Radio Wave Propagation &Atmosphere
- overview of radio signals and radio-wave propagation and how different areas of the atmosphere affect radio communications.
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:
- Troposphere: As a very approximate rule of thumb, this area of the atmosphere tends to affect signals more above 30 MHz or so.
- Ionosphere: The ionosphere is the area that enables signals on the short wave bands to traverse major distances. It crosses over the meteorological boundaries and extends from altitudes around 60 km to 700 km. The region gains its name because the air in this region becomes ionised by radiation primarily from the sun. Free electrons in this region have affect radio signals and may be able to refract them back to Earth dependent upon a variety of factors.
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.
Description of ionospheric regions
- D region: The D layer or D region is the lowest of the regions that affects radio signals. It exists at altitudes between about 60 and 90 km. It is present during the day when radiation is being received from the sun, but because of the density of molecules at this altitude, free electrons and ions quickly recombine after sunset when there is no radiation to retain the ionisation levels. The main effect of the D region is to attenuate signals that pass through it, although the level of attenuation decreases with increasing frequency. Accordingly its effects are very obvious on the medium wave broadcast band - during the day when the D region is present, few signals are heard beyond that provided by ground wave coverage. At night when the region is not present, signals are reflected from higher layers and signals are heard from much further afield.
- E region: Above the D region, the next region is the E region or E layer. This exists at an altitude of between 100 and 125 km. The main effect of this region is to reflect radio signals although they still undergo some attenuation.
- F region: The F region or F layer is higher than both the D and E regions and it the most important region for long distance HF communications. During the day it often splits into two regions known as the F1 and F2 regions, the F1 layer being the lower of the two.
At night these two regions merge as a result of the reduction in level of radiation to give one region called the F region. The altitudes of the F regions vary considerably. Time of day, season and the state of the sun all have major effects on the F region. As a result any figures for altitude are very variable and the following figures should only be used as a very rough guide. Typical summer altitudes for the F1 region may be approximately 300 km with the F2 layer at about 400 km or even higher. Winter figures may see the altitudes reduced to about 200 km and 300 km. Night time altitudes may be around 250 to 300 km.
Like the D and E regions, the level of ionisation fort he F region falls at night, but in view of the much lower air density, the ions and electrons combine much more slowly and the F layer decays much more slowly. As a result it is able to support radio communications at night, although changes are experienced because of the lessening of the ionisation levels.
In view of its altitude and the density of the air, electrons and positive ions recombine relatively quickly. This means that after sunset when the source of radiation is removed, the layer reduces in strength very considerably although some residual ionisation does remain.
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.
By Ian Poole
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