Power-combining devices and, more particularly, antenna structures for spatial power-combining devices are disclosed. Spatial power-combining devices include amplifier assemblies with amplifiers and antenna structures supported by body structures. Body structures may include inner surfaces that are closer to a center axis of the spatial power-combining device where the inner surfaces have peripheral edges that are inset from remainders of the body structures. Antenna structures are disclosed that are arranged within inset portions such that the antenna structures are bounded on one end by the body structures. When assembled, a portion of a coaxial waveguide section that engages with the amplifier assemblies may bound opposing ends of the antenna structures. By not having body structures bound both ends of antenna structures, amplifier assembly design may be improved to allow smaller sizes for higher frequency operation along with improved manufacturability.