If you have two antenna elements and want to create a pattern similar to that in the picture(maximum signal perpendicular to a line between the elements), what would the spacing (D) and the phase shift between the elements need to be?

spacing λ/4, phase 90 deg

spacing λ/4, phase 0 deg

spacing λ/2, phase 90 deg

spacing λ/2, phase 0 deg

Answer: spacing λ/2, phase 0 deg.