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We stack Yagis in order to increase the gain over that obtainable from one Yagi and/or to decrease the beamwidth. The increase in gain is due to the reduction in beamwidth and it should be noted that the beamwidth is reduced in the plane of stacking only. If we stack vertically the beamwidth is decreased in the vertical or “H” plane of a horizontally polarised Yagi. Stacking horizontally results in a narrower beamwidth in the horizontal or “E “plane of a horizontally polarised Yagi. In some applications, such as interference from or to points off to one side or below the main lobe, the reduction in beam width is a more important consideration than the gain increase. However most people stack to get more gain.Yes of course we can. The increase in gain due to stacking two Yagis approaches the limit
of 3dB. We will see that this limit is overly optimistic in practice. Nevertheless, it is theoretically possible. So how much bigger would we have to make a Yagi in order to increase its gain by 3dB? If you think about it, the answer is obviously about twice as big. Consider Figs. 1a and 1b. This is a fairly big 2m Yagi with 13 elements on a six metre boom. It has a gain of 12.74dBd and a clean pattern. That is a pattern which has low side lobes in relation to the main lobe.

Answer No.1