Radio-Wave Absorbing Material

any nonmetallic material whose composition and structure permits efficient absorption (with negligible reflection) of electromagnetic radiation over a certain range of radio wavelengths. Radio-wave absorbing materials are used, for example, to prevent radar detection by reducing the effectiveness of the reflecting surfaces of land and marine objects and aircraft, to outfit testing chambers in which antenna systems are studied, and to absorb electromagnetic radiation in terminal and other absorbing elements of superhigh-frequency equipment.

When electromagnetic radiation interacts with radio-wave absorbing materials, the waves experience absorption (dielectric and magnetic losses), scattering (due to the structural inho-mogeneities of the material), and interference. Nonmagnetic absorbing materials are subdivided into interference, gradient, and mixed types.

Interference materials are composed of alternate dielectric and conducting layers. Interference occurs between the waves reflected from the electrically conducting layers and the metallic surface of the object being protected. Gradient materials, which are the commonest type, have a layered structure in which the complex dielectric constant varies either in continuous or stepwise fashion throughout the thickness and is usually governed by a hyperbolic law. The thicknesses of these materials are relatively large and amount to more than 0.12 to 0.15Aλ_max, where λ_max is the maximum operating wavelength. The outer (matching) layer is made of a solid dielectric having a high content of trapped air, such as a foam plastic, and a dielectric constant close to unity; the other (absorbing) layers are made of dielectrics having high dielectric constants, such as fiber-glass laminates, and an absorbing conducting filler, such as carbon black and graphite. Also among the gradient materials, by convention, are those with outer surfaces having such projections as pins, cones, and pyramids, that is, spiniferous

materials. The multiple reflection of waves from the surfaces of the projections, with absorption of energy from the wave at each reflection, assists in reducing the reflection coefficient.

The mixed types of absorbing materials are composites of the gradient and interference types and are distinguished by their efficacy over a wide range of wavelengths. Ferrites form a group of magnetic absorbing materials in which the required thickness of the layer is characteristically small (1 to 10 mm).

A distinction is made between broad-band (λ_max/λ_min > 3 to 5), narrow-band (λ_max/λ_min ~ 1.5 to 2.0), and those materials designed for a fixed (discrete) wavelength (bandwidth < 10 to 15 percent of λ₀), where λ_min and λ₀ are, respectively, the minimum and the operating wavelengths. Ordinarily, absorbing materials reflect between 1 and 5 percent of the electromagnetic radiation; some, however, reflect no more than 0.01 percent. They are capable of absorbing flux densities ranging from 0.15 to 1.5 watts/cm² and, in the case of foam ceramics, up to 8 watts/cm². The operating temperature range of the materials with air cooling is from - 60° up to 650°C, with some materials offering a range up to 1315°C.