Complex frequency poles of radar scattering from coated conducting spheres

The radar scattering amplitude, as a function of frequency, possesses poles whose importance was recognized in the framework of the singularity expansion method (SEM); they are located at the scatterer´s complex eigenfrequencies and are characteristic for its shape and composition. For perfectly conducting targets, these poles lead to little pronounced, broad frequency resonances in the radar echoes while for dielectrically coated objects, such resonances can be very prominent. We here present pole patterns in the complex-frequency plane for dielectrically-coated spheres, and obtain the movement of the poles under changes of the thickness and dielectric constant of the coating. The results explain the appearance and disappearance of resonance peaks in previously calculated radar cross sections.