Radio Wave Propagation
An energy wave generated by a transmitter is called a radio wave.The radio wave radiated into space by the transmitting antenna is a very complex form of energy containing both electric and magnetic fields. Because of this combination of fields, radio waves are also referred to as electromagnetic radiation
Radio Wave Propagation:
Figure 1 Radio Wave Propagation
Radio waves can exhibit three fundamental propagation behaviors depending on their frequency.
Ground wave (<2 MHz): Waves with low frequencies follow the earth’s surface and can propagate long distances. These waves are used for, e.g., submarine communication or AM radio.
Sky wave (2–30 MHz): Many international broadcasts and amateur radio use these short waves that are reflected2 at the ionosphere. This way the waves can bounce back and forth between the ionosphere and the earth’s surface, travelling around the world.
Line-of-sight (>30 MHz): Mobile phone systems, satellite systems, cordless telephones etc. use even higher frequencies. The emitted waves follow a (more or less) straight line of sight. This enables direct communication with satellites (no reflection at the ionosphere) or microwave links on the ground.
Radio propagation categories:
There are a number of categories into which different types of radio propagation can be placed. These relate to the effects of the media through which the signals propagate
Free space propagation: Here the radio signals travel in free space, or away from other objects which influence the way in which they travel. It is only the distance from the source which affects the way in which the field strength reduces. This type of radio propagation is encountered with signals travelling to and from satellites.
Ground wave propagation: When signals travel via the ground wave they are modified by the ground or terrain over which they travel. They also tend to follow the earth's curvature. Signals heard on the medium wave band during the day use this form of propagation. Read more about Ground wave propagation
Ionospheric propagation: Here the radio signals are modified and influenced by the action of the free electrons in the upper reaches of the earth's atmosphere called the ionosphere. This form of radio propagation is used by stations on the short wave bands for their signals to be heard around the globe. Read more about Ionosphere propagation
Tropospheric propagation: Here the signals are influenced by the variations of refractive index in the troposphere just above the earth's surface. Tropospheric radio propagation is often the means by which signals at VHF and above are heard over extended distances. Read more about Tropospheric propagation