Sunday, January 16, 2011

【 Weak current College 】 electromagnetic spectrum (2) 】

Keywords: electromagnetic wave spectrum
Time: location: user: label type: operating frequency: standard: implementation of the company: solution providers: hardware providers:
【 Summary 】 electromagnetic spectrum (2)

The following will be divided by the band to discuss the features and frequency bands.

1.10 ~ 200 kHz band
The band are very long-wave and long wave band, because of their similar characteristics, and discuss together.
The band can spread with SkyWave and ground wave, but mainly to ground wave propagation mode. As a result of ground wave propagation, the higher the frequency, the greater the absorption of the Earth, so early in the radio to the development of low-frequency. SkyWave is electromagnetic wave on the ground and ionospheric reflection and transmission between back and forth.
The band is characterized by: (1) transmission distance, in number of seawater on the application of power can achieve kilowatt 3000 kilometres of communication. So there are still many marine radio communication using the longwave (30 ~ 200 kHz). 10 ~ 30 kHz can achieve extremely long distance communication. (2) the impact of ionospheric disturbances. Long-wave propagation and stable, no fading phenomenon. (3) the longer wavelength, land or sea water absorption of less, therefore suitable for underwater and underground communications.
But the disadvantage is obvious: (1) capacity. Long-wave the entire bandwidth only 200 kHz, the capacity is limited and cannot accommodate more than one station in the same area. (2) large atmospheric noise. Because of the frequency, the lower the more atmospheric noise (also atmospheric disturbances and geographical location, near the equator, the greater interference). (3) require large antennas.
The allocation of frequency bands. According to international requirements, 10 to 200 kHz is used mainly for radio navigation (aviation and marine), targeted communications, maritime mobile communications and broadcasting.
Be specified navigation frequency is 10 ~ 14 kHz and 70 ~ 130 kHz. This is as of the long range navigation, mainly because of the long-wave propagation, and without blind spots. In the navigation system, blind spot is not allowed. In 70-130 kHz work Ang LAN-C system and sets of cards (Decca) system.
Maritime mobile communications is mainly used for shore-ship traffic. Due to the long wave of high reliability, therefore, when capacity is not a major, which require high reliability and long distance communication, the use of this band, and is especially suitable in the polar regions of shore-ship traffic. Ship-shore communication typically do not have this band, because the boat position is limited, you cannot get high efficiency of the antenna.
Almost the entire band are allocated for fixed-point communications, which is currently used as a backup using the shortwave communication, so that when you use the ionosphere disturbance. Now it seems that the need has been gradually reduced, with the exception of a few areas, most regions have no, last such use will be discarded.
In Europe and Africa, with 150 ~ 200 kHz as broadcasting frequencies. This long-wave radio features, both day and night are considerable and stable service area.
There is also a standard frequency transmissions require 20 kHz.
Military, Longwave is useful mainly underground (trenches) communication can consider using this frequency band. The band's main drawback is that the capacity of the small dimensions of the antenna. Therefore, the majority of underground communications now still using short-wave. Second, 200 ~ 3000 kHz band
The band was mainly in the waves. Electromagnetic wave in the wave, the main mode of transmission is ground wave propagation. At the low end of the band than high-end communication better. For frequencies increase, the ground of the distinction is not very significant. In the daytime sky basically to the ionosphere (D) the absorption and therefore cannot rely on SkyWave propagation. But the night, because of the D-layer disappear, by E-reflection, SkyWave propagation can reach a considerable distance, but it is also used for ground wave propagation disturbance those business a source of interference.
The band features: mainly rely on ground wave propagation, secondary transmission distance (tens to hundreds of kilometres), signal stability.
The band is mainly used for broadcasting, radio navigation, maritime mobile communications, air traffic. Due to the characteristics of wave propagation in, especially suitable for regional broadcasting business. 535-1605 kHz is the international requirements of the broadcasting segment.
In the band, 200 ~ 415 kHz for short distances with a radio-navigation system, of which 285 ~ 325 kHz, 405-415 kHz for maritime navigation, are air navigation. In addition, 1800-2000 kHz as Roland A system available.
The band's maritime mobile communications, is in the early days of radio is arranged, 415 ~ 525 kHz, 500 kHz fixed frequency for SOS distress at sea, the other shall not be used for any business. In order to use smaller antenna equipment, sea, boats with this band of high-end, 1800-2000 kHz.
