An assembly for confining electromagnetic radiation in a waveguide. The assembly comprises a waveguide, comprising walls surrounding a cavity and an aperture in the walls that opens to the cavity, and a substrate assembly disposed in the aperture. The substrate assembly comprises a substrate comprising an antenna, wherein the antenna is located within the cavity and is configured for transmission of radiation within the cavity. The substrate assembly comprises an integrated circuit (IC) electrically connected to the substrate, where the IC comprises semi-conductor components and a ground plane on one side of the IC. The ground plane is located between the IC semi-conductor components and the antenna. The ground plane is located across the aperture to reduce the area of the aperture and to reflect some of the radiation directed to the aperture back into the cavity.