Patent GB-1074898-A (GB)
Title: Improvements in devices for absorbing elector-magnetic waves
IPC: H01B3/30
Filed: 1961-12-19
Abstract: 1,074,898. Laminates. ELTRO G.m.b.H. Dec. 19, 1961 [Feb. 2, 1961], No. 45546/61. Heading B5N. [Also in Division H4] An absorber device for absorbing impinging electromagnetic waves by interference comprises an outer surface on which one part of the impinging waves is reflected, a base layer reflecting the other part of the waves penetrating into the absorber device, and an absorber laminate superimposed on the reflecting base layer, the laminate having a thickness where # 0  = wavelength on free space K<SP>1</SP> = relative dielectric constant of the absorber material Km<SP>1</SP> = relative magnetic permeability of the absorber material and consisting of at least two layers each consisting of a material whose high frequency characteristics K<SP>1</SP>, Km<SP>1</SP> vary in dependence on the wavelength according to the formula where x is a non-dimensional correction factor, the variations being such that they compensate each other over a wide frequency band. Interference results from the reflected wave and the wave emanating from the absorber material being of equal amplitude and in phase opposition, but a residual reflection up to 30% is admissible in a wide frequency band By selective combination of layers, laminates may be constructed which completely absorb waves in a broad band as long as they satisfy the following equation: where Tan #dfy(#) = dielectric loss tangent dependent on wavelength Tan #mfy(#) = magnetic loss tangent dependent on wavelength K<SP>1</SP>mfy(#) = relative magnetic permeability of the material dependent on the wavelength K<SP>1</SP>fy(#) = relative dielectric constant of the material dependent on the wavelength y = a correction factor. A layer which has no high-frequency losses only shifts the phase of the wave, and is termed the " phase-shifting layer ". The term " absorptive layer " is used to designate a layer whose high-frequency losses enable the wave amplitude to be reduced. In Fig. 5 an absorber laminate is bonded to a metallic reflecting base 20 and consists of phase-shifting layers 21a, 21b and absorptive layers 22a, 22b. The high-frequency characteristics satisfy equation V over a wide frequency band. A substantial part of wave 24 is absorbed by interference between partial beams 25 and 26<SP>1</SP>. Also the other reflected waves are largely cancelled. In many instances, the boundary surface reflection is so great and the attenuation within the absorber so strong, that the former beam has a much larger amplitude than the latter. The amplitudes are equalized by providing at least one surface layer which incorporates an impedance matching layer. In Fig. 6 wave- 34 is reflected by the impedance matching layer 38. A larger part 35<SP>1</SP> of the wave is. reflected at interface 33<SP>1</SP>. The penetrating beam 36 is reflected as beams 36 and 36<SP>1</SP>. The energy of beams 35, 35<SP>1</SP> is smaller than the energy of beams reflected in the absence of impedance matching layer 38 and the amplitude of beams. 35 and 35<SP>1</SP> can be made equal to the amplitude of beams. 36 and 36<SP>1</SP>. The amplitude of the beam. reflected at the boundary surface may also be reduced, and thus matched with the amplitude. of the beam emerging from the absorber,. by constructing the. laminate surface so that a portion of the impinging wave is. scattered. In Fig. 7 (not shown) the. surface is in the shape of a waffletype network with rows of pyramids defining rows of recesses therebetween. A similar network is shown in Fig. 8 in which the protuberances are frustocones. The metallic reflecting base. 50 carrier the. phase shifting layer 51, absorber layer 52a and 52b and the impedance matching layer 58 and protective covering layer 59. Beams, such as 54b and 54c are scattered as beams 55b and 55c and. will not be received by radar receivers. The material of the layers consist of an embedding mass filled with fillers having a grain size. and form which control the high. frequency characteristics so that they vary in dependence on the.wavelength. The embedding mass may consist of natural or synthetic rubbers, plastics, concrete, mortar, bitumen, tar, magnetic material consisting, of ferrite or iron powders having a grain size of 0À1 to 100 microns, or magnetic powders consisting of mixed crystals of ferrite with magnetites. The fillers partially consist of a material controlling only the relative dielectric constants. These fillers may be non-magnetic metal powders, semi-conductor materials, strontium bismuth-titanate, graphite, or mixtures of at least two of these. Dipoles may be embedded on the material to aid the variation of high-frequency characteristics with wavelength, and the length of the dipoles is given by where y = wavelength on the material. At least some of the dipoles may be made of metal filaments having a high electric resistance, and the filaments may be monocrystals; plastics coated with electrically conductive metal that remains solid at service temperatures, such as nickel, iron, aluminium, copper, plastics coated with carbon black or graphite; or an electrically conductive plastics compound. The absorber laminate comprises one or more sheets of any suitable commercial material, such as pulp sheet wood or cardboard, having a loss tangent tan #d + tan #m of below 0À1, and at least one layer consisting of a compound of an embedding mass filled with fillers, which control the highfrequency characteristics, of rubber or bitumen. The sheet is coated on both upper and lower surfaces with a layer or layers consisting of an embedding mass and fillers, each layer having a relative dielectric constant of between 20 and 600 and a relative permeability of between 0À9 and 10 so that the layer coating applied to the lower surface is effective as reflecting base layer. The absorber device may consist of a camouflage net (Fig. 20) consisting of a thin reflecting base layer 180 and other bonded flexible layers. The net has apertures occupying up to 40% of the whole surface area and the average width of the apertures is <SP>1</SP>/ 5 -<SP>1</SP>/ 10  of the wavelengths. The apertures may be circular, triangular or quadrangular, and the average widths differ from each other in accordance with the differing wavelengths to be absorbed. The apertures are in groups so that larger full regions of the material remain among the groups. Fig. 24 shows a camouflaging net applied to an object 229. The net is composed of a reflecting base 220, absorptive and phase-shifting layers 221 and 222, and apertures 228. Beams 224a and 224b are interfered by beams 225a, 226a and 226b. The beam 224c which passes through the apertures is reflected back and forth between the base 220 and the object 229, the oscillating beams interfering with each other to cause extinction. Similarly the beam 224b also becomes extinct. Fig. 16 shows an absorber laminate for camouflaging airfield landing strips. Layer 140 is a concrete body and layer 141 is a wire mesh with a mesh size and a wire diameter of 2-3 mm. Layers 142 and 143 are the phase shifting and absorbing layers. The reflecting base layer consists of an embedding mass which may be a lacquer filled with a metallic powder, the base layer being applied to the absorber laminate by painting, spraying, puttying, or by means of an adhesive.