Comprehensive understanding of antennas, the knowledge you don’t know!
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1 Development History of Mobile Base Station Antennas
2 Basic knowledge of electromagnetic wave propagation
Definition of Radio Waves
Radio waves are a form of signal and energy propagation. During the propagation process, the electric field and magnetic field are perpendicular to each other in space and are perpendicular to the propagation direction.
Direction of radio wave propagation
Orthogonal characteristics; electricity generates magnetism and magnetism generates electricity.
The relationship between the wavelength, frequency and propagation speed of radio waves
Where: wavelength λ = C/f (where C is the speed of light, f is the operating frequency, and λ is the wavelength.)
In the same medium, at different frequencies, the antenna's operating wavelength is different. The higher the frequency, the shorter the wavelength.
The electrical performance of the antenna corresponds to the electrical length (wavelength). The physical length needs to be converted.
Polarization of radio waves
When radio waves propagate in space, the direction of their electric field changes according to a certain rule. This phenomenon is called the polarization of radio waves. The polarization of radio waves is determined by the trajectory of the electric field vector moving in space. If the electric field direction of the radio wave is perpendicular to the ground, we call it a vertically polarized wave. If the electric field direction of the radio wave is parallel to the ground, it is called a horizontally polarized wave.
Circular polarization <— Elliptical polarization —> Linear polarization
Left-handed, right-handed; vertical, horizontal
Antenna Polarization
It refers to the trajectory of the electric field vector moving in space.
Dual polarization antenna
It consists of two groups of orthogonal radiation units.
1) Complementary (completely unrelated. Orthogonal/90 degrees) (planning work)
2) Equivalent (balanced work. +45/-45) (competent for the job)
3) High efficiency (XPD reduces losses) (Focus on work)
Multipath propagation
In the process of propagation, radio waves, in addition to direct propagation, will also produce reflection and diffraction when encountering obstacles (such as hills, forests, ground or tall buildings). Therefore, the electromagnetic waves reaching the receiving antenna include not only direct waves, but also reflected waves, diffracted waves and transmitted waves. This phenomenon is called multipath transmission.
Due to multipath propagation, the signal intensity distribution is complicated and fluctuates greatly. Due to the influence of multipath transmission, the polarization direction of the radio wave will change (twist). Therefore, the signal intensity is enhanced in some places and weakened in some places. In addition, different obstacles have different reflection capabilities for radio waves. In order to reduce the influence of multipath transmission effects, spatial diversity or polarization diversity is generally used for reception.
Spatial diversity: single polarized antenna
Polarization diversity: dual polarized antenna
3 Antenna Radiation Principle
Definition of Antenna
A device that can effectively radiate electromagnetic waves in a specific direction in space or can effectively receive electromagnetic waves from a specific direction in space.
Antenna half wave dipole
The half-wave dipole is the basic radiation unit of the antenna. The longer the wavelength, the larger the half-wave dipole of the antenna.
Half-wave oscillator example:
Antenna Radiation Pattern
It is used to describe the ability of an antenna to transmit and receive electromagnetic waves in all directions in space. It is usually a three-dimensional radiation stereogram.
In actual evaluation, it is converted into a two-dimensional plane figure, namely the horizontal plane direction diagram and the vertical plane direction diagram.
Antenna components
There are multiple design solutions for the same base station antenna. The design solution involves the following four parts of the antenna:
1) Radiating unit (symmetrical oscillator or patch [array element])
2) Reflector (bottom plate)
3) Power distribution network (feeding network)
4) Packaging protection (radome)
4 Antenna main performance parameters
Antenna operating frequency
Whether it is an antenna or other communication products, they always work within a certain frequency range (bandwidth), which depends on the requirements of the indicators. Usually, the frequency range that meets the requirements of the indicators can be the working frequency of the antenna.
Generally speaking, the antenna performance is different at each frequency point within the working frequency bandwidth. Therefore, under the same performance requirements, the wider the working frequency bandwidth, the more difficult the antenna design is.
Radiation parameters
Main lobe;
Side lobe;
Half-power beamwidth;
Gain;
Beam downtilt;
front-to-back ratio;
Cross-polarization discrimination;
Upper sidelobe suppression;
Lower zero fill;
According to the degree of influence of antenna radiation parameters on network performance, they can be classified as follows:
Half power beam width
The angular width when the power density in the main lobe of the radiation pattern drops to half relative to the maximum radiation direction is also called the 3dB beamwidth.
