Scatter reducing step: theory and measurements

A physical (or dielectric filled) step of the proper height introduced at the junction of two materials with different surface impedances will reduce electromagnetic backscattering from the impedance discontinuity that occurs at the material junction. The optimum backscatter-reducing step height at normal incidence is that which minimizes the difference between the imaginary parts of the first-order surface impedances of the two materials. At oblique angles of incidence, the optimum step height is smaller and must be determined using less approximate techniques. Analysis of the impact of steps of various heights on backscatter at all angles of incidence indicates that large steps do not increase backscatter, even at oblique angles of incidence Raising a 6.5 Ω-cm silicon sample 475 μm (0.019 inch) above the surface of an aluminum fixture was shown both analytically and experimentally to have a negligible impact on the backscatter at an elevation angle of 18°. This result has important implications for specifying the flushness of joints between dissimilar materials on low scatter targets