2.1 Omni-directional antenna Omni-directional antenna, that is, it is uniformly radiated in 360 ° in the horizontal direction diagram, which is usually called non-directional, and is expressed as a beam with a certain width in the vertical direction diagram. In general, the lobe The smaller the width, the greater the gain. The omnidirectional antenna is generally used in the mobile communication system and the station type of the suburban county system, and the coverage is large.
2.2 Directional antenna The directional antenna is radiated in a certain angle range in the horizontal direction diagram, which is usually called directional, and in the vertical direction diagram is expressed as a beam with a certain width. Like the omnidirectional antenna, the lobes The smaller the width, the greater the gain. Directional antennas are generally used in urban cell-type stations in mobile communication systems, with small coverage, high user density, and high frequency utilization.
Different types of base stations are established according to the requirements of networking, and different types of base stations can choose different types of antennas as needed. The basis for selection is the above technical parameters. For example, an omnidirectional station uses omnidirectional antennas with substantially the same horizontal gain, while a directional station uses a directional antenna with a significant change in horizontal gain. Generally, an antenna with a horizontal beam width B of 65 ° is selected in the urban area, and an antenna with a horizontal beam width B of 65 °, 90 ° or 120 ° in the suburbs (depending on the station configuration and local geographical environment), while in the rural areas Choosing an omnidirectional antenna that can achieve a wide range of coverage is the most economical.
2.3 Mechanical antenna The so-called mechanical antenna refers to a mobile antenna that uses mechanical adjustment of the downtilt angle.
After the mechanical antenna is installed perpendicular to the ground, if the network optimization is required, it is necessary to adjust the position of the bracket on the back of the antenna to change the tilt angle of the antenna. During the adjustment process, although the coverage distance in the direction of the main lobe of the antenna changes significantly, the amplitudes of the vertical and horizontal components of the antenna do not change, so the antenna pattern is easily deformed.
Practice has proved that the best downtilt angle of mechanical antenna is 1 ° -5 °; when the downtilt angle changes from 5 ° -10 °, the antenna pattern is slightly deformed but the change is not big; when the downtilt angle is 10 ° -15 When the ° changes, the antenna pattern changes greatly; when the mechanical antenna tilts down 15 °, the shape of the antenna pattern changes greatly, from a duck pear shape without a tilt to a spindle shape, although the main lobe direction covers the distance Obviously shortened, but the entire antenna pattern is not in the base station sector, and the signal of the base station will also be received in the adjacent base station sector, resulting in serious intra-system interference.
In addition, in daily maintenance, if you want to adjust the downtilt angle of the mechanical antenna, the entire system should be shut down. You cannot monitor the antenna while adjusting the inclination angle; adjusting the downtilt angle of the mechanical antenna is very troublesome, and generally requires maintenance personnel to climb to the antenna placement Make adjustments; the downtilt angle of the mechanical antenna is the theoretical value calculated by computer simulation analysis software, and has a certain deviation from the actual optimal downtilt angle; the step angle of the mechanical antenna to adjust the inclination angle is 1 °, and the third-order intermodulation index is -120dBc.
2.4 Electrically adjustable antenna The so-called electrically adjustable antenna refers to a mobile antenna that electronically adjusts the downtilt angle.
The principle of electronic downtilt is to change the phase of the collinear array antenna element, the amplitude of the vertical component and the horizontal component, and the field strength of the combined component, thereby tilting the antenna's vertical pattern. Since the field strength of the antenna increases and decreases in all directions at the same time, it ensures that the antenna pattern does not change much after changing the inclination, which shortens the main lobe direction coverage distance, and at the same time reduces the coverage of the entire directivity pattern in the serving cell sector Area but without interference. Practice has proved that when the downtilt angle of the ESC changes from 1 ° to 5 °, the antenna pattern is roughly the same as that of the mechanical antenna; when the downtilt angle changes from 5 ° to 10 °, the antenna pattern is slightly smaller than that of the mechanical antenna. Improved; when the downtilt angle changes between 10 ° -15 °, the antenna pattern changes more than the mechanical antenna; when the mechanical antenna tilts down 15 °, the antenna pattern is significantly different from the mechanical antenna, then the antenna The shape of the pattern does not change much, and the coverage distance in the direction of the main lobe is significantly shortened. The entire antenna pattern is within the sector of the base station. Increasing the downtilt angle can reduce the sector coverage area, but does not cause interference. Such a pattern is What we need, so the use of electrically tuned antennas can reduce call loss and reduce interference.
In addition, the electrically adjustable antenna allows the system to adjust the downtilt angle of the vertical directivity graph without stopping the machine, and monitor the effect of the adjustment in real time. The stepping accuracy of adjusting the inclination angle is also high (0.1 °), so the network can be refined. Adjustment: The third-order intermodulation index of the electrically adjustable antenna is -150dBc, which is 30dBc different from that of the mechanical antenna, which is beneficial to eliminate adjacent frequency interference and spurious interference.
2.5 Dual-polarized antenna Dual-polarized antenna is a new type of antenna technology. It combines + 45 ° and -45 ° antennas with mutually orthogonal polarization directions and works simultaneously in the transceiver duplex mode, so its most prominent The advantage is to save the number of antennas of a single directional base station; generally, the directional base station (three sectors) of the GSM digital mobile communication network needs to use 9 antennas, and each sector uses 3 antennas (spatial diversity, one transmission and two receptions). Polarized antenna, each sector only needs one antenna; at the same time, in a dual-polarized antenna, the polarization orthogonality of ± 45 ° can ensure that the isolation between the two antennas of + 45 ° and -45 ° meets each other Adjust the isolation requirements between the antennas (≥30dB), so the space between the dual-polarized antennas only needs 20-30cm; in addition, the dual-polarized antennas have the advantages of electrically tuned antennas, the use of bipolar in mobile communication networks The antenna is the same as the ESC antenna, which can reduce call loss, reduce interference, and improve the service quality of the entire network. If a dual-polarized antenna is used, since the dual-polarized antenna does not have high requirements for erection and installation, no land acquisition is required to build a tower, only an iron column with a diameter of 20 cm is required to fix the dual-polarized antenna on the iron column according to the corresponding coverage direction That is, it saves capital construction investment, at the same time makes the layout of the base station more reasonable, and the selection of the base station site is easier.
For the choice of antenna, we should choose the mobile antenna suitable for the needs of the mobile network in the region according to the actual situation of our mobile network coverage, traffic, interference and network service quality:
--- In high traffic areas with dense base stations, dual polarized antennas and electrically adjustable antennas should be used as much as possible;
--- In areas where the traffic volume is not high, such as borders and suburbs, where base stations are not dense and areas that only require coverage, traditional mechanical antennas can be used.
China's current mobile communication network has high call loss and high interference in areas with high traffic density. One of the important reasons is that the mechanical antenna downtilt angle is too large, the antenna downtilt angle is too large, and the antenna pattern is severely deformed. To solve the problem of insufficient capacity in the high traffic area, it is necessary to shorten the station distance and increase the antenna downtilt angle. However, when using a mechanical antenna, when the downtilt angle is greater than 5 °, the antenna pattern begins to deform. When it exceeds 10 °, the antenna pattern Severe deformation, so using mechanical antennas, it is difficult to solve the problem of high call loss and high interference in high-density areas of users. Therefore, it is recommended to replace the mechanical antenna with an electrically adjustable antenna or a dual-polarized antenna in the high traffic density area. The replaced mechanical antenna can be installed in rural areas, suburbs, and other areas with low traffic density.
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