The half-power beamwidth in the horizontal plane is called the horizontal beamwidth; the half-power beamwidth in the vertical plane is called the vertical beamwidth.
The relationship between antenna gain and beam width:
Horizontal beam width
When the maximum radiation direction of each sector antenna deviates from ±60°, it reaches the edge of coverage and needs to switch to the adjacent sector. In the switching angle domain of ±60°, the directional pattern level should have a reasonable drop. If the level drops too much, it is easy to cause call drops in the coverage blind area near the switching angle domain; if the level drops too little, the coverage overlaps near the switching angle domain, resulting in increased interference between adjacent sectors.
The results of theoretical simulation and practical application show that in densely built urban areas, due to severe multipath reflection, in order to reduce mutual interference between adjacent sectors, the level at ±60° is preferably reduced to about -10dB, and the reverse half-power width is about 65°; in open suburban areas, due to less multipath reflection, in order to ensure good coverage, the level at ±60° is preferably reduced to about -6dB, and the reverse half-power width is about 90°.
The horizontal plane beam width, beam deflection and pattern consistency determine the performance of the coverage area in azimuth.
Multipath reflection propagation:
P ~~ 1/R^n
n = 2~4
±60 level design:
------------------
Urban area n=3~3.5
9~10.5dB down
Countryside: n=2
6 dB down
Vertical beam width and electrical downtilt accuracy
Determines the quality of the distance performance in the network coverage area.
Observe the vertical plane pattern in the figure below. The beam should be appropriately tilted down, and the best downtilt angle is to make the maximum radiation point to the edge of the target service area in the figure. If the downtilt is too much (yellow), the coverage level at the far end of the service area will drop sharply; if the downtilt is too little, the coverage is outside the service area and co-channel interference problems will occur.
Electrical down tilt angle
The angle between the maximum radiation direction and the antenna normal.
Front-to-back ratio
An important indicator for suppressing co-channel interference or pilot pollution.
Usually, only the front-to-back ratio of the horizontal plane pattern needs to be examined, and specifically the worst value within the range of ±30° in the rear direction.
The worse the front-to-back ratio is, the greater the backward radiation is, and the greater the possibility of causing interference to the coverage area behind the antenna.
The front-to-back ratio of the vertical plane pattern is only considered in special applications, such as when there are super-high-rise buildings in the area behind the base station.
Antenna gain
It refers to the ratio of the radiated power flux density of an antenna in a specified direction to the maximum radiated power flux density of a reference antenna (usually an ideal point source) at the same input power.
Relationship between antenna gain, radiation pattern and antenna size
Antenna gain is used to measure the ability of an antenna to send and receive signals in a specific direction. It is one of the important parameters for selecting base station antennas.
The higher the antenna gain, the better the directivity, the more concentrated the energy, and the narrower the lobe.
The higher the gain, the longer the antenna length.
Several key points of antenna gain:
1) Antennas are passive devices that cannot generate energy. Antenna gain is simply the ability to effectively concentrate energy to radiate or receive electromagnetic waves in a specific direction.
2) The gain of the antenna is generated by the superposition of the oscillators. The higher the gain, the longer the antenna.
3) The higher the antenna gain, the better the directivity, the more concentrated the energy, and the narrower the lobe.
Gain affects the coverage distance index, choose the gain reasonably! ! !
Increasing antenna gain increases the coverage distance, but at the same time it will narrow the beam width, resulting in poor coverage uniformity. The selection of antenna gain should be based on the matching of the beam and the target area. It is not advisable to excessively narrow the vertical beam width in order to increase the gain. The only correct way to increase the antenna gain is to optimize the solution, achieve a rapid drop in the level outside the service area, reduce the side lobes and back lobes, reduce the cross-polarization level, and use a low-loss, no surface wave parasitic radiation, low VSWR feed network.
