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.
![Layers in the atmosphere including the ionosphere](/images/electronics/atmos01.gif)
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
- 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.
- 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.
- 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.
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