Abstract: Provided herein are various enhancements for antenna systems and directed radio frequency energy structures. In one example, an apparatus includes an antenna array comprising a plurality of antenna elements formed by waveguide structures embedded within a substrate and positioned about a longitudinal axis of the substrate to form at least two concentric ring arrangements of antenna elements. Apertures of the waveguide structures are configured to emit or receive radio frequency (RF) energy generally along the longitudinal axis. Feed elements are coupled to each of the waveguide structures on an end opposite of the apertures, and configured to couple the RF energy for the antenna array.