I stumbled on a great video on the internet which shows exactly how a Yagi directional antenna works. This video was produced by MakeMagasine.com and presented by Diana Eng.
In short order, it demonstrate how a dipole antenna transfers energy thru the air and how to give direction to this energy. A superbly well made and simple video that also demonstrates why Nikola Tesla was right when he said he could transport electricity wirelessly. Since a picture is worth a thousand words, I will let you enjoy this video.