Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere long waves can diffract around obstacles like mountains and follow the contour of the earth ( ground waves), shorter waves can reflect off the ionosphere and return to earth beyond the horizon ( skywaves), while much shorter wavelengths bend or diffract very little and travel on a line of sight, so their propagation distances are limited to the visual horizon. Radio waves are very widely used in modern technology for fixed and mobile radio communication, broadcasting, radar and radio navigation systems, communications satellites, wireless computer networks and many other applications. They are received by another antenna connected to a radio receiver, which processes the received signal. Radio waves are generated artificially by an electronic device called a transmitter, which is connected to an antenna which radiates the waves. Naturally occurring radio waves are emitted by lightning and astronomical objects, and are part of the blackbody radiation emitted by all warm objects. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents.
Like all electromagnetic waves, radio waves in a vacuum travel at the speed of light, and in the Earth's atmosphere at a slightly slower speed. Wavelength of a radio wave is inversely proportional to its frequency, because its velocity is constant. At 30 Hz the corresponding wavelength is ~10,000 kilometers (6,200 miles), which is longer than the radius of the Earth. At 300 GHz, the corresponding wavelength is 1mm, which is shorter than the diameter of a grain of rice. Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. In this animation the action is shown slowed down tremendously. Loops of electric field leave the antenna and travel away at the speed of light these are the radio waves. The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. For the British broadcasting station, see Radio Wave 96.5. For the electronics, see Radio frequency engineering. For the generic oscillation, see Radio frequency.
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