What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?

Phased array antenna patterns are determined by the combination of physical spacing and electrical phase difference. In this setup, the antennas are physically spaced 1/2-wavelength apart, which equates to 180 degrees of travel time for a radio wave. Because they are fed 180 degrees out of phase, the signal from the first antenna reaches the second antenna exactly in phase with the second antenna's own feed. This results in maximum constructive interference (signal addition) along the axis drawn between the two antennas, known as an end-fire pattern. Conversely, in the broadside direction (perpendicular to the array), the signals travel the same distance to a receiver but start 180 degrees out of phase, perfectly canceling each other out. This combination of end-fire addition and broadside cancellation results in a figure-eight pattern oriented along the axis of the array.