In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?
Why Stacking two identical antennas and feeding them in phase doubles the effective aperture, and a doubling of power or aperture is 3 dB. So two three-element Yagis stacked a half wavelength apart give about 3 dB more gain than one of them alone, with the extra gain coming from a narrower vertical pattern.
Watch out The choice saying about 6 dB corresponds to four stacked antennas (two doublings of aperture), not two; each additional doubling adds only another 3 dB.
Double the antennas, double the power: 2x = 3 dB. Four stacked would be 6 dB.