Singular Phase Characteristics of Electromagnetic Absorbers and a Framework for Low-RCS Target Detection

We conceptualize, numerically validate, and experimentally demonstrate a scheme for distant line-of-sight detection of low-observable targets, such as those covered by metamaterial absorbers, by solely relying on their nontrivial phase characteristics. Relying upon the temporal coupled-mode theory (TCMT), first, we put forward a generalized description of open resonators where we identify the presence of a spectral inflection point in the phase response function as the signature of interest. Thereafter, a realistic phase response function has been modeled and analyzed by incorporating the propagation aspects as well as band-limited Gaussian noise. Change of curvature information owing to the resonant absorption is then ascertained from this involute phase response by developing an algorithm. For pragmatic scrutiny of our proposition, we design a singly resonant (at 12 GHz) wideband resistive sheet-based metamaterial absorber providing
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10 dB absorption across the X- and Ku-bands and employ its de-embedded scattering parameters to confirm the existence of inflection point around 12 GHz. As a final validation, the metamaterial absorber has been fabricated using ITO/PET sheets, and its phase characteristics have been measured in a pseudomonostatic configuration. The information regarding the presence of resonant absorption has been then ascertained by the algorithm extracted phase, thereby validating our proposition.