Near Vertical Incidence Skywave
Encyclopedia
Near Vertical Incidence Skywave, or NVIS, is a radio-wave propagation method that provides usable signals in the range between groundwave and skywave
distances (usually 30 to 400 miles, or 50 to 650 km). It is used mainly for military and paramilitary
communications and by radio amateurs. The radio waves travel upwards into the ionosphere
, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction fails to occur and if it is too low absorption reduces the signal strength.
The usable frequencies for NVIS communications are between 1.8 MHz and 15 MHz. The most common bands used in amateur radio are 3.5 MHz and 7 MHz, with experimental use of 5 MHz (60 meters) frequencies. Military NVIS communications mostly take place on 2-4 MHz at night and on 5-7 MHz during daylight. The lowest layer of the ionosphere, called the D layer, causes attenuation of low frequencies during the day. This layer disappears at night enabling improved communications at the lower frequencies during this time
An NVIS antenna configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from 1/20th wavelength
(λ) to 1/8 wavelength above the ground. That proximity to the ground forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground
wire slightly longer than the antenna parallel to and directly underneath the antenna. While the ground wire is not necessary under good to excellent propagation
conditions, antenna gain
in the 3 dB to 6 dB range are common when the ground wire is used.
Significant increases in communication will be realized when both the transmitting station and the receiving station use NVIS configuration for their antennas.
NVIS is most useful in mountainous areas where line-of-sight propagation
at VHF or UHF frequencies is ineffective or when the communication distance is beyond ground wave (more than 50 miles, 80 km) and less than sky-wave (300 to 1500 miles, 500 to 2500 km).
Skywave
Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...
distances (usually 30 to 400 miles, or 50 to 650 km). It is used mainly for military and paramilitary
Paramilitary
A paramilitary is a force whose function and organization are similar to those of a professional military, but which is not considered part of a state's formal armed forces....
communications and by radio amateurs. The radio waves travel upwards into the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction fails to occur and if it is too low absorption reduces the signal strength.
The usable frequencies for NVIS communications are between 1.8 MHz and 15 MHz. The most common bands used in amateur radio are 3.5 MHz and 7 MHz, with experimental use of 5 MHz (60 meters) frequencies. Military NVIS communications mostly take place on 2-4 MHz at night and on 5-7 MHz during daylight. The lowest layer of the ionosphere, called the D layer, causes attenuation of low frequencies during the day. This layer disappears at night enabling improved communications at the lower frequencies during this time
An NVIS antenna configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from 1/20th wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
(λ) to 1/8 wavelength above the ground. That proximity to the ground forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
wire slightly longer than the antenna parallel to and directly underneath the antenna. While the ground wire is not necessary under good to excellent propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
conditions, antenna gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
in the 3 dB to 6 dB range are common when the ground wire is used.
Significant increases in communication will be realized when both the transmitting station and the receiving station use NVIS configuration for their antennas.
NVIS is most useful in mountainous areas where line-of-sight propagation
Line-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
at VHF or UHF frequencies is ineffective or when the communication distance is beyond ground wave (more than 50 miles, 80 km) and less than sky-wave (300 to 1500 miles, 500 to 2500 km).