Hi,
best is to take a look into the CWNA.
The perfect antenna pattern is like a round balloon. If you now need more gain to one site, you deform the balloon. A high gain omni antenna as example, if you push from top and bottom you get more to the outside. This is how you can get a high gain omni antenna. But your gain to top and bottom is not as big as it was. As more as you pressing booth sides, as more gain you get to the outside. But at the same time you'll lose the gain to top and bottom.
If you mounting a high gain antenna at a high of 15 meters, you maybe have no connection under the antenna. Based on the angle the signal is transmitted above you.
Same for a patch antenna, you pressing left, right, bottom, tot and behind the balloon and get more gain to the front.
I hope my english isn't too bad to describe this.