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A Near Vertical Incident Skywave antenna, or NVIS antenna, transmits a radio wave up into the sky at an angle that is nearly vertical. Transmitting a radio wave in this manner allows it to bounce off the Earth’s ionosphere and back toward the ground, which makes it less susceptible to the limiting effects of the Earth‘s curvature. As a result, a radio signal transmitted with an NVIS antenna is receivable at a much greater distance than one transmitted with a conventional antenna.
Conventional transmitter antennas transmit radio waves horizontally, roughly parallel to the ground, in a straight line. In shorter-range applications, such as television, this provides maximum reception power in the immediate vicinity of the transmitter. As the Earth’s surface curves and the transmitted signal travels in a straight line, the two grow farther apart as the distance from the transmitter increases.
Eventually, usually 50 to 70 miles depending on local topography, the signal will no longer reach the surface of the Earth and will not be receivable. Placing the transmitter on a tall tower will allow it to transmit at a slight downward angle. This will compensate for some of the Earth’s curvature; however, it usually adds only a few miles to the range and is more useful in overcoming local topography than the Earth‘s curvature.
When transmitting with an NVIS antenna, the signal does not travel horizontally. Instead, the signal travels upward at a 75 to 90 degree angle. When the signal reaches the Earth’s ionosphere, the ionosphere reflects the signal back to the surface of the Earth at an angle corresponding to the transmission angle. As a result, a signal from an NVIS antenna is receivable at a range of approximately 100 to 250 miles from the transmitter, depending on the exact angle of the original transmission.
Despite its seemingly superior performance, the transmission signal from an NVIS antenna cannot be of just any frequency. As the Earth’s ionosphere constantly varies, it will consistently and reliably only reflect radio waves in the 3.5 to 7.3 megahertz range. Under ideal conditions, this range may extend from 2 to 10 megahertz; however, such conditions are always temporary. These frequencies are too low for television and other broadband signals, which are typically transmitted above 30 megahertz.
If a signal’s transmission frequency is outside the ionosphere’s reflective range, the ionosphere will either absorb the signal or allow the signal to pass through it and off into deep space. Despite the limitations of NVIS transmissions, they can establish reliable, long-range voice communications, which are useful to the military and amateur radio operators. Researchers also use NVIS antennas to study and measure the Earth’s ionosphere by analyzing how it reflects radio signals.