GB1030483A - Structures of low-reflectivity for attenuating electromagnetic waves - Google Patents

Abstract

1,030,483. Radio wave absorbers. SIEMENS & HALSKE A.G. March 10, 1964 [March 12, 1963; March 22, 1963; March 25, 1963; March 26, 1963], No. 9954/64. Heading H4A. [Also in Division H1] . A radio wave attenuating structure consists of a slab 1 of material of low dielectric constant having a plurality of conical or pyramid-shaped recesses 2 the surfaces 3 of which are coated with electrically-resistive material, and a layer 4 of electrically-conducting material which makes contact with all the surfaces 3. In the arrangement shown in Fig. 1, the slab 1 may be of a foamed synthetic material which may have a dielectric filler and the coating of the surfaces 3 may be a carbon-graphite varnish. The layer 4 may be a cover-plate, metal foil such as aluminium or copper or a mesh of copper wire or copper-covered wire, the mesh width being less than one-tenth of the shortest wavelength to be attenuated. In another embodiment (Figs. 4 and 5, not shown), a plurality of recesses of two different diameters and depths are used and the coatings of the smaller recesses have a higher specific conductivity than those of the layer. In another embodiment (Fig. 6, not shown), the slab 1 is made of polystyrol or phenol resin and the recesses 2 are completely filled with polystyrol, polytetrafluorethylene or a thermoplastic synthetic material containing TiO 2 which forms a filler having a higher dielectric constant than the material of the slab 1. In another embodiment (Fig. 8, not shown), each recess 2 is partly filled with fillers consisting of cones or pyramids, each of smaller depth than the recesses and consisting of material of higher dielectric constant than the slab 1 and coated with carbon-graphite varnish. These fillers may each have a cross-sectional area which increases exponentially from the apex of the recesses to the layer 4 and (Fig. 7, not shown). The fillers may each be formed of a cone having a conical recess in which a smaller cone is fitted (Fig. 9, not shown), the smaller cone having a larger dielectric constant than the larger cone and having its outer surface covered with a varnish of higher conductivity than that covering the larger cone. The structures may be incorporated in a honeycomb waveguide arrangement, Figs. 11 and 12 (not shown), in which they are used as an inner backing for cells formed by waveguide matrices. The centre to centre distance between individual waveguides progressively reduces by half from W3 to W1 and about 1/6 to 1/3 of the depth of each of the smallest cells is filled with a structure 1 as described above. A conductive cover 4 is provided at the rear and the slab material may comprise polyurethene or cross-linked polyurethene such as " Moltopren " (Trade Mark). The permeabilities and dielectric constants of all the materials used are given.