An antenna includes an elliptical waveguide having a plurality of length-tapered multi-slot arrays of elongate slots therein at respective spaced-apart locations along a length thereof. The plurality of length-tapered multi-slot arrays of elongate slots can include at least first and second length-tapered multi-slot arrays of elongate slots, which are spaced apart from each other along the length of the elliptical waveguide. The first length-tapered multi-slot array of elongate slots can include: (i) a first elongate slot having a first length and a first width, and (ii) a second elongate slot having a second length less than the first length and a second width that may be greater than the first width.