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Antenna classification and selection [Copy link]

The technology of mobile communication antennas has developed rapidly. At first, China mainly used ordinary directional and omnidirectional mobile antennas. Later, mechanical antennas were widely used. Now, some provinces and cities have begun to use electrically adjustable antennas and dual-polarized mobile antennas. Since the frequency of use, gain, front-to-back ratio and other indicators of various antennas used in mobile communication systems are not much different, and they all meet the network indicator requirements, we will focus on the impact of the change of the downtilt angle of the mobile antenna on the antenna pattern and the wireless network, and analyze and compare the above-mentioned antennas.

2.1 Omnidirectional Antenna
Omnidirectional antenna, that is, it radiates uniformly at 360° on the horizontal pattern, which is usually called non-directional, and it shows a beam with a certain width on the vertical pattern. Generally speaking, the smaller the lobe width, the greater the gain. Omnidirectional antennas are generally used in suburban and large-area stations in mobile communication systems, with a large coverage area.

2.2 Directional Antenna
Directional antennas radiate within a certain angle range on the horizontal pattern, which is usually called directional, and it shows a beam with a certain width on the vertical pattern. Like omnidirectional antennas, the smaller the lobe width, the greater the gain. Directional antennas are generally used in urban cell-based stations in mobile communication systems. They have a small coverage area, a large user density, and a high frequency utilization rate.

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 according to their needs. The basis for selection is the above technical parameters. For example, an omnidirectional station uses an omnidirectional antenna with basically the same gain in each horizontal direction, while a directional station uses a directional antenna with a significant change in gain in the horizontal direction. Generally, an antenna with a horizontal beam width B of 65° is selected in urban areas, and an antenna with a horizontal beam width B of 65°, 90°, or 120° can be selected in suburban areas (depending on the station configuration and local geographical environment). In rural areas, it is most economical to choose an omnidirectional antenna that can achieve wide coverage.

2.3 Mechanical antennas
The so-called mechanical antennas refer to mobile antennas that use mechanical adjustment of the downtilt angle.

After the mechanical antenna is installed vertically to the ground, if the network optimization requirements require the position of the bracket on the back of the antenna to change the antenna's tilt angle, it is achieved. During the adjustment process, although the coverage distance in the main lobe direction of the antenna changes significantly, the amplitudes of the vertical and horizontal components of the antenna remain unchanged, so the antenna pattern is easily deformed.

Practice has proved that the best downtilt angle of the mechanical antenna is 1°-5°; when the downtilt angle changes from 5°-10°, its antenna pattern is slightly deformed but not much; when the downtilt angle changes from 10°-15°, its antenna pattern changes greatly; when the mechanical antenna is downtilted by 15°, the shape of the antenna pattern changes greatly, from the pear shape when there is no downtilt to the spindle shape. At this time, although the coverage distance of the main lobe direction is significantly shortened, the entire antenna pattern is not all within the sector of the base station, and the signal of the base station will also be received in the adjacent base station sector, causing serious interference within the system.

In addition, in daily maintenance, if the downtilt angle of the mechanical antenna is to be adjusted, the entire system must be shut down, and monitoring cannot be performed while adjusting the antenna tilt angle; it is very troublesome to adjust the downtilt angle of the mechanical antenna, and generally maintenance personnel need to climb to the antenna placement to adjust it; the downtilt angle of the mechanical antenna is a theoretical value calculated by computer simulation analysis software, which has a certain deviation from the actual optimal downtilt angle; the step number of mechanical antenna adjustment tilt angle is 1°, and the third-order intermodulation index is -120dBc.

2.4 Electrically adjustable antennas
Electrically adjustable antennas refer to mobile antennas that use electronic adjustment of the downtilt angle.

