For wireless communication, signals can travel from source to destination in several ways they are ground propagation, sky propagation and line-of-sight propagation.
Radio waves travel through the lowest portion of the atmosphere. These low frequency signal emanate in all directions from the transmitting antenna and follow the curvature of the planet. Distance depends on the amount of power in the signal. This type of propagation is called Ground Propagation.
Higher frequency radio waves propagate upward into the layer of the atmosphere where particles exist as ions (ionosphere). This type of transmission allows greater distances with lower power output and is called as Sky Propagation.
When very high frequency signals are transmitted in straight lines directly from antenna to antenna, it is called line-of-sight propagation. Antennas must be directional, facing each other, and either tall enough or close enough together not to be affected by the curvature of the earth.