we first design a thin microwave cavity exhibiting a phase discontinuity roughly at the midway of the operating band. This cavity is then employed as an effective ground plane, which leads to the emergence of zeroth and first-order modes at widely separated frequencies of 8 and 37 GHz, effectuating an ultrabroadband absorption. Experimental verification was performed by fabricating a 60 × 60 unit cell sample using laser micromachining (on indium tin oxide/polyethylene terephthalate resistive sheets) and microwave measurements were performed that agree well with simulations.