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Classification of radio broadcasts
- Apr 02, 2018 -

Radio broadcasts vary in frequency (RF) frequencies, and can be divided into medium wave broadcasts (526.5~1606.5 khz), Short-wave broadcasts (2.3~26.1 MHz), meter-wave broadcasts (48.5~223 MHz), Decimeter broadcasts (470~958 MHz). The values in parentheses are the frequencies of the current broadcast bands in China. The division of broadcasting bands varies from country to world.

Radio waves are an electromagnetic wave that travels at a speed of 3x108 m/s. The higher the frequency of radio waves, the shorter the wavelength, the product of the frequency (khz) and the wavelength (m) equals the propagation speed of the electromagnetic wave.  The above broadcast bands are named after the length of their wavelength. Amplitude and short wave segments are used for AM broadcasts. FM broadcasts use a portion of the meter band, and China prescribes FM radio frequencies of 87.5~108 MHz.  Due to its signal bandwidth of up to several MHz, television broadcasts use the M-band and Decimeter bands. There are two kinds of wave propagation in medium wave broadcasting, such as seismic waves propagation and sky sky propagation. During the daytime, mainly rely on the ground wave propagating along the surface of the earth, because the surface of seismic waves is absorbed by the media, the attenuation is larger, the transmission distance is relatively, but the transmission is more stable and the receiving effect is better. In the evening, in addition to seismic waves, there is also a part of the High-altitude ionosphere reflected in the sky wave, the wave propagation distance farther.  So for the medium wave broadcast, the service in the evening is larger than the day, some areas can not receive the medium-wave radio programs, in the evening but received. Shortwave broadcasts rely mainly on the wave propagation by the ionospheric reflection.  Because the High-altitude ionosphere changes with time, day and night, and season, the propagation is very unstable, the sound of hearing is light, but the transmission distance is far, mainly for central to remote areas of radio and external broadcasting. The propagation of meter-wave and decimeter broadcasts is limited to the transmission of line-of-sight distance, and its spread is very close. In order to expand the scope of services, the need to increase the height of the transmitting antenna, or the launch pad built on the mountain. If the receiving point exceeds the visual range, the program can be transmitted by means of microwave relay.

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