Characteristics of radar absorbers with tapered thickness

The design and characterisation of radar absorbing material (RAM) is usually carried out under the assumption that the latter is in the form of a sheet of constant electrical or physical thickness. There are occasions, however, when this is not so. Although there is some discussion in the literature on scattering from impedance wedges, this is mainly concerned with edge diffraction effects. In this paper, however, the wedge lateral dimensions are assumed to be large compared to /spl lambda/, so that specular phenomena predominate. For simplicity, the paper deals only with Salisbury screen and Jaumann type RAM, where either or both front and back faces of the panel are inclined with respect to the incident radar signal. Because electrically large panels are being considered, first order predictions of the RAM scattering characteristics can be obtained using quasi-optical techniques and these may be used to check the more detailed results found using 2D FDTD analysis. Results are presented in the form of bistatic RCS plots. The 2D FDTD computer code used to obtain the RCS plots is based on the work of Yee and Taflove (1995). In all the cases considered, a scatterer discretisation of /spl lambda//20 was used, together with second order Mur (1981) boundary conditions. Because inclined surfaces were being modelled, "staircasing" effects might be important, but in the 2D case these are only significant for TE polarised illumination. Far-field calculations of the scattered fields were performed using a technique based on the surface equivalence theorem.