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

Explicación

Broadside to the array the two path lengths are equal, so elements fed in phase reinforce there. Along the axis the half-wavelength spacing puts the signals 180 degrees apart and they cancel, so the pattern is a figure-eight perpendicular to the line of the array.