Singularities of Floquet scattering solution for large cell spacings

The Floquet moment method (FMM) is useful for computing the scattering by periodic structures composed of complex shaped cells. However, when the period s is larger than 0.5 λ, the computed field is singular at the grating lobe angles. In this paper, we demonstrate that these singularities have nothing to do with grating lobes, which are a physical phenomenon. Rather, they are a mathematical artifice of the FMM. This is demonstrated by modeling a flat ground plane with the FMM. We use a strip grating for the transverse magnetic case. The physical structure under investigation does not have any grating lobes, however the singular behavior can be made to appear and disappear by adjusting the cell spacing of the periodic structure.