When electromagnetic waves travel through a medium they can interact with that medium in a variety of ways. The first type of interaction is reflection. Radio waves can be reflected by a solid object much as light waves are. Whenever a radio wave move across a boundary from one medium to another (assuming that the media have different refractive indices) there will be a reflection. The incidence and reflection angles are equal, and the magnitude and phase of the reflected wave depend on the properties of the reflecting medium.. A perfect reflector that reflects all RF incident on it has a reflection coefficient of 1.0. Metals and sea water are examples of good RF reflectors.
The reflection of radio waves by a solid object is affected by their polarization. If the incident wave has its electric field vector parallel to the reflecting surface, the electric field will be shorted out by the conductivity of the surface. If the electric field vector is perpendicular to the reflecting surface, it is reflected.
The second type of interaction is refraction. When radio waves pass from one material to another, they change direction at the interface between the two materials. This is called refraction. The angles of incidence and refraction are related to the refractive indices of the two media by Snellís law:
n1sinq1 = n2sinq2
Variables n1 and q1 are the refractive index and direction of travel in the incident medium and n2 and q2 are the refractive index and direction of travel in the refracting medium. Refraction is an important aspect of radio wave propagation. At frequencies between 30 and 30 MHz, the ionosphere refracts RF and redirects the waves back towards the earth's surface. Above 100 MHz. The refractive index of air is dependent on the temperature and relative humidity of the air. A temperature inversion can cause RF waves to be bent just enough to follow the curvature of the earth and travel for hundreds of miles with little loss.
A third type of interaction is diffraction. When radio waves encounter an obstacle, the obstacle casts a shadow, just as it would when illuminated with light. However, the shadow region is not completely void of radio waves, because some radio waves are scattered around the edge of the object. As one gets farther from the object, one eventually reaches a point where the scattered waves have completely filled in the shadow. The amount of scattering depends on the size of the electromagnetic wave relative to the size of the object. For example, an interstate underpass is dark underneath, because its size (~10m) is millions of times larger than light waves (~0.5 mm). The bridge casts a sharp shadow and there is little illumination. However, FM radio waves, whose wavelength is about 3m are diffracted significantly by the bridge and it is possible to receive FM signals on a car radio while driving under the bridge. There is so much diffraction that the shadow zone is completely washed out.
The degree of diffraction also depends on the sharpness of the edges of the object. A gradually sloping hill does not diffract radio waves much and the shadow zone behind it is quite small. On the other hand, a sharply defined cliff or mountain causes significant diffraction and a sizeable shadow zone.
There are 3 basic modes of propagation for radio waves in the vicinity of the earth, which will be discussed in more detail in the next three sections::
Ground wave propagation
Space wave (direct wave) propagation
Sky wave propagation