General exam question G9C09

WhyTwo three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically have a gain approximately 3 dB higher than a single three-element Yagi, in free space. Stacking two identical Yagis a half wavelength apart and feeding them in phase combines their radiated fields constructively in the favored direction, and the classic rule of thumb for a well-designed stack like this is about a 3 dB gain improvement over a single Yagi, answer B. The smaller 1.5 dB figure understates a well-executed stack's typical gain, and 6 dB or 9 dB would require a much larger array than just two stacked antennas, so A, C, and D are wrong.