What is NVIS? - inlandnvis

NVIS stands for Near Vertical Incidence Skywave, and what that means is that the radio operator bounces a radio signal off the ionosphere more or less directly overhead, and is able to send and receive signals to other stations out to perhaps 300 to 500 miles away, depending on the time of day and the state of the ionosphere

Near Vertical Incidence Skywave is a specific operational mode for HF radio communication and the distinguishing characteristics are:

Intentional limitation of communication range;
Use of high angle lower HF radio emissions reflected off the F layer(s) of the ionosphere; NVIS is a type of skywave communication.
Generally, use of low power and reduced height antennas.
Use of frequencies from 1.8 to perhaps as high as 10 mHz in North America (possibly higher during solar maxima or close to the equator); frequency used depends on solar flux, time of day and other factors.

To understand HF radio communications you must understand that there are three main types of radio propagation:

Ground wave, where the signal is usually vertically polarized (perpendicular to the surface of the Earth) and usually low frequency. Ground wave is the propagation mode for most daylight AM radio broadcast stations; the radio waves actually hug the surface of the Earth and travel along the ground. Geologic discontinuities like mountains, rivers, and deep gorges attenuate ground wave propagation, as does the absorption of the RF by the ground. This propagation is inversely proportional to frequency; the higher the frequency, the quicker it attenuates.

Skywave, where the signal travels from the transmitting antenna to the receiving antenna by reflecting one or more times off any one of several layers of the ionosphere. It may include reflection from the ground in between as well. This includes NVIS as well as skip communication, auroral reflection, Sporadic E skip, and tropospheric ducting, among others. This type of propagation is the most common propagation mode for HF radio communication. While the signals may be horizontally, vertically or circularly polarized by the transmitting antenna, the reflected signal can arrive at the receiving antenna with any polarization due to their reflections from the ionosphere and/or the earth. To reiterate, NVIS is a skywave propagation type, because the signal bounces off the ionosphere and comes back to Earth.

Line of sight, (LOS) where the transmitting antenna and the receiving antenna are in view of one another, and propagation is not by means of ionospheric reflections or by ground wave. This propagation can be horizontally, vertically or circularly polarized. LOS signals can be bounced off structures or geologic features like cliffs, too, or refracted off mountain tops, and while the purist might argue that these are not line of sight anymore, the generally accepted definition includes such reflections. Much VHF and UHF communication is LOS, although there are exceptions.

NVIS does not use VHF or UHF frequencies, does not use multiple hops or cover great distances, does not require high antennas, and does not typically use high power.

Propagation

NVIS is a communications system that permits coverage up to 500 km (300 miles) or so using relatively low power equipment. To do so, the operating channels must be below the Critical Frequency, the highest frequency where signals radiated straight up will be returned to Earth by the ionosphere. Above that frequency, signals pass off into space, even though they may be reflected back when striking the ionosphere at flatter angles. Generally the critical frequency may get as high as 12 MHz in the tropics or 9 MHz at higher latitudes, depending on the current ionospheric conditions, so 160m, 80m, and 40m (and 60m where it is available) are the most likely bands to use. It varies throughout the day, with the seasons, and with the 11-year sunspot cycle, as well as with latitude.

At the other end of the spectrum, the minimum frequency is generally determined by RF absorption in the D layer, which increases with ionization. Unlike the maximum frequency, increasing power may allow sufficient signals in marginal conditions. When D layer absorption is excessive at the Critical Frequency, then NVIS won’t work.

Successful NVIS operation requires being able to change frequency to suit current conditions, rather than making assumptions and hoping the ionosphere will cooperate.