The sum signal is created by a feedhorn structure positioned to maximize signal at the center of the antenna beam. The delta RF signals are created by pairs of antenna feed-horns located adjacent to the sum feed-horn (sum feed-horn not shown in the figure). The output of each pair of delta feed-horns are added together, and this creates zero output signal when the incoming RF signal is located at the center of the antenna beam. The signal strength from each delta beam increases as the aircraft drifts further away from the antenna center-line.