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?

Explication

Two identical antennas correctly spaced and fed in phase double the array's effective aperture, and doubling power gain is 10 × log₁₀(2) = 3.0 dB. Reaching 6 dB would require four antennas, not two.