Radar protection of impedance sphere and other nonplanar surfaces

It is known that the sphere is invisible for the radar if its surface has the unit impedance, similarly as the plane does not reflect the normally falling plane wave. Here the question is investigated which concerns the existence of other impedances which have the same property. Two other values of the complex impedance are obtained, which provide the radar invisibility of the sphere. These values have negative real parts, that is, the surface having such impedance is analogous to an active medium. Numerical results describing the frequency dependences of these impedances are presented. For nonplanar surfaces of complicated shapes the relation between the impedance tensor elements is found which provide the protection, at least, for two orthogonal polarizations of the incident plane wave.