A method for making a horn antenna array includes the steps of making planar boards with surface conductor or metallization defining a plurality of side-by-side horns, and with horn feed conductors extending to an edge of the boards. The edges of the board are metallized in a pattern to define feed pads in contact with the feed conductors. Slots are cut in the boards on the axes of the horns so that two orthogonal boards can be joined together for “radiation” in mutually orthogonal planes. A surface-conducive dielectric support defines surface pads in a pattern that matches the pattern of feed pads in a set of joined boards, and through vias connect from the surface pads to lower layers, which may include a beamformer, for making individual connections to the horns.