- an overview or summary the different modes or ways for the propagation of radio waves

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. Read more about Ground wave 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. Read more about Ionospheric propagation
    * 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. Read more about Tropospheric propagation

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.

There are also a number of other, more specialised forms of radio propagation that are used in a number of instances:

    * Meteor scatter / Meteor Burst: This form of radio propagation is often used for links of up to 1500 km or more where real time communications are not needed. It is often used for relaying data from remote sites to a base station. Read more about Meteor burst communications
    * NVIS: This is actually a form of ionospheric propagation. Signals are transmitted with a very high angle of radiation, and returned to earth over a limited area. It is particularly useful in hilly or forested regions where normal ground wave propagation may not be able to access all areas needed. Read more about NVIS - near vertical incidence skywave

Areas that affect radio propagation

It is also necessary to understand the differnet areas of the atmosphere, or other areas that affect radio propagation and radio communications signals. Read more about Areas of the atmosphere that affect radio propagation

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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