If an object is standing free, so that an RF signal traveling parallel with the ground is diffracted around it on both sides, the signal will often fill in the object’s “shadow” as it continues to propagate. However, if a standing object such as a building or a mountain obstructs the signal, the signal can be adversely affected in the vertical direction.
In Figure 3-14, a building partially obstructs the path of the signal. Because of diffraction along the front and top of the building, the signal is bent and also attenuated. This causes the signal to be masked behind most of the height of the building.