Radio wave propagation

Radio waves propagate in space according to several different physical mechanisms: free-space propagation or line-of-sight propagation, reflection, transmission, diffraction, scattering and wave guiding.

In free space a wave propagates without encountering any obstacle. The attenuation in free space results from the scattering of energy which occurs as the wave propagates away from the transmitter. Free-space attenuation is a function of the distance and the frequency. The excess attenuation compared to free-space attenuation is defined as the difference between the path loss and free-space attenuation (atmospheric absorption, hydrometeor attenuation, vegetation attenuation, attenuation due to diffraction, etc).

Reflection is the phenomenon whereby vibrations or waves are reflected at a surface. This phenomenon occurs when a propagating wave impinges upon a surface with large dimensions compared to the wavelength. A distinction is commonly drawn between specular reflection, occurring in the presence of a perfectly plane, homogeneous surface, and diffuse reflection, which takes place in the presence of a rough surface, i.e. a surface presenting irregularities. The reflection coefficient is defined as the ratio between the received energy flux and the incident energy flux.

The phenomenon of transmission is the process whereby vibrations or waves propagate through a medium, for instance vacuum, the air or an obstacle, without a change of frequency. Different types of transmission are usually distinguished. In regular transmission, the wave propagates through an object without diffusion. In diffuse transmission a phenomenon of diffusion occurs at a macroscopic scale independently of the refraction laws: the incident wave, while being transmitted, is scattered over a range of different angles. At last, mixed transmission is a partly regular and partly diffuse transmission. The transmission coefficient is defined as the ratio between the transmitted energy flux and the incident energy flux.

The phenomenon of diffraction occurs when waves impinge upon an obstacle or an aperture with large dimensions compared to the wavelength. This phenomenon is one of the most important factors in the propagation of radio waves.

Scattering is the phenomenon whereby the energy of an electromagnetic wave is distributed in a propagation medium along several directions after meeting a rough surface or heterogeneities with small dimensions compared to the wavelength.

The emitted energy can be channeled along a given direction using a waveguide.

Interferences result from the superposition of oscillations or waves of same nature and equal frequency.

The Earth’s atmosphere

The atmosphere of the Earth is the gaseous envelope surrounding it and interdependent with the different movements of the Earth. The atmosphere may be regarded as a series of concentric layers delimiting different regions, the two main such regions being the homosphere, which extends from the surface of the Earth up to approximately 80 or 90 kilometres, and the heterosphere, extending beyond these altitudes. The two main components of the homosphere, nitrogen and oxygen, are present in this layer in constant proportions, whereas light gases, such as nitrogen, hydrogen and helium are prevailing in the heterosphere.

Three principal atmospheric layers are defined in the homosphere: the troposphere, the stratosphere and the mesosphere. These layers are differentiated by their temperature with respect to the altitude.

The troposphere is the lowest layer of the atmosphere, and is characterised by the regular decrease of temperature as altitude rises, at the average rate of - 5 to - 6° C per kilometer. This is where most meteorological phenomena, including the formation of clouds, develop. The troposphere itself is subdivided into two layers: the turbulent layer and the free atmosphere. The turbulent layer extends from the surface up to the attitude of 1500 meters over plains, and to higher altitudes over high-relief areas.

The altitude of the upper limit of the troposphere varies between 8 kilometres high at the poles and 18 kilometres high at the equator depending on the geographical latitude as well as on seasons and meteorological conditions. In this region, known as the tropopause, temperature varies from 190 K at the equator and 220 K at the poles.

The region extending above the tropopause, known as the stratosphere, is a layer where the temperature rises with increasing temperature, first slowly up to 20 kilometers, then at a slower rate until it reaches a maximum value approaching 290 K at approximately the altitude of 50 kilometers. The region where temperature is the highest is referred to as the stratopause.

The increase in temperature observed in the stratosphere is caused by the absorption by this layer of a part of the ultraviolet radiation emitted by the Sun. The stratosphere forms therefore a regulating filter which creates the conditions for life to exist at the surface of the Earth.

The layer of the atmosphere lying above the stratopause is known as the mesosphere. In this region, the infrared emission by carbon dioxide, a component present in the atmosphere in very small quantities, is enough to cause a reduction of the temperature down to a minimum value between 150° and 210° K. The region where this minimum temperature occurs is called the mesopause, and extends at altitudes between 80 and 90 kilometers.

Дата добавления: 2015-11-14