An antenna array used in radio astronomy consists of a set of antennae equally-spaced along a north-south line. By reciprocity, the detection pattern of the antenna array is just the same as the radiation pattern as if the antennae were radiating at the same wavelength. The spacing between the antennae is smaller than the wavelength. Show that the array only receives radiation from a specific direction in the N-S plane, and that the direction can be chosen by int