The oldest aviation mobile communication, namely the communication for aircraft, is the use of 2850-3025 kHz. Now, many countries have to move it to the very high frequency (VHF) band. But this is still a band needs, especially when the communications path contains a large amount of water.
Standard frequency in the band 2500 kHz.
Due to historical reasons, the band radio seem overcrowded, so you should put some business transferred to other more suitable frequency. For example, aviation and maritime mobile communications should move out, and more rational allocation of the frequency that updates. [NextPage]
3, 3 ~ 30 MHz band
The band for short-wave band. HF ionospheric communication simple, easy-to-implement, low cost, low power available and the smaller antenna for long range communication, which is its advantages.
However, short-wave also has serious flaws, that communication is not stable. To remain fully communicate several frequencies must be replaced. Because of the ionosphere 11-year cycle of solar activity, when large, can be used to 3-30 MHz, and when the Sun's activity in the smallest time only 3 ~ 15 MHz can be applied. So HF communication must have full-band frequencies in order to adapt.
At the same time, there is a serious fall short, it is necessary to adopt diversity reception in order to get more stable communication. Usually adopts frequency diversity, this means that the need to take two frequencies, this already crowded shortwave bands is a difficult problem.
In addition it also affected by the impact of ionospheric disturbances, natural disturbance, such as the atmosphere is relatively large.
HF communication, using a frequency, using SkyWave can only reach a certain distance away (because if the distance to close, you must increase the elevation when electromagnetic waves will pass through the ionosphere and not reflected), while the ground wave propagation and can only be reached relatively close. Therefore, the distance between the two, both have not received the ground wave and sky wave, does not get called blind spot or a quiet zone (Figure 1). This is typical of the HF bands. So the short wave bands should not be used for navigation.
In short-wave band, the use of ground wave propagation traffic is scarce, because the short wave bands of ground wave propagation close, distant point attenuation is great. Therefore, in addition to military tactical HF radio also uses small ground-wave communication, elsewhere is little used.
Due to historical reasons, shortwave problems is a band too crowded. At the same time, due to short-wave transmission distance, and prone to interfere with each other, therefore, spectrum utilization is an issue. The current solution is: (1)use of SSB, the existing short-wave radio station are replaced by SSB. CCIR recommended in 1973 before all of the mobile station is replaced by the single sideband., fixed more than 100-Watt radio station in 1967 has been completely replaced by SSB. (2) is not required in this band of fixed-point communications business has moved out of the band. Short range radio does not allow the creation of the band.
Shortwave is mainly used for fixed-point communications, maritime and aeronautical mobile telecommunication, broadcasting, tropical broadcasting and amateur radio.
1. in a fixed-point communications
Allocation to the band's more, a total of 25, distributed throughout your band, successes, not here. Because of this band radio too crowded, interfere with each other and, therefore, measures have been taken. For example, more than 100 Watt radio are replaced by single sideband system, or to independent side-band system. And that will be part of the high-frequency fixed communications business for more than 100 MHz instead of multiple systems or use cable communications.
2. maritime and aeronautical mobile communication
Navigation with mobile communication between for ship-shore, from 4063 kHz to 27.50 MHz, divided into 10 segments. Mainly used for long distance communication. Aviation remote communication using high-frequency frequency, as on an airplane remote communication must rely on the ionosphere reflection, using the shortwave cannot achieve this far. Use a lower frequency, the aircraft did not allow loading of large-volume of antenna, generally used the HF bands (divided into airline business and non-airline operations), ranging from 2.852 MHz to l 8 MHz. In order not to interfere with any other business, through international agreements, mostly restricted to a narrow range is not large in-band. This business-to-use radio requirements all in 1973, replaced by SSB.
3. broadcasting business
Short-wave-band remote broadcast only suitable band.International agreements are designated to short-wave broadcasts of seven segments, i.e. 5.95 ~ 6.2 MHz, 9.5 ~ 9.77 MHz, 11.7 ~ 11.975 MHz, 15.1 ~ 15.45 MHz, 17.7 ~ 17.9 MHz, 21.45 ~ 21.75 MHz, 25.6 ~ 26.1 MHz. 7.1 ~ 7.3 MHz is used by old world countries. This band is particularly heavy, according to statistics, in 1963, the band has a total of 130 countries and regions in the 2000 Department of short wave radio transmitter. Of which 55% were foreign broadcast, now even more crowded.
