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?

Erklärung

Along the line joining the two elements the half-wavelength spacing adds 180 degrees of travel to the 180-degree feed difference, so the two signals arrive back in phase and reinforce. Broadside there is no path difference at all, so the out-of-phase currents cancel, leaving a figure-eight aligned with the axis of the array.