To illustrate the fact that earth relatively close to the antenna usually plays the dominant role in creating vertical radiation patterns, let's try an HFTA run where the reflecting plane extends only 1000ft from the tower base.
This is kind of like building a 100 foot tower 1000 feet from the edge of the Grand Canyon, hence the name of this example. The antenna only has 1000 feet of reflecting surface with which to work. According to the earlier chart we can see that a reflection point distance of 1000 feet for a 100 foot tower over flat earth corresponds to an elevation angle of approximately 6 to 7 degrees. We would expect, then, for this antenna system to perform normally for this elevation angle and higher, but experience problems with lower elevation angles.
To see if this is true, we run HFTA and obtain the following:
Sure enough, signals arriving at and above 6 degrees are properly supported by the 1000 foot reflecting surface. Signals at elevation angles of less than 6 degrees are not.