Radiowave Propagation and the Atmosphere

- overview of radio signals and radiowave 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. This is governed to a great degree by the regions of the atmosphere through which they 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.

Layers of the Atmosphere

The atmosphere can be split up into a variety of different layers according to their properties. As different aspects of science look at different properties there is no single nomenclature for the layers. The system that is most widely used is that associated with. Lowest is the troposphere that extends to a height of 10 km. Above this at altitudes between 10 and 50 km is found the stratosphere. This contains the ozone layer at a height of around 20 km. Above the stratosphere, there is the mesosphere extending from an altitude of 50 km to 80 km, and above this is the thermosphere where temperatures rise dramatically.

There are two main layers that are of interest from a radio communications viewpoint. The first is the troposphere that tends to affect radio frequencies above 30 MHz. The second is the ionosphere. This is a region which crosses over the boundaries of the meteorological layers and extends from around 60 km up to 700 km. Here the air becomes ionised, producing ions and free electrons. The free electrons affect radio communications and radio signals at certain frequencies, typically those radio frequencies below 30 MHz, often bending them back to Earth so that they can be heard over vast distances around the world.

Regions of the Atmosphere

Troposphere

The lowest of the layers of the atmosphere is the troposphere. This extends from ground level to an altitude of 10 km. It is within this region that the effects that govern our weather occur. To give an idea of the altitudes involved it is found that low clouds occur at altitudes of up to 2 km whereas 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 15 km.

Within the troposphere there is generally a steady fall in temperature with height and this has a distinct bearing on some radio propagation modes and radio communications that occur in this region. The fall in temperature continues in the troposphere until the tropopause is reached. This is the area where the temperature gradient levels out and then the temperature starts to rise. At this point the temperature is around -50 �C.

The refractive index of the air in the troposphere plays a dominant role in radio signal propagation and the radio communications applications that use tropospheric radiowave 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

The ionosphere is an area where there is a very high level of free electrons and ions. It is found that the free electrons affect radio waves and hence they have a marked effect on radio communications in many instances. Although there are low levels of ions and electrons at all altitudes, the number starts to rise noticeably at an altitude of around 30 km. However it is not until an altitude of approximately 60 km is reached that the it rises to a sufficient degree to have a major effect on radio signals.

The overall way in which the ionosphere is very complicated. It involves radiation from the sun striking the molecules in the upper atmosphere. This radiation is sufficiently intense that when it strikes the gas molecules some electrons are given sufficient energy to leave the molecular structure. This leaves a molecule with a deficit of one electron that is called an ion, and a free electron. As might be expected the most common molecules to be ionised are nitrogen and oxygen.

Most of the ionisation is caused by radiation in the form of ultraviolet light. At very high altitudes the gases are very thin and only low levels of ionisation are created. As the radiation penetrates further into the atmosphere the density of the gases increases and accordingly the numbers of molecules being ionised increase. However when molecules are ionised the energy in the radiation is reduced, and even though the gas density is higher at lower altitudes the degree of ionisation becomes less because of the reduction of the level of ultraviolet light.

At the lower levels of the ionosphere where the intensity of the ultraviolet light has been reduced most of the ionisation is caused by x-rays and cosmic rays which are able to penetrate further into the atmosphere. In this way an area of maximum radiation exists with the level of ionisation falling below and above it.

In terms of its radio communications properties, the ionosphere is often thought of as a number of distinct layers. Whilst it is very convenient to think of the layers as separate, in reality this is not quite true. Each layer overlaps the others with the whole of the ionosphere having some level of ionisation. The layers are best thought of as peaks in the level of ionisation. These layers are given designations D, E, and F1 and F2.

Description of the layers in the ionosphere

1. D layer:   The D layer is the lowest of the layers of the ionosphere. It exists at altitudes around 60 to 90 km. It is present during the day when radiation is received from the sun. However the density of the air at this altitude means that ions and electrons recombine relatively quickly. This means that after sunset, electron levels fall and the layer effectively disappears. This layer is typically produced as the result of X-ray and cosmic ray ionisation. It is found that this layer tends to attenuate signals that pass through it.
   
   
2. E layer:   The next layer beyond the D layer is called the E layer. This exists at an altitude of between 100 and 125 km. Instead of acting chiefly as an attenuator, this layer reflects radio signals although they still undergo some attenuation.
   
   In view of its altitude and the density of the air, electrons and positive ions recombine relatively quickly. This occurs at a rate of about four times that of the F layers that are higher up where the air is less dense. This means that after nightfall the layer virtually disappears although there is still some residual ionisation, especially in the years around the sunspot maximum that will be discussed later.
   
   There are a number of methods by which the ionisation in this layer is generated. It depends on factors including the altitude within the layer, the state of the sun, and the latitude. However X-rays and ultraviolet produce a large amount of the ionisation light, especially that with very short wavelengths.
   
   
3. F layer:   The F layer is the most important region for long distance HF communications. During the day it splits into two separate layers. These are called the F₁ and F₂ layers, the F₁ layer being the lower of the two. At night these two layers merge to give one layer called the F layer. The altitudes of the layers vary considerably with the time of day, season and the state of the sun. Typically in summer the F₁ layer may be around 300 km with the F₂ layer at about 400 km or even higher. In winter these figures may be reduced to about 300 km and 200 km. Then at night the F layer is generally around 250 to 300 km. Like the D and E layers, the level of ionisation 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 less. Accordingly it is able to support radio communications, although changes are experienced because of the lessening of the ionisation levels. The figures for the altitude of the F layers are far more variable than those for the lower layers. They change greatly with the time of day, the season and the state of the Sun. As a result the figures which are given must only be taken as an approximate guide.
   
   Most of the ionisation in this region of the ionosphere is caused by ultraviolet light, both in the middle of the UV spectrum and those portions with very short wavelengths.
   

Summary

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.