Spatial power-combining devices, and in particular, antenna structures for spatial power-combining devices are disclosed. A spatial power-combining device includes a plurality of amplifier assemblies, and each amplifier assembly includes an input antenna structure, an amplifier, and an output antenna structure. At least one of the input antenna structure and the output antenna structure may have a profile that includes tuning features, such as steps or other shapes, configured to tune or match with a desired operating frequency range. The tuning features may be configured with one or both of a signal conductor and a ground conductor of at least one of the input and output antenna structures. The tuning features may be non-symmetric across a particular signal conductor or a ground conductor, and the tuning features of a signal conductor may be non-symmetric with the tuning features of a ground conductor.