Question

A radar array consists of five vertical antennae arranged in a line, each emitting radio waves of frequency 120MHz in phase with one another uniformly in all directions. The spacing between each of the antennae is 8.0m.
Apart from the normal direction perpendicular to the line joining the antennae, at what other angles will the wave fronts from the 5 antennae arrive in phase at a distant location to produce a strong radar signal?