  We report a theoretical analysis of total absorption conditions in a
structure consisting of a lossy coating layer on top of a specular metal
substrate, which is known as a Dallenbach absorber. All possible combinations
of complex refractive indices of the coating material and thicknesses of the
coating layer providing complete absorption are described for the case of
normal incidence of a plane wave. The work of coatings based on
epsilon-near-zero (ENZ) materials is analyzed in detail. Simple analytical
relations are obtained for the conditions of total absorption. It is shown that
the range of angles where strong absorption occurs is limited by the phenomenon
of total external reflection. The characteristics of coatings based on
indium-tin oxide, which is an example of an ENZ material, are analyzed.
