Radio-Wave Transparent Material

a type of in-homogeneous dielectric structural material, made up of one or several layers, which does not significantly alter the amplitude and phase of radio-frequency electromagnetic waves transmitted through it. This type of material is used mainly in the manufacture of antenna housings, or radomes, which protect the antennas of radar sets from the effects of the surrounding medium. The transparency of the material to radio waves is assured by choosing dielectrics with small volumes of the dielectric loss tangent (tan δ ≤ 0.02), by selecting the dielectric constant of the separate layers (∊ = 1.1–9.0), and by suitable electrodynamic design of the layer thicknesses.

Single-layer transparent materials are by convention classified as thin-walled materials, with a thickness equal to 0.02–0.05 of the working wavelength λ0 in the dielectric, half-wave materials, with a thickness equal to λ0/2 or a multiple thereof, and compensating materials, with an intermediate thickness. Single-layer compensating materials also contain a metal structure in the form of a lattice, which exhibits an inductive or capacitive reactance to the passing electromagnetic wave. Single-layer materials provide good transparency only in a relatively narrow frequency band; its width is 3–4 percent of the mean working frequency. The application of thin-walled and compensating materials is limited in many cases by their inadequate rigidity and durability.

Multilayer (two, three, five, and seven layers) transparent materials are constructed so as to maintain a certain law of variation in the dielectric constants of alternate layers. These materials, which may also contain metal structures, are characterized by a broad range of working frequencies.

Monolithic and porous substances are used to obtain radio-wave transparent materials. The monolithic substances, namely, ceramics, glass, and plastics in the form of fiberglass laminates, are used in single-layer materials. They also find use as strengthening and matching layers in multilayer materials. Their density is 1,300–2,800 kg/m³ or more, ∊ = 3–9, and tan S ≤ 0.02. Their long-term operating temperature is 200°–350°C; the short-term range is 400°–1400°C. Porous substances, which include cellular and foam plastics, are used in multilayer materials to increase the rigidity and to provide matching layers and layers with a low dielectric constant. Their density is 20–400 kg/m³, ∊ = 1.1–2.5, and tan δ ≤ 0.01; the long-term operating temperature is 150°–350°C.