4. tropical broadcasting business
In the tropics, the relative atmospheric disturbances are large, medium-wave broadcast interference noise level is high, so local broadcasting also used the HF bands (5 MHz). There are three-band allocated to them, namely 3.2 ~ 3.4 MHz, 3.9 ~ 4.0 MHz, 4.75 ~ 5.06 MHz. In order to guarantee their regional without interference from other radio communications, the maximum radiation direction must be upward.
5. vehicle mobile radio
This section only short-wave spectrum of very small parts. It is usually as close communication, it is recommended that this part of the business moved to VHF or UHF (m spread decimeter) band.
6. the amateur radio service
Amateur radio services in a foreign country is unique, so part of the International Division also for amateur radio bands. This portion of the radio station has also been suggested to move to the UHF band.
7. industrial, scientific, and medical business
In this band for engineering, science, medicine and business set aside two frequencies, 13.56 MHz and 27.12 MHz (both are the harmonic relationship), and is strictly limited to the frequency.
Secondly, in the band also has a standard frequency. Its frequency to 5 MHz, 10 MHz and 15 MHz, 20 MHz and 25 MHz. Fourth, 30-1000 MHz band
This band is the very high frequency (VHF) and super high frequency (UHF). This band is an "intermediate" band. It basically cannot be reflected in the ionosphere, but in the VHF band of low-end can also bereflected (in solar activity is high year and E1 layer), usually 60 MHz. This is often a cause of interference.
Ground wave propagation of the short distance, but there are military tactical radio wave for short distance communication used. Mainly use the low end of the band.
In the band play a major role in transmission is the apparent distance of space wave propagation, and tropospheric scatter and ionospheric scatter.
And high-frequency band, the band has the advantage that, for low capacity systems can use the small size of the antenna. Obviously, this feature is particularly suitable for mobile communication. In the radio-relay systems, with a higher frequency, while the propagation lossIncrease, but the higher the antenna gain can compensate for this part of the loss. Therefore, the use of this band high-frequency end is appropriate, and capacity can be increased through more channels.
Tropospheric scatter in some occasions the place of radio relay systems, because it can not hop, a skip several hundred kilometers, while also has a large capacity (multiplexed), and this in low frequency is not possible.
The band frequency assignments. Main distribution in broadcasting, land-based mobile communications, aerospace, mobile communications, maritime mobile communications, fixed communications, space communications, radar, etc.
1. the broadcasting business
FM broadcasting distribution in the 88-108 MHz, while the TV broadcast is assigned in the 41-100 MHz, 170 ~ 216 MHz, 470 ~ 960 MHz (each country vary).
Land-based mobile communications mainly vehicles radio or carry radio use, its main band at 500 MHz. In the lower-frequency end, due to atmospheric noise interference is large, it is not appropriate in a city application (city people-noise level is high). The city used in the 450 MHz or so.
2. aeronautical mobile communication
Air to ground communication use 118 ~ 139 MHz. They are close to Visual range of mobile communication. When the altitude of 1500 meters, sight distance is approximately 130 km/h. When the height is 12000 m, about 320 km/h. This communication are using pre channel way through mobile phones.
Maritime mobile communications mainly for inland waterway, port range or high seas vessel (Sprint) communication. 156.8 MHz to international requirements of VHF frequency section for help.
3. in a fixed-point communications
In almost 30 to 1000 MHz for the entire range. It works with line-of-sight, tropospheric scatter, and a variety of ionospheric scatter. But judging from the current look, work in this frequency range radio increases quickly. The reason for this is that if you work in more than 1000 MHz frequency, antenna gain boost, and large capacity, easy multi-channel, also on interference control easier. So the frequency used in more than 1 stem MHz more some of radio.
Work on fpis is 30 ~ 60 MHz range, minimal communication distance is 1200 km. It requires high power and large antenna, which is its shortcomings, but it can be compared with high-frequency of SkyWave propagation provides more reliable communication.
Tropospheric scatter the VHF and UHF for horizon, long-distance communication, it has superior than HF channel, it can be a long jump up to 800 km (several sessions), or in more recent distance on route 120 voice circuits.
4. space communications
The current allocation in the band are:
136-137 MHz — space research of the telemetry and tracking purposes.
