Frequency band

Frequency band, is a communication related and sound streaks of words, communications aspects band means certain radio frequency range.

Communication frequency

concept

Band is certain frequency range.

For example, we use the radio, some can reap medium-wave, some can reap medium-wave, shortwave, and FM. People purchase the radio and total first to make sure it can receive several band, the band is equivalent to what we call tuner.

According to international radio rules, the existing radio communication was divided into aviation communication, sailing communication, land communications, satellite communication, radio, TV, radio navigation, positioning and remote measure, remote control, space exploration etc 50 kinds of different business, and for each business have stipulated certain frequencies.

Communication frequency division

Various communications system to use channel frequencies and a selective and rationality of the assignment problem, so reasonable use and try to save frequency resource, meet efficient and reliable transmission requirements.

For the cable channel, important is to choose a different transmission media and broadband media channel frequency reuse. General according to the channel business requirements, considering each they required of the foregoing cable channel (constant) the performance features of parameter, such as loss, delay and phase shift characteristics, and minimum and maximum cutoff frequency etc, to determine the tuner.

Undersea communication is suitable for extremely low frequency band, it has very good transmission performance, Any baseband signal transmission using baseband signal bandwidth for all the cutoff frequency low frequency band, analog voice low frequency transmission using only 300 ~ 3400Hz or quality sound (music) from 50Hz to 15kHz bandwidth.

Complex problem is, all kinds of wireless communication according to the space electromagnetic wave propagation characteristics, to choose and proper distribution working band. Itu-t R of spectrum allocation made specific rules, in various countries and various departments shall scientific and strict control frequency point use.

Electromagnetic wave by launching to receiving way can be divided into three kinds: one is to rely on through the ground; called "ground" 2 it is by space between two straight line dissemination called a "space; wave" Three is to rely on earth above the ionosphere reflection to the ground one hop or multiple hops strightline, called "TianBo".

Along the surface of the ground spread, because along the ground electromagnetic wave propagation induced current, jumping by ground this kind of good conductors attenuation, and the higher the frequency, the greater the integrating skin effect loss is bigger. Therefore for ground in mw (i.e. longwave and hundreds of KHZ to a few MHZ), such as broadcasting from 535kHz to 1605kHz frequency (each 10kHz a program) is an example.

Few MHZ to dozens of MHZ of short-wave (FFT) suitable for TianBo spread, sending the distance between the far outweigh the ground, can reach hundreds of kilometers to thousands of kilometers, depending on the antenna Angle size. Had mentioned above, the ionosphere to the reflection of the electromagnetic absorption, the attenuation, ionization concentration is bigger, and the loss caused by the ionosphere random changes in electromagnetic wave attenuation is undulating decline phenomena.

If a shorter wavelengths, i.e. higher frequency range, such as hundreds of megahertz to several JiHe (109Hz) above, the into microwave band. This frequency electromagnetic wave, the ionosphere, and no longer absorb rarely be reflected back to the surface. Such as satellite communications, electromagnetic wave can penetrate the ionosphere spread to a satellite. This kind of space wave propagation and light have analogue, not only straight line dissemination, and electromagnetic wave also have diffraction (diffraction) function, and can be bypassed some local obstacles. For example, microwave relay apanage pastry and between points straight line dissemination, besides should suffer ground environment (swamp, mountain, forest etc) certain influence outside, antenna, so too inconvenience erection relay (relay) section but four, 50 kilometers, usually called "over" communication.

Wireless communication all needs to transceiver antenna length and wavelength lambda matched antenna size for 1/4 · lambda, so use omni-directional antenna with the broadcasting radio antenna may not stable erect 100 meters. Using the GSM mobile phones 900MHz band antennas, can be short as a few centimeters long, for the mobile phone miniaturization portable bring great convenience.

All wireless communication are spread through free space, in order to reasonable use frequency, regions of communication and from mutual interference, ITU scientifically distribution of various communication system of applicable frequencies, each band frequencies and wavelength corresponding values and name, by the international telecommunication union (ITU wireless committee - R) promulgated, the countries and regions and cities are equipped with corresponding radio management committee for homeland and this district wireless fixed-frequency reasonable coordination.