Steering of an electromagnetic beam of energy in the upper plate of a plate waveguide of a traveling wave antenna concurrently with the formation of a flat phase front and collimation of the electromagnetic beam is achieved by providing a second waveguide beneath the lower plate of the first waveguide and providing a 180% bend parabolic main reflector to reflect the energy beam to the upper plate of the upper waveguide. A feed horn is located in the lower waveguide and illuminates a pivotal subreflector which reflects the energy to the parabolic main reflector. By rotating the subreflector about its pivot point, the beam which is radiated to the upper waveguide is angularly shifted or steered.