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Antenna main performance parameters
Feb 28, 2018

Antenna operating frequency

Regardless of the antenna or other communications products, always work in a certain frequency range (bandwidth), depending on the requirements of the indicator. Under normal circumstances, to meet the requirements of the target frequency range for the antenna operating frequency.

In general, the antenna performance is different at each frequency point within the operating bandwidth. Therefore, under the same index requirements, the wider the operating frequency band, the greater the antenna design difficulty.


Radiation parameters

 

Main lobe

Sidelobes;

Half power beamwidth

Gain

Beam downtilt

Before and after;

Cross polarization identification rate;

Sidelobe inhibition;

Fill in at zero


According to the antenna radiation parameters on the degree of network performance, can be classified as follows:

 

Half power beamwidth

In the main lobe range of the pattern, the width of the angular domain when the power density relative to the maximum radiation direction is reduced to half, also called 3dB beamwidth.


The horizontal half-power beamwidth is called the horizontal beamwidth and the vertical half-power beamwidth is the vertical beamwidth.

 

The relationship between antenna gain and beamwidth:

Horizontal beam width

The antenna of each sector reaches the coverage edge when the maximum radiation deviates from ± 60º and needs to be switched to work with adjacent sectors. At a switching angle of ± 60º, there should be a reasonable drop in the pattern level. When the level drops too much, it is easy to cause coverage drop in the vicinity of the switching angle domain. When the level drops too little, overlap occurs in the vicinity of the switching angle domain, resulting in an increase in the interference of adjacent sectors.

Theoretical simulation and practical application show that in the densely-built urban area, due to the serious multipath reflections, it is better to reduce to about -10 dB at the level of ± 60 ° in order to reduce the mutual interference between adjacent sectors. The power width is about 65º. However, in the open suburbs, due to the low multipath reflections, it is better to drop to about -6dB at ± 60º to ensure good coverage. The reverