The half-wave oscillator is the basic radiation unit of the antenna. The longer the wavelength, the larger the antenna half-wave oscillator.
Example of a half-wave oscillator:
Antenna Radiation Pattern
It is used to express the ability of an antenna to transmit and receive electromagnetic waves in all directions of space. Generally, it is a three-dimensional radiation stereogram.
In the actual evaluation, it is the two-dimensional plane figure transformed into it, that is, the horizontal plane pattern and the vertical plane pattern.
Antenna Components
The same base station antenna has a variety of design schemes to achieve. The design scheme involves the following four parts of the antenna:
1) Radiation unit (symmetric oscillator or patch [array element])
2) Reflector (bottom plate)
3) Power distribution network (feeder network)
4) Encapsulation protection (radome)
4 Main performance parameters of the antenna
Antenna operating frequency
Regardless of the antenna or other communication products, it always works within a certain frequency range (bandwidth), which depends on the requirements of the indicators. Usually, the frequency range that meets the requirements of the index can be the working frequency of the antenna.
Generally speaking, the antenna performance is different at each frequency point within the operating frequency bandwidth. Therefore, under the same index requirements, the wider the working frequency band, the more difficult the antenna design will be.
Radiation parameters
main lobe;
side lobe;
half-power beamwidth;
gain;
Beam downtilt angle;
front to back ratio;
Cross-polarization discrimination rate;
upper sidelobe suppression;
bottom zero fill;
According to the degree of influence of antenna radiation parameters on network performance, they can be classified as follows:
Half power beamwidth
In the range of the main lobe of the pattern, the angular domain width when the power density relative to the maximum radiation direction drops to half, also called the 3dB beam width.
The half-power beamwidth of the horizontal plane is called the horizontal beamwidth; the half-power beamwidth of the vertical plane is called the vertical beamwidth.
The relationship between antenna gain and beamwidth:
Horizontal beam width
The antenna of each sector reaches the edge of coverage when the maximum radiation direction deviates ±60º, and needs to switch to the adjacent sector to work. In the switching angle range of ±60º, there should be a reasonable drop in pattern level. When the level drops too much, it is easy to cause call drop in the coverage blind area near the handover angle; when the level drops too little, the coverage overlaps near the handover angle, resulting in increased interference in adjacent sectors.
Theoretical simulation and practical application results show that: in urban areas with dense buildings, due to serious multipath reflection, in order to reduce the mutual interference between adjacent sectors, it is better to drop the level at ±60º to about -10dB, and reverse the halfway. The power width is about 65º; while in the open suburbs, due to less multipath reflection, in order to ensure good coverage, it is better to drop the level to about -6dB at ±60º, and the reverse half power width is about 90º.
The horizontal beamwidth, beam skew, and pattern consistency determine the azimuth performance of the coverage area.
Multipath reflection propagation:
P ~~ 1/R^n
n = 2~4
±60º level design:
------------------
Urban n=3~3.5
9~10.5dB drop
Countryside: n=2
6 dB drop
Vertical beamwidth and electrical downtilt accuracy
Determines the distance performance in the network coverage area.
Observe the vertical plane orientation diagram below. The beam should be properly downtilted, preferably at an angle such that the maximum radiation is directed towards the edge of the target service area in the map. If the dip is too much (yellow), the coverage level at the far end of the service area will drop sharply; if the dip is too little, the coverage will be outside the service area and cause co-channel interference problems.
Electric downtilt angle
The angle between the maximum radiation point 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 should be considered, and the worst value within the range of ±30° of the rearward direction is specified.
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 cell behind the antenna.
The front-to-back ratio of the vertical plane pattern is only examined in special applications, such as super high-rise buildings in the area facing away from the base station.
Antenna gain
Refers to the ratio of the radiated power flux density of the antenna in a specified direction to the maximum radiated power flux density of the reference antenna (usually using an ideal point source) at the same input power.
Relationship between Antenna Gain, Pattern and Antenna Size
Antenna gain is used to measure the ability of the antenna to send and receive signals in a specific direction, and it is one of the important parameters for selecting a base station antenna.
The higher the antenna gain, the better the directivity, the more concentrated the energy, and the narrower the lobes.
The higher the gain, the longer the antenna length.
A few key points about antenna gain:
1) Antennas are passive devices and 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 length.
3) The higher the antenna gain, the better the directivity, the more concentrated the energy, and the narrower the lobe.
The gain affects the coverage distance index, so choose the gain reasonably! ! !
