A spatial power-combining device and an antenna structure designed for high efficiency, high frequency, and ultra-wide bandwidth operation. The antenna structure may include a signal conductor and a ground conductor that are entirely separated by air. A spatial power-combining device may include a plurality of amplifier assemblies including multiple output antenna structures and an output coaxial waveguide section configured to concurrently combine signals received from each output antenna structure of the plurality of amplifier assemblies. The plurality of amplifier assemblies may also include multiple input antenna structures and an input coaxial waveguide configured to provide an input signal concurrently to each input antenna structure of the plurality of amplifier assemblies.