A flat-aperture waveguide sidewall-emitting antenna in a compact low-profile configuration and with the capability of radiating a beam of extremely high-power microwave (HPM) pulses in a directional manner is provided. High-power microwave antennas are essential technologies to microwave-based directed energy weapons (DEW). The flat-aperture waveguide sidewall-emitting antenna is especially well-suited to high-power microwave operation because of its relatively large aperture, which distributes the output power evenly over a large area, thus reducing the risk of microwave-induced air-breakdown or surface-breakdown that would other wise impede proper operation and degrade output beam formation.