I have heard that stacking two or more antennas help in increasing the reception gain. Will this be true for transmission? For example, in transmission using double stacked yagis, wouldn't each antenna in the stack receive half of the power fed to the stack?
I have heard that stacking two or more antennas help in increasing the reception gain. Will this be true for transmission?
Yes! Electromagnetism as it applies to typical antennas has a property called reciprocity, which says that everything works the same when you reverse the directions of motion of the waves (or reverse time, equivalently). Transmission and reception are always symmetrical, as far as the antenna's concerned.
For example, in transmission using double stacked yagis, wouldn't each antenna in the stack receive half of the power fed to the stack?
Yes, but that's okay; all of the power is still being transmitted. If the goal here were to feed more total power in, you could do that by using a bigger amplifier instead.
The benefits of an antenna array of any sort are, largely, not because it is able to gather or emit more power than it would otherwise. Rather, it is because the power is more focused in the desired direction — the array has more gain than the individual antennas.
This occurs because the elements of the stacked antenna array are transmitting in phase, so as long as their distances to the receiver are equal — i.e. the array is perpendicular to the direction to the receiver — the signals add up. And they add up to more than the power in that direction for a single antenna, because there are many other directions in which they cancel out instead, saving that wasted power.
Thus, the array has gain in the direction perpendicular to it. The effect of the individual antennas being Yagis, rather than more omnidirectional antennas, is just to add their own gain; maximum gain in a single direction is obtained by making sure the Yagis' individual beam directions are exactly perpendicular to the line of the array.