Wave propagation, in general, is defined as the manner in which waves travel from the source to the destination. Waves are categorized into longitudinal waves and transverse waves depending on the direction of oscillation of the waves to the direction of this wave propagation. A transmission material medium is needed for all kinds of waves, except electromagnetic waves, for their propagation. Electromagnetic waves, however, are the only waves that do not necessarily require a material medium; they can propagate even through the vacuum. Space wave propagation is when an electromagnetic wave, like radio waves, travels through the Earth’s atmosphere from a source antenna to a receiving antenna.
Radio propagation is when radio waves are transmitted from the source to the destination or when radio waves travel through the environment. Being a kind of electromagnetic wave, radio waves do not require a material medium for their propagation. The propagation of radio waves, much like other types of electromagnetic waves, are affected by phenomena like reflection, refraction, diffraction, absorption, polarization, and scattering. The study of radio wave propagation is extremely important because of the employment of radio waves in modern communication technologies. A better understanding of radio wave propagation allows us to design effective mobile telephone systems, radio navigation and communication systems, create international shortwave broadcasters, and choose the appropriate frequency ranges for ham radio communication purposes.
Among several types of radio wave propagation, line of sight propagation is the most commonly used. In line of sight propagation, the radio waves transmitted from the source antenna travel in a straight line through the medium or vacuum to the receiver antenna. This does not necessarily mean that its path of propagation should be completely clear and should not contain any obstacles. The line of sight propagation of radio waves with shorter frequencies can pass through obstacles like trees, buildings, and other concrete structures. Line of sight propagation is limited by the visual horizon to about 40 miles (64 km), and it is the only method of propagation for microwave frequencies and above. Radio wave propagation can be categorized further based on the mode of propagation, as follows.
Space wave propagation refers to the propagation of radio waves within the troposphere layer of the Earth’s atmosphere, that is, within 20 km of the atmosphere. It comprises both direct and reflected waves. Direct waves are those that reach the receiving antenna at the destination directly from the source antenna without any interruptions or obstructions. Reflected waves are those that eventually reach the receiving antenna at the destination after getting reflected off of the ground and obstructions like buildings. Space wave propagation is also known as tropospheric propagation, as the radio waves involved can travel directly to the troposphere layer of the atmosphere.
Line of sight propagation, discussed earlier, is another name for space wave propagation. Space wave propagation is mostly used for VHF (Very High Frequency) or higher, which deals with frequency values in the range of 30 and 300 megahertz (MHz). As the name suggests, line of sight propagation is limited by the line of sight, which is about 64 km and so is affected by the curvature of the Earth. Space wave propagation is used by the radio waves in mobile phones, walkie-talkies, FM and television broadcasting systems. Satellite communication also makes use of line of sight propagation, but those are longer. Satellite dishes found in households used for television receive signals from communication satellites that are 22,000 miles (35,000 km) above the Earth.
The antennas in space wave propagation are usually small in size and can be placed at heights of several wavelengths above the ground to pick up the radio waves more accurately and efficiently. Thus, the range of communication can be effectively increased by placing the antennas at appropriate heights. And because of many possible obstacles in the path of the radio waves, which can lead to a loss in signal strength, the heights at which these antennas are placed must be calculated accurately, which can be done using the following expression.
Dm = (2RhT)-½ + (2RhR)-½
Space wave propagation is used in several different applications, some of which are listed below.