NVIS Propagation: Near Vertical Incidence Skywave

NVIS propagation, Near Vertical Incidence Skywave propagation is a form of radio wave propagation used on the MF and HF bands to provide radio communications and broadcasting coverage over short distances, especially where the terrain contains obstacles.

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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. It fills the gap between line of sight and the longer distance skip type communications that are normally used at HF.

The use of NVIS propagation requires careful frequency selection, particular styles of antennas and often a good degree of operator skill.

NVIS applications

NVIS propagation 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 propagation 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.

What is NVIS propagation

NVIS propagation requires a high angle or near vertical signal to be 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. Typically it is just below the critical frequency for the ionospheric layer or region that is to be used.

The critical frequency varies according to ionisation density in the relevant ionospheric layer or region which in itself is dependent upon the radiation received from the Sun.

Accordingly it is dependent upon the sunspot cycle, time of day, season and a variety of other factors.

Near Vertical Incidence Skywave, NVIS, propagation
Near Vertical Incidence Skywave, NVIS propagation

When NVIS propagation is being used, 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.

Accordingly NVIS 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.

NVIS frequencies & angles

Typically radiation should be at angles greater than 75° or 80° to the horizontal so that good local coverage is achieved. Typically coverage areas may have a radius of between 35 and 350 km.

It is possible to show geometrically that with an radiation angle of 50° and using F2 reflections, it is possible to achieve ranges up to around 450 miles, i.e. around 727 km.

The frequency needs to be chosen carefully for NVIS. The frequency selection is normally a balance between the reducing the D layer losses and achieving high angle reflection. Too low, and although good reflection might be achieved from the higher layers of the ionosphere, the D layer losses will be much greater. Too high and the reflections from the F layers may not be achievable.

Also by keeping the angle of radiation as high as possible the path through the D region is shorter and this reduces the losses and improves the local coverage signal significantly.

 NVIS, propagation losses lower for high angle radiation
NVIS, propagation losses lower for high angle radiation

Frequencies used for NVIS are normally 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.

NVIS antennas

When using NVIS propagation it is necessary to use a suitable form of antenna.

In order to be able to make use of NVIS propagation, it is necessary to radiate the majority of power at a high angle, 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.

Often radio antennas specifically designed to enable the required high angle of radiation to be achieved for NVIS propagation. Typically this is achieved by using a horizontally polarised antenna. Horizontal dipoles are a favourite type.

In addition to this the antenna should be mounted at an elevation that will maximise the high angle of radiation. Elevation heights of between 0.1λ and 0.25λ are generally used because dipoles that these heights radiate most of their energy upwards.

NVIS propagation is a way of obtaining local coverage. It has been used effectively for broadcasting in what are termed the “Tropical Bands” where local coverage is often difficult to achieve by more traditional means because of mountainous terrain and forests.

NVIS propagation is also used in many other applications to obtain short range coverage when other techniques are not applicable.

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