Radio propagation or radiowave propagation overview
- an overview or summary the different modes of radio propagation or
radiowave propagation that can be experienced.
Radio signals are affected in many ways by objects in their
path and by the media through which they travel. This means that radio signal
propagation is of vital importance to anyone designing or operating a radio
system. The properties of the path by which the radio signals will propagate
governs the level and quality of the received signal. Reflection, refraction and
diffraction may occur. The resultant signal may also be a combination of several
signals that have travelled by different paths. These may add constructively or
destructively, and in addition to this the signals travelling via different
paths may be delayed causing distorting of the resultant signal. It is therefore
very important to know the likely radio propagation characteristics that are
likely to prevail.
The distances over which radio signals may propagate varies
considerably. For some applications only a short range may be needed. For
example a wi-fi link may only need to be established over a distance of a few
metres. On the other hand a short wave broadcast station, or a satellite link
would need the signals to travel over much greater distances. Even for these
last two examples of the short wave broadcast station and the satellite link,
the radio propagation characteristics would be completely different, the signals
reaching their final destinations having been affected in very different ways by
the media through which the signals have travelled.
Radio propagation categories
There are a number of categories into which different types
of radio propagation can be placed. These relate to the effects of the media
through which the signals propagate.
- Free space propagation: Here the radio signals travel in
free space, or away from other objects which influence the way in which they
travel. It is only the distance from the source which affects the way in
which the field strength reduces. This type of radio propagation is
encountered with signals travelling to and from satellites.
- Ground wave propagation: When signals travel via the
ground wave they are modified by the ground or terrain over which they
travel. They also tend to follow the earth's curvature. Signals heard on the
medium wave band during the day use this form of propagation.
- Ionospheric propagation: Here the radio signals are
modified and influenced by the action of the free electrons in the upper
reaches of the earth's atmosphere called the ionosphere. This form of radio
propagation is used by stations on the short wave bands for their signals to
be heard around the globe.
- Tropospheric propagation: Here the signals are
influenced by the variations of refractive index in the troposphere just
above the earth's surface. Tropospheric radio propagation is often the means
by which signals at VHF and above are heard over extended distances.
In addition to these categories, many short range radio
communications or wireless systems have radio propagation scenarios that do not
fit neatly into these categories. Many mobile communications systems along with
wi-fi and cellular systems for example need to have their radio propagation
models generated for office, or urban situations. Under these circumstances the
"free space" propagation is modified by multiple reflections, refractions and
diffractions. Despite these complications it is still possible to generate rough
guidelines and models for these radio propagation scenarios.
Summary
There are many radio propagation, or radiowave propagation
scenarios in real life. Often radio signals may travel by several means, signals
travelling using one type of propagation interacting with another. However to
build up an understanding of how a signal reaches a receiver, it is necessary to
have a good understanding of all the possible methods. By understanding these,
the interactions can be better understood and it is then possible ti understand
some of the reasons why mobile radio communications systems, or two way radio
communications systems work in the way they do..
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