The various effects like reflection, refraction, diffraction, etc all come together in a real way as radio signals propagate through the atmosphere. The signals are affected by a variety of factors enabling signals to be detected near and far dependent upon a variety of factors.
The way that radio signals propagate, or travel from the radio transmitter to the radio receiver is of great importance when planning a radio communications network or system.
In many instances, terrestrial radio propagation is governed to a great degree by the regions of the atmosphere through which the signals pass. Without the action of the atmosphere it would not be possible for radio communications signals to travel around the globe on the short wave bands, or travel greater than only the line of sight distance at higher frequencies.
In fact the way in which the atmosphere affects radio communications is of tremendous importance for anyone associated with radio communications, whether they are for two way radio communications links, mobile radio communications, radio broadcasting, point to point radio communications or any other radio.
In view of the importance of the atmosphere to radio communications, an overview of its make-up is given here.
The atmosphere can be split up into a variety of different layers according to their properties.
Although there are is a number of different ways of classifying the different atmospheric regions - typically different scientific displaces may have their own nomenclature as a result of their interest in different properties.
The lowest area in the meteorological system is referred to as the Troposphere. This extends to altitudes of around 10km above the Earth's surface. Above this is the Stratosphere that extends from altitudes around 10 to 50km. Above this at altitudes between 50 and 80 km is the Mesosphere and above this is the Themosphere: named because of the dramatic rise in temperatures here.
From the viewpoint of radio propagation, there are two main areas of interest:
The lowest of the layers of the atmosphere is called the troposphere. The troposphere extends from ground level to an altitude of 10 km.
It is within the tropospheric region that what we term the weather, occurs. Low clouds occur at altitudes of up to 2 km and medium level clouds extend to about 4 km. The highest clouds are found at altitudes up to 10 km whereas modern jet airliners fly above this at altitudes of up to 12 km.
Within this region of the atmosphere there is generally a steady fall in temperature with height. This affects radio propagation because it affects the refractive index of the air. This plays a dominant role in radio signal propagation and the radio communications applications that use tropospheric radio-wave propagation. This depends on the temperature, pressure and humidity. When radio communications signals are affected this often occurs at altitudes up to 2 km.
The ionosphere is the area that is traditionally thought of as providing the means by which long distance communications can be made. It has a major effect on what are normally thought of as the short wave bands, providing a means by which signals appear to be reflected back to earth from layers high above the ground.
The ionosphere has a high level of free electrons and ions - hence the name ionosphere. It is found that the level of electrons sharply increases at altitudes of around 30 km, but it is not until altitudes of around 60km are reached that the free electrons are sufficiently dense to significantly affect radio signals.
The ionisation occurs as a result of radiation, mainly from the sun, striking molecules of air with sufficient energy to release electrons and leave positive ions.
Obviously when ions and free electrons meet, then they are likely to recombine, so a state of dynamic equilibrium is set up, but the higher the level of radiation, the more electrons will be freed.
Much of the ionisation is caused by ultraviolet light. As it reaches the higher reaches of the atmosphere it will be at its strongest, but as it hits molecules in there upper reaches where the air is very thin, it will ionise much of the gas. In doing this, the intensity of the radiation is reduced
At the lower levels of the ionosphere, the intensity of the ultraviolet light his much reduced and more penetrating radiation including x-rays and cosmic rays gives rise to much of the ionisation.
As a result of many factors it is found that the level of free electrons varies over the ionosphere and there are areas that affect radio signals more than others. These are often referred to as layers, but are possibly more correctly thought of a regions as they are quite indistinct in many respects. These layers are given designations D, E, and F1 and F2.
The way in which the various regions in the atmosphere affect radiowave propagation and radio communications is a fascinating study. There are very many factors that influence radio propagation and the resulting radio communications links that can be established. Predicting the ways in which this occurs is complicated and difficult, however it is possible to gain a good idea of the likely radio communications conditions using some simple indicators. Further pages in this section of the website detail many of these aspects.
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