Introduction Radio communication was made by radio waves which are electromagnetic waves like light 10 waves. These radio waves travel in space with the speed of light which is 3×10 cm/s. The radio waves get attenuated or weakened as they travel out in space, partly due to the absorption of their energy by reflection and refraction in the ionosphere, partly by the ground and partly by other objects. The propagation characteristics of radio waves of different frequencies mainly decide the use to which these waves can be put to for communication purposes. Radio waves are accordingly been divided into various categories or classes with regard to their frequencies and propagation characteristics. Classification of radio waves and their propagation characteristics Class Frequency range Propagation characteristics and typical uses Very Low 10 to 30 kHz Low attenuation and propagation characteristics reliable all day – used for long distance Frequency (VLF) communication Low Frequency 30 to 300 kHz Day time absorption more than VLF – used for marine communication and navigation aids (LF) Medium 300 to 3000 kHz High attenuation during day and less attenuation at night – suitable for broadcasting and marine Frequency (MF) communication High Frequency 3 to 30 MHz Propagation characteristics vary with time of day, season and frequency – used for long distance (HF) communication Very High 30 to 300 MHz Line of sight propagation, not affected by ionosphere – used for television, FM transmission, Frequency (VHF) radar etc. Ultra High 300 to 3000 MHz Line of sight propagation – used for television and short distance communication Frequency (UHF) Super High 3000 to 30000 MHz The same as UHF Frequency (SHF)
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