Radio signals can travel over vast distances. However radio signals are affected by the medium in which they travel and this can affect the radio propagation or RF propagation and the distances over which the signals can propagate. Some radio signals can travel or propagate around the globe, whereas other radio signals may only propagate over much shorter distances.
Radio propagation, or the way in which radio signals travel can be an interesting topic to study. RF propagation is a particularly important topic for any radio communications system. The radio propagation will depend on many factors, and the choice of the radio frequency will determine many aspects of radio propagation for the radio communications system.
Accordingly it is often necessary to have a good understanding of what is radio propagation, its principles, and the different forms to understand how a radio communications system will work, and to choose the best radio frequencies.
Radio propagation definition
Radio propagation is the way radio waves travel or propagate when they are transmitted from one point to another and affected by the medium in which they travel and in particular the way they propagate around the Earth in various parts of the atmosphere.
Factors affecting radio propagation
There are many factors that affect the way in which radio signals or radio waves propagate. These are determined by the medium through which the radio waves travel and the various objects that may appear in the path. 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 radio signal may also be a combination of several signals that have travelled by different paths. These may add together or subtract from one another, 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 radio communications 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 radio waves 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.
Types of radio propagation
There are a number of categories into which different types of RF propagation can be placed. These relate to the effects of the media through which the signals propagate.
- Free space propagation: Here the radio waves 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 signal strength reduces. This type of radio propagation is encountered with radio communications systems including satellites where the signals travel up to the satellite from the ground and back down again. Typically there is little influence from elements such as the atmosphere, etc. . Read more about free space propagation.
- 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 RF propagation. Read more about ground wave propagation
- Ionospheric propagation: Here the radio signals are modified and influenced by a region high in the earth's atmosphere known as the ionosphere. This form of radio propagation is used by radio communications systems that transmit on the HF or short wave bands. Using this form of propagation, stations may be heard from the other side of the globe dependent upon many factors including the radio frequencies used, the time of day, and a variety of other factors. 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
Some of these other types of niche forms of radio propagation include:
- Sporadic E: This form of propagation is often heard on the VHF FM band, typically in summer and it can cause disruption to services as distant stations are heard. Read more about sporadic E propagation.
- Meteor scatter communications: As the name indicates, this form of radio propagation uses the ionised trails left by meteors as they enter the earth’s atmosphere. When data is not required instantly, it is an ideal form of communications for distances around 1500km or so for commercial applications. Radio amateurs also use it, especially when meteor showers are present. Read more about meteor scatter communications.
- Transequatorial propagation, TEP: Transequatorial propagation occurs under some distinct conditions and enables signals to propagate under circmstances when normal ionospheric propagation paths would not be anticipated.Read more about transequatorial propagation.
- Near Vertical Incidence Skywave, NVIS: This form of propagation launches skywaves at a high angle and they are returned to Earth relatively close by. It provides local coverage in hilly terrain. Read more about NVIS propagation.
- Auroral backscatter: The aurora borealis (Northern Lights) and Aurora Australis (Southern Lights) are indicators of solar activity which can disrupt normal ionospheric propagation. This type of propagation is rarely used for commercial communications as it is not predictable but radio amateurs often take advantage of it. Read more about auroral backscatter propagation.
- Moonbounce EME: When high power transmissions are directed towards the moon, feint reflections can be heard if the antennas have sufficient gain. This form of propagation can enable radio amateurs to communicate globally at frequencies of 140 MHz and above, effectively using the Moon as a giant reflector satellite.
In addition to these categories, many short range wireless or radio communications systems have RF propagation scenarios that do not fit neatly into these categories. Wi-Fi systems, for example, may be considered to have a form of free space radio propagation, but there will be will be very heavily modified because of multiple reflections, refractions and diffractions. Despite these complications it is still possible to generate rough guidelines and models for these radio propagation scenarios.
RF propagation summary
There are many radio propagation scenarios in real life. Often signals may travel by several means, radio waves travelling using one type of radio propagation interacting with another. However to build up an understanding of how a radio signal reaches a receiver, it is necessary to have a good understanding of all the possible methods of radio propagation. By understanding these, the interactions can be better understood along with the performance of any radio communications systems that are used.