Cut a 4" circle out of a piece of black construction paper. Now, go outside in the sun and put your hand underneath the circle. This is raw sun power.
Now, put a 4" magnifying glass in that hole and put your hand underneath. Feel the burn? You just learned about antennas.
Antennas don't change the amount of power, they just focus it.
ad5mb - Your comment about lower power changing the vertical pattern... I didn't read any post that said that. All I read was that lower gain changes vertical pattern. Of course, antennas sole purpose in life is to change the horizontal and vertical pattern of a signal. Input power does not change that pattern.