An antenna apparatus comprises a lower assembly and an upper assembly, which together forming a cavity to contain an RF circuit device. The upper assembly comprises a waveguide flange interface at an external surface of the upper assembly. The waveguide flange interface comprises a waveguide channel extending from the external surface to an internal surface forming a surface of the cavity. An opening of the waveguide channel at the internal surface is substantially centered about a first centerline of the upper assembly parallel with the external surface and offset from a second centerline of the upper assembly parallel with the external surface, whereby the second centerline perpendicular is to the first centerline. The upper assembly is removably attachable to the lower assembly in either of a first orientation or a second orientation, whereby the second orientation represents a 180 degree rotation of the upper assembly relative to the first orientation.