Theoretical investigations of scattering from plastic foams

The use of cellular or foamed plastics in various microwave applications, such as supports at radar ranges, makes it desirable to know the back scattering properties of such materials. Since the cell structure is of a random nature with some predictable average properties such as cell size and density, it is modeled by an aggregate of randomly distributed spherical shells. Assemblies of scatterers will in general have a coherent and an incoherent scatter. Coherent scattering comes primarily from sudden particle density changes such as that at the boundaries of a particle system. Since coherent scattering comes only from the boundaries of a constant density material, it can sometimes be reduced by appropriate shaping. Incoherent scattering is the result of the contribution of all the particles in the system, i.e, a volume or an interior effect. It represents the irreducible scattering contribution to the total back scatter. As such it can be looked upon as the minimum cross section that can be obtained from a foam structure provided all coherent scatter has been removed. The magnitude of the incoherent scattering is illustrated by calculating radar cross sections for a cylinder made of styrofoam. Since the compressive strength of styrofoam is known, the maximum load that a styrofoam structure can support and the minimum achievable cross section from it can be easily calculated.