Cross-polarization ratio
Indicators of polarization diversity effect
In order to obtain good uplink diversity gain, the dual-polarization antenna should have good orthogonal polarization characteristics, that is, in the sector service area of ±60°, the cross-polarization pattern level should be significantly lower than the main polarization level at the corresponding angle, and the difference (cross-polarization ratio) should be 15dB greater in the maximum radiation direction, greater than 10dB within ±60°, and the minimum threshold should be greater than 7dB, as shown in the figure. Only in this way can it be considered that the signals received by the two polarizations are unrelated.
Sidelobe Suppression
Auxiliary indicators for suppressing co-channel interference or pilot pollution
For application scenarios with dense buildings in urban areas, on the one hand, the large communication capacity requires the cell to be smaller, and on the other hand, it is difficult to achieve long-distance coverage due to building obstruction and multipath reflection. Usually, a low-gain antenna with a gain of 13~15dBi and a large downtilt angle is used for micro-cell coverage. As a result, the first and second side lobes on the upper side of the main beam are likely to point to the same-frequency cell in front. This requires that when designing the antenna, the upper side lobes should be suppressed to reduce interference.
Lower zero fill
Auxiliary indicators to reduce blind spots in some special scenarios
When designing an antenna, filling the lower zero point appropriately may reduce the call drop rate. However, zero point filling should be done in moderation. When the requirement for zero point filling is high, the gain loss is large, which is not worth the loss. For low-gain antennas, due to the wide lobe, the downward tilt angle is usually large when used, and the lower side lobe does not participate in the coverage, so zero point filling is not required.
The influence of multipath causes the close-range null effect to be inconspicuous or disappear.
Directional pattern circularity
Indicators for evaluating uniform coverage of omnidirectional antennas
Only the circularity of the horizontal plane pattern needs to be examined. Evaluation example: The index is ±1dB, and all frequency points need to be better than this index.
VSWR
Voltage Standing Wave Ratio (VSWR): The ratio of the maximum voltage to the minimum voltage on the transmission line.
When there is no reflection at the antenna port, it is an ideal match and the standing wave ratio is 1; when the antenna port is fully reflected, the standing wave ratio is infinite.
Voltage standing wave ratio is a basic indicator requirement for high-efficiency radiation of antennas.
Examine VSWR in the entire frequency band and take the maximum value as the indicator.
Evaluation example: The indicator is 1.5, and all frequency points need to be better than this indicator.
Isolation
It refers to the ratio of a certain polarization signal received by another polarization signal.
Generally refers to the direct isolation of the two polarizations in a dual-polarized antenna.
Third-Order Intermodulation
Ensure that intermodulation interference from antenna transmission does not affect the sensitivity of the receiver
PIM3 is examined in the entire frequency band and the maximum value is taken as the indicator.
The intermodulation index can reflect the comprehensive level of the supplier's antenna products, especially the quality control capabilities of the material production and assembly processes.
Necessary conditions for intermodulation interference: sufficiently strong intermodulation signal level + able to fall into the system receiving band
Antenna main parameters measurement unit
Unit of measurement
1) dB
Relative value, which represents the relative size relationship between two quantities, such as the power of A is greater or less than the power of B.
When it is dB, it can be calculated as 10log(A power value/B power value).
For example: The power value of A is 2W, and the power value of B is 1W, that is, A is twice as much as B. The conversion into dB unit is:
10log(2W/1W) ≈3dB
2) dBm
The quantity that characterizes the absolute value of power can also be considered as a ratio based on 1mw power, calculated as: 10log(power value/1mw).
For example: the power value is 10w, which is converted into dBm as 10log(10w/1mw)=40dBm.
3) dBi and dBd
Both represent the quantity of antenna gain and are relative values, similar to dB, except that dBi and dBd have fixed reference standards: the reference standard for dBi is an omnidirectional ideal point source, and the reference standard for dBd is a half-wave oscillator.
For example: 0dBd=2.15dBi.
5 The Future of Antenna Technology
High performance antenna
In the face of growing traffic demand, antenna technology is the key to improving network capacity. Since capacity is limited by SINR, to improve SINR through antenna technology, it is necessary to minimize inter-sector interference and maximize the centralized antenna radiation energy.
Multi-beam antenna technology
Use multi-beam antennas to split sectors to increase capacity, such as 2 x 9 x 6° 18-beam antennas.
RF and antenna integration
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提升卡
变色卡
千斤顶
Such great information.