Increasing the antenna gain increases the coverage distance, but at the same time narrows the beam width, resulting in poor coverage uniformity. The selection of the antenna gain should be based on the matching of the beam and the target area. In order to improve the gain, it is not advisable to over-compress the vertical beam width. Only by optimizing the scheme, the level outside the service area can be rapidly dropped, and the side lobes and back lobes can be reduced. It is correct to reduce the cross-polarization level and use a feed network with low loss, no surface wave parasitic radiation, and low VSWR to increase the antenna gain.
cross polarization ratio
The index of the pros and cons of polarization diversity effect
In order to obtain good uplink diversity gain, it is required that the dual-polarized antenna should have good orthogonal polarization characteristics, that is, in the sector service area of ±60º, the cross-polarization pattern level should be higher than that of the main polarization at the corresponding angle. The difference (cross-polarization ratio) should be greater than 15dB in the maximum radiation direction, greater than 10dB within ±60º, and the minimum threshold should also be greater than 7dB, as shown in the figure. In this way, it can be considered that the signals received by the two polarizations are not correlated with each other.
Sidelobe suppression
Auxiliary index for suppressing co-channel interference or pilot pollution
For application scenarios with dense buildings in urban areas, on the one hand, due to the large communication capacity, it is required to reduce the size of the cell, on the other hand, due to building occlusion and multipath reflection, it is difficult to achieve long-distance coverage. Usually a low-gain antenna with a gain of 13~15dBi is used, and a large downtilt angle is used for microcellular coverage. Therefore, the first and second side lobes on the upper side of the main beam are very likely to point to the forward co-frequency cell, which requires the design of the antenna. , try to suppress the upper side lobes, thereby reducing interference.
Bottom zero padding
Auxiliary indicators for limited reduction of blind spots in some special scenarios
In the antenna design, it is possible to reduce the call drop rate by properly filling the lower zero point. However, the zero-point filling should be done in moderation. When the requirements for zero-point filling are high, the gain loss will be large, and the gain will outweigh the gain. For low-gain antennas, due to the wide lobes, the down-tilt angle is usually large in application, and the lower side lobes do not participate in coverage, and zero-point filling is not required.
Due to the influence of multipath, the close-range zero-point effect is not obvious or disappears.
Directional circularity
Metrics for evaluating the uniform coverage effect of omnidirectional antennas
It is only necessary to examine the roundness of the horizontal plane pattern. Evaluation example: the index is ±1dB, and all frequency points need to be better than this index.
VSWR
Voltage Standing Wave Ratio (VSWR): It is 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 totally reflected, the standing wave ratio is infinite.
The voltage standing wave ratio is the basic index requirement for the high-efficiency radiation of the antenna.
The VSWR is investigated in the whole frequency band, and the maximum value is taken as the indicator.
Evaluation example: the indicator is 1.5, and all frequency points need to be better than this indicator.
isolation
Refers to the proportion of the received signal of one polarization of the other polarization.
Generally refers to the direct isolation of two polarizations in a dual-polarized antenna.
third-order intermodulation
Ensure that the intermodulation interference emitted by the antenna does not affect the sensitivity of the receiver
Investigate PIM3 in the whole frequency band, and take the maximum value as the indicator.
The intermodulation index can reflect the comprehensive level of the supplier's antenna products, especially the quality control ability of the material production and assembly process.
Necessary conditions for intermodulation interference: strong enough intermodulation signal level + can fall into the system receiving frequency band
Antenna main parameters measurement unit
Measurement unit description
1） dB
Relative value, characterizing the relative magnitude relationship between two quantities, such as the power of A is greater or less than the power of B
How many dB can be calculated by 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, and the unit of conversion to dB 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, and is calculated as: 10log(power value/1mw).
For example: the power value is 10w, and the conversion to dBm is 10log(10w/1mw)=40dBm.
3) dBi and dBd
Both represent the amount of antenna gain, which is also a relative value, similar to dB, except that dBi and dBd have a fixed reference: the reference of dBi is an ideal omnidirectional point source, and the reference of dBd is a half-wave oscillator.
Example: 0dBd=2.15dBi.
5 The future of antenna technology
high performance antenna
Facing the ever-increasing traffic demand and increasing network capacity, antenna technology is the key. Since the capacity is limited by the SINR, to improve the SINR through antenna technology, it is necessary to minimize the inter-sector interference and maximize the centralized antenna radiation energy.
Multibeam Antenna Technology
Use multi-beam antennas to split sectors to increase capacity, such as 2 x 9 x 6° 18 beam antennas.
RF Section and Antenna Fusion