Minimum Reflection Properties of Planar Impedance-Matched Boundaries

Reflection of plane electromagnetic waves from an impedance boundary with the surface impedance equal to the wave impedance of the surrounding medium is addressed. It is shown that compared with other impedance boundaries, the impedance-matched boundary guarantees the lowest mean reflectance when the incidence direction and polarization of the incident wave are random and uniformly distributed. In addition to the classical realization as a thick layer of a lossy magnetic material, the impedance-matched boundary can be realized as a thin layer of metamaterial absorber.