Spatial power-combining devices and antenna assemblies for spatial power-combining devices are disclosed. A spatial power-combining device may include an input coaxial waveguide section, an output coaxial waveguide section, and a center waveguide section. The center waveguide section may include an input center waveguide section, an output center waveguide section, and a core section. The core section may form an integral single component with an input inner housing of the input center waveguide section and an output inner housing of the output center waveguide section. Alternatively, the core section may be attached to the input inner housing and the output inner housing. The plurality of amplifiers may be registered with the core section. Antenna assemblies may include antennas with signal and ground conductors that are separated by air. Representative spatial power-combining devices may be designed with high efficiency, high or low frequency ranges, ultra-wide bandwidth operation, and high output power.