137-138 MHz a operating system for telemetry and tracking purposes.
400 ~ 401 MHz — for meteorological satellite.
401-402 MHz — for space telemetry.
5. the air navigation applications
108-118 MHz allocated to blind landing system. 75 MHz for aviation airport beacon. 420 ~ 460 MHz for radio altimeter.
6. the use of radio astronomy
Only specify several narrow-band for radio astronomy use, i.e., 38, 80, 405, 610 MHz, etc. Other there is radar (specified in 216-225, 400 ~ 450, 890-942 MHz), amateur radio, and standard frequency and time signal service. Engineering, science, medical, the frequency is specified as 40.68 MHz. [NextPage]
5.1000 ~ 10000 MHz band
The band belongs to the decimeter wave band to cm (30 cm to 3 cm). In 1959, Geneva's radio session, this band has been allocated to the fixed and mobile communications, navigation, radar, weather, radio astronomy, space communications, amateur radio and workers, branch, hospital use.
The band of communication characteristics are apparent distance communications, atmosphere and low noise, but in certain frequency ranges (3 cm wavelength), absorption of atmospheric (water vapor). In addition, the band also disseminated by scattering. Since the band is not too crowded, so that the current distribution is not a big problem.
The band frequency assignments. Targeted communication and mobile communication service in the frequency range, mainly radio microwave relay system, many with great capacity. In addition, because the development of tropospheric scatter communications, there are also a lot of fixed communication station using tropospheric scatter.
Mobile communication in the frequency bands to vehicle radio in the form of relatively small, most are temporary fixed-point communications (i.e. belong to transportation-equipment), the movement of traffic is rarely used this band.
In the band, because there is no atmospheric noise interference, at the same wavelength short antenna beam easily done very narrow, radio navigation and radar particularly appropriate. In l000 ~ 10000 MHz range, in 18 per cent to navigation, and 30 per cent to radar and positioning. In fact, in the frequency band of navigation, basically it is the use of radar technology.
1. space communications
The band is the most important space communications frequencies, this is because the band frequency bandwidth, can accommodate a large number of channels. At the same time, the current radio technology development and dissemination of the electromagnetic waves are suitable for the band. In the 3400-8500 MHz, 200 MHz allocated for satellite use, and low-power mobile communication.
2. the radio astronomy service
Assigned to 1400 ~ l427 MHz and 16644 ~ 16684 MHz. This difference is on the atomic hydrogen and hydroxyl (OH) ion radiation observation of the band. Additionally allocated 2690 ~ 2700 and 4990 ~ 5000 MHz for the observation of the continuous spectrum of radiation.
Engineering, science, medical frequency assignment 2450 ± 50 MHz. More than six, 10000 MHz band
The frequencyParagraph essentially millimeter-wave band. Currently only assigned to 40 GHz (8 millimeter wave length), then high has not made the assignment.
10 ~ 30 gigahertz spread situation basically light propagation, but its propagation loss in high frequency range of high-end loss than a low-end to large, and are subject to the influence of rain. However high antenna gain can compensate for this loss. Currently, this part of the low end is more suitable for radio-relay (relay) communication, space communications, radar, navigation, radio astronomy, and other applications.
Stem from 40 MHz to 30 GHz (this is the wave of lower limit), in addition to the laser, still not very good. Pending subsequent research and development. VII. conclusions
Frequency allocation is based on the characteristics of electromagnetic wave propagation (that is, the characteristics of a variety of channels) and a variety of device communications business requirements, but also some other factors to consider. For example, historical development, international agreements, country-specific policies, the current situation and to avoid interference. Therefore, although the international frequency allocations was identified, but also you can on this basis based on your own country's specific circumstances and policies, to the specific assignment.
Military Division of the frequency, the general principle isthe same. However, due to military requirements for a wide range of high, therefore mainly based on the means of communication and transmission characteristics, and is not fully supported by the international frequency allocations. But some frequency, such as crying for help, and so on, should not be occupied and interference. In addition, because civil have used these frequencies, so the selected frequency, also consider the frequency of interference. The primary information source for references:
[1] jianglin. Summary of radio wave propagation and spectrum utilization. Beijing: defense industry press, 1974
[2] Liu Yang Wang Yu-Min. automatic train identification system principles and applications-Beijing: China railway Publishing House, 2003

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