On the physical limit of radar absorbers

A previous investigation has shown that at normal angle of incidence, the integral of the reflectance over wavelength is bounded for a flat metal backed absorber. The bound is applicable to any absorber made of linear, time-invariant, causal and passive materials. We generalize the physical bound to arbitrary angle of incidence and polarization. Different design examples and numerical calculations are provided to investigate the inequalities. It is shown that the theoretical limit for TE polarization results in fair approximations of the integral of the reflectance over wavelength but the TM polarization overestimates the integral value. A simple relation for estimating the optimal thickness of a nonmagnetic absorber is suggested for arbitrary angle of incidence.