What is the antenna nvis?
Antenna almost vertical incident Skywave or NVIS antenna transmits a radio wave to the sky at an angle that is almost vertical. The transmission of radio waves in this way allows to bounce off the Earth's ionosphere and back to the ground, which is less susceptible to the limiting effects of the Earth's curvature. As a result, the radio signal transmitted by the NVIS antenna is viewable at a much greater distance than the transmitted conventional antenna. In shorter range applications, such as television, it provides maximum energy energy in the immediate vicinity of the transmitter. As the ground surface curves and the transmitted signal travel on a straight line, two continue to grow apart, as the distance from the transmitter increases. Darwing reach to the surface of the Earth and will not have a claim. Placing the transmitter on the high tower allows him to broadcast at a slight angle down. This will compensate for some of the curvature of the country; Usually, however, it adds only a few kilometers to the range and is more useful in overcoming local topography than the curvature of the country.
When transmitting the antenna NVIS signal does not travel horizontally. Instead, the signal travels up at an angle of 75 to 90 degrees. When the signal reaches the Earth's ionosphere, the ionosphere reflects the signal back to the ground surface at an angle corresponding to the transmission angle. As a result, the NVIS antenna signal is a view of approximately 100 to 250 miles from the transmitter, depending on the exact angle of the original transmission.
Despite its seemingly superior performance, the NSMis from the NVIS antenna can not only have any frequency. Since the Earth's ionosphere is constantly different, only radio waves will be consistently and reliably reflect only radio waves of 3.5 to 7.3 megahertz. Under ideal conditions, this range can expand from 2 to 10 megahertz; However, such conditions are always temporary. These frequencies are too low for TV and other broadband signals that are usually transmitted above 30 megahertz.
If the transmission frequency of the signal is outside the reflective range IoNosfers, the ionosphere either absorbs the signal, or allows the signal to pass through it and turn off into a deep space. Despite the limits of NVVA transmission, they can create reliable voice communication with long distance, which is useful for the army and amateur radio operators. Scientists also use NVIS antennas to study and measure the Earth's ionosphere by analyzing how radio signals reflect.