  We present a semianalytical scheme for the design of broad-angle multichannel
metagratings (MG), sparse periodic arrangements of loaded conducting strips
(meta-atoms), embedded in a multilayer printed circuit board configuration. By
judicious choice of periodicity and angles of incidence, scattering off such a
MG can be described via a multi-port network, where the input and output ports
correspond to different illumination and reflection directions associated with
the same set of propagating Floquet-Bloch modes. Since each of these possible
scattering scenarios can be modelled analytically, constraints can be
conveniently applied on the modal reflection coefficients (scattering matrix
entries) to yield a diffusive response, which, when resolved, produce the
required MG geometry. We show that by demanding a symmetric MG configuration,
the number of independent S parameters can be dramatically reduced, enabling
satisfaction of multiple such constraints using a single sparse MG. Without any
full-wave optimization, this procedure results in a fabrication-ready layout of
a multichannel MG, enabling retroreflection suppression and diffusive
scattering from numerous angles of incidence simultaneously. This concept,
verified experimentally via a five-channel prototype, offers an innovative
solution to both monostatic and bistatic radar cross section reduction,
avoiding design and implementation challenges associated with dense
metasurfaces used for this purpose.
