Waveguide assemblies are described that utilize a surface-mount waveguide for vertical transitions of a printed circuit board (PCB). The surface-mount waveguide enables low transmission-loss (e.g., increased return-loss bandwidth) by utilizing a waveguide cavity positioned over a plated slot to efficiently transfer electromagnetic energy from one side of the PCB to another side. The waveguide cavity is designed to excite two resonant peaks of the EM energy to reduce a return-loss of power and increase power delivered to an antenna while supporting a high bandwidth of EM energy. Furthermore, the surface-mount waveguide does not require precise fabrication often required for vertical transitions, allowing the surface-mount waveguide to be compatible with low-cost PCB materials (e.g., hybrid PCB stack-ups).