A radar system in an autonomous vehicle may be operated in various modes and with various configurations. The autonomous vehicle features a radar system having a waveguide with a first waveguide section, a second waveguide section, and a seam between the first and the second waveguide sections. The first waveguide section and the second waveguide section form a waveguide cavity. Additionally, the seam corresponds to a low surface current location of a propagation mode of the waveguide and is formed where the first waveguide section is coupled to the second waveguide section. The height of the first waveguide section may be equal to the height of the second waveguide section. The waveguide also may include a feed configured to introduce a wave with the propagation mode into the waveguide. Moreover, the waveguide may also include more than one cavity. Each cavity may lie on a plane defined by the seam.