- overview, article, tutorial about NVIS, Near Vertical Incidence Skywave propagation used for radio communications and radio broadcasting.
As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.
NVIS, or Near Vertical Incidence Skywave propagation is a form of ionospheric radio propagation that can be used where radio communications links are needed to cover short distances. NVIS is particularly useful where radio communications coverage is required in regions where the ground is mountainous or rough because other modes relying on more direct coverage have significant areas where the radio signal is masked or shadowed. If MF or HF radio communications were used, the ground wave would be attenuated by the hills and mountains, and similarly line of sight radio communications using VHF or above would not be possible. Using Near Vertical Incidence Sky Wave (NVIS) it is possible to have a signal with a high angle of elevation that is not shielded by the terrain.
In view of this, NVIS is particularly suited to professional radio communications applications in geographical regions that are poorly served by land lines or line of sight repeaters, and it can provide a very useful and cost effective alternative to satellite links. It also has advantages over VHF radio in many applications for built up or forested regions as the buildings and trees introduce very significant levels of loss making line of sight VHF radio communications links virtually unworkable in many instances.
Use of NVIS
Using NVIS propagation, a high angle or near vertical signal is transmitted towards the ionosphere. This must be at a frequency that is below the critical frequency, i.e. the maximum frequency at which a vertically incident signal is "reflected" by the ionosphere. The near vertical incident signal is "reflected" by the ionosphere and returned to the Earth over an area of many kilometres either side of the transmitter. In this way good local coverage can be obtained. This form of propagation is particularly useful in rough terrain because the coverage area is illuminated from above and undulations in the Earth's surface do not create shadow areas that cannot be reached.
Typically radiation should be at angles greater than 75 or 80 degrees to the horizontal so that good local coverage is achieved. Typically coverage areas may have a radius of between 35 and 350 km. The frequency needs to be chosen carefully. Usually this is between about 2 and 10 MHz, although during the periods of the sunspot minimum, maximum frequencies may be limited to 6 to 7 MHz. By using these frequencies, the losses from the D region can be overcome, and the higher layers of the ionosphere are still able to reflect the high angle signals without them passing right through the ionosphere.
Antennas for NVIS
In order to be able to make use of NVIS for radio communications systems, it is necessary to radiate the majority of power at a high angle of radiation, i.e. near to the vertical. This requires the antennas to be designed specifically with this application in mind. In many instances HF radio antennas are designed to provide a much lower angle of radiation. This requires radio antennas specifically designed to enable the required high angle of radiation to be achieved. Typically this is achieved by using a horizontally polarised antenna. In addition to this the antenna should be mounted at an elevation that will maximise the high angle of radiation. Normally this means that it will not be particularly high.
Although NVIS is has not been particularly widely used as a concept until recently, its use is growing, especially for specialised applications such as broadcasting in what are termed the "Tropical" broadcast bands where it is possible to achieve a relatively local coverage area, especially in mountainous or forested regions. In addition to this, NVIS can also be used for a number specialist two way radio communications or mobile radio communications applications.
By Ian Poole
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