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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. Now look at Figs.2a and 2b. This is the same basic Yagi with its boom extended to twelve metres in length, elements added as required and the whole subjected to some optimisation adjustments to clean it up a bit. Note that it too has a “clean” pattern. Its gain of 15.55dBd is not quite a 3dB increase over the original, but close (2.81dB). So, would you put that up? That boom is 40 feet long. How is your rotator going to stand up to the extra torque when the westerlies hit it? How will the neighbours feel about it hanging over their backyard? No thanks! I would rather stack two of the six metre jobs. Still, the choice is yours.
Someone may suggest that there are additional feeder losses in the stacked arrangement due to the need to connect the two Yagis together. This is true but there is also extra feeder loss due to the need to connect your feeder to the dipole which will be further out from the mast with the single Yagi. Not much in this argument.
Answer No.2