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The old rule of thumb was to stack at two thirds of the boom length. This idea was presumably based on the aim of achieving a 3dB increase in gain over one Yagi. Look at Figs. 3a and 3b. This is the Yagi of Fig. 1 stacked at half and at two thirds the boom length. Note that at half the boom length large lobes have appeared on each side of the main lobe and 14dB down. These are called “grating” lobes and they are due to the pattern multiplication process so that they appear within the area of the main lobe of a single Yagi. The gain increase is 2.85dB

Answer No.3