RADAR sensor assemblies/modules, particularly those for vehicles. In some embodiments, the assembly may comprise a plurality of waveguides, each waveguide of the plurality of waveguides being defined by a waveguide groove. A slot may be positioned to extend along an axis of each of the plurality of waveguide grooves. Each of the waveguides may be further defined, at least in part, by a periodic feature that extends back and forth in a periodic manner along at least a portion of its respective waveguide and a plurality of periodic signal confinement structures, a first periodic signal confinement structure of which may extend adjacent to a first side of each of the plurality of waveguides, and a second periodic signal confinement structure which may extend along a second side of each of the plurality of waveguides opposite the first side.