RADIO WAVE PROPOGATION :TYPES

Types of radio propagation

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.

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. 

• 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.

• 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. 

• 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 main categories, radio signals may also be affected in slightly different ways. Sometimes these may be considered as sub-categories, or they may be quite interesting on their own.

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.

• 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. 

• 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.

• 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.

• 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.

• 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.