The principle of electronic downtilt is to change the phase of the colinear array antenna oscillator, change the amplitude of the vertical component and the horizontal component, and change the field strength of the composite component, so as to make the vertical directivity diagram of the antenna downtilt. Since the field strength in all directions of the antenna increases and decreases at the same time, the antenna directivity diagram does not change much after changing the tilt angle, so that the coverage distance in the main lobe direction is shortened, and at the same time, the entire directivity diagram reduces the coverage area in the service cell sector without causing interference. Practice has proved that when the downtilt angle of the electrically adjustable antenna changes from 1° to 5°, its antenna pattern is roughly the same as that of the mechanical antenna; when the downtilt angle changes from 5° to 10°, its antenna pattern is slightly improved compared with the mechanical antenna; when the downtilt angle changes from 10° to 15°, its antenna pattern changes more significantly than that of the mechanical antenna; when the mechanical antenna is downtilted by 15°, its antenna pattern is significantly different from that of the mechanical antenna. At this time, the shape of the antenna pattern does not change much, and the coverage distance in the main lobe direction is significantly shortened. The entire antenna pattern is within the sector of this base station. Increasing the downtilt angle can reduce the sector coverage area but does not cause interference. This pattern is what we need, so the use of electrically adjustable antennas can reduce call losses and reduce interference.

In addition, the electrically adjustable antenna allows the system to adjust the downtilt angle of the vertical directivity diagram without shutting down, and monitor the adjustment effect in real time. The step accuracy of adjusting the tilt angle is also high (0.1°), so fine adjustment of the network can be achieved; the third-order intermodulation index of the electrically adjustable antenna is -150dBc, which is 30dBc different from that of the mechanical antenna, which is conducive to eliminating adjacent channel interference and stray interference.

2.5 Dual-polarized antenna
Dual-polarized antenna is a new antenna technology that combines two mutually orthogonal antennas with polarization directions of +45° and -45° and works in the transmit-receive duplex mode at the same time. Therefore, its most prominent advantage is that it saves the number of antennas of a single directional base station. Generally, a directional base station (three sectors) of a GSM digital mobile communication network uses 9 antennas, 3 antennas for each sector (spatial diversity, one transmit and two receive). If a dual-polarized antenna is used, only one antenna is needed for each sector. 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 the intermodulation requirement for antenna isolation (≥30dB), so the spatial interval between the dual-polarized antennas only needs to be 20-30cm. In addition, dual-polarized antennas have the advantages of electrically adjustable antennas. Using dual-polarized antennas in mobile communication networks can reduce call loss, reduce interference, and improve the service quality of the entire network, just like electrically adjustable antennas. If dual-polarized antennas are used, since dual-polarized antennas do not require high requirements for installation and installation, there is no need to requisition land for tower construction. It is only necessary to erect an iron pole with a diameter of 20 cm and fix the dual-polarized antenna on the iron pole according to the corresponding coverage direction, thereby saving infrastructure investment, making the base station layout more reasonable, and making the selection of base station sites easier.


For the selection of antennas, we should choose mobile antennas that are suitable for the needs of the local mobile network according to the actual conditions such as the coverage, traffic volume, interference and network service quality of our own mobile network:
--- In high-traffic areas with dense base stations, dual-polarized antennas and electrically adjustable antennas should be used as much as possible;
--- In areas with low traffic volume, sparse base stations and other areas such as borders and suburbs, and areas where only coverage is required, traditional mechanical antennas can be used.

China's current mobile communication network has high call loss and interference in high-traffic density areas. 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 seriously deformed. To solve the problem of insufficient capacity in high-traffic areas, the site distance must be shortened and the antenna downtilt angle must be increased. However, when using a mechanical antenna, the antenna pattern begins to deform when the downtilt angle is greater than 5°, and the antenna pattern is seriously deformed when it exceeds 10°. Therefore, it is difficult to solve the problem of high call loss and interference in high-density user areas by using a mechanical antenna. Therefore, it is recommended to use an electrically adjustable antenna or a dual-polarized antenna to replace the mechanical antenna in high-traffic density areas. The replaced mechanical antenna can be installed in rural areas, suburbs and other areas with low traffic density. [*-)]

This post is from RF/Wirelessly
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