Nonreflecting absorbers for microwave radiation

The absorption of very short electromagnetic waves by absorbing systems, which avoid reflection of the incident wave is a problem of practical interest. Three different methods are applicable: 1. Complete absorption of the incident energy can be obtained for one wavelength by using resonance systems of relatively small thickness; e.g., a resistance card having a surface resistivity equal to the wave impedance of free space and placed a quarter of the wavelength in front of a metal sheet; a dielectric layer of lossy material on a metal sheet, with the thickness of the layer equal to about a quarter of the wavelength in the material; a two-dimensional periodic structure of concentric resonant circuits arranged within the metal sheet itself. 2. The reflecting object can be covered by a thick layer of absorbing material, so that in a wide wavelength range most energy of the incident wave will be absorbed before reaching the reflecting surface. To avoid reflection, the absorption material can be tapered or arranged in different layers in such a manner that the loss tangent steadily increases towards the base plate. 3. The bandwidth of resonance absorbers can be widened without an increase of its thickness by combination of two specially dimensioned resonant circuits.