Gap-fill exercise

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Radio a system of communication employing electromagnetic waves propagated through space. Because of their varying characteristics, radio waves of different lengths employed for different purposes and usually identified by their frequency. The shortest waves the highest frequency, or number of cycles per second; the longest waves the lowest frequency, or fewest cycles per second. The name of the German radio pioneer Heinrich Hertz been given to the cycle per second (hertz, Hz). 1 kilohertz (kHz) is 1,000 cycles per sec, 1 megahertz (MHz) is 1 million cycles per sec, and 1 gigahertz (GHz) is 1 billion cycles per sec. Radio waves from a few kilohertz to several gigahertz. Waves of visible light much shorter. In a vacuum, all electromagnetic radiation as waves at a uniform speed of about 300,000 km (186,000 mi) per second.
Radio waves used not only in radio broadcasting but in wireless telegraphy, telephone transmission, television, radar, navigation systems, and space communication. In the atmosphere, the physical characteristics of the air slight variations in wave motion, which sources of error in such radio-communications systems as radar. Also, storms or electrical disturbances anomalous phenomena in the propagation of radio waves.
Because electromagnetic waves in a uniform atmosphere in straight lines and because the earth's surface approximately spherical, long-distance radio communication made possible by the reflection of radio waves from the ionosphere. Radio waves shorter than about 10 m (33 ft) in wavelength - designated as very high, ultrahigh, and superhigh frequencies (VHF, UHF, and SHF) - usually not reflected by the ionosphere; thus, in normal practice, such very short waves received only within line-of-sight distances. Wavelengths shorter than a few centimetres absorbed by water droplets or clouds; those shorter than 1.5 cm (0.6 in) may be absorbed selectively by the water vapour present in a clear atmosphere.
A typical radio-communication system two main components, a transmitter and a receiver. The transmitter electrical oscillations at a radio frequency called the carrier frequency. Either the amplitude or the frequency itself may be modulated to vary the carrier wave. An amplitude-modulated signal of the carrier frequency plus two sidebands resulting from the modulation. Frequency modulation (FM) more than one pair of sidebands for each modulation frequency. These the complex variations that as speech or other sound in radio broadcasting, and in the alterations of light and darkness in television broadcasting.