Spatial power-combining devices and antenna assemblies for spatial power-combining devices are disclosed. The disclosure relates to spatial power-combining devices with segmented waveguides and antennas. The spatial power-combining devices may be designed for high efficiency, high or low frequency ranges, ultra-wide bandwidth operation, and high output power. A spatial power-combining device may include a plurality of amplifiers, an output center waveguide including an output inner housing and an output outer housing, and an output coaxial waveguide. The output center waveguide may form a plurality of output center waveguide segments that are discontinuous with each other. Each output center waveguide segment may include a different portion of the output inner housing and the output outer housing. An antenna for a spatial power-combining device may include a plurality of antenna segments, each of which includes a different portion of a signal conductor and a ground conductor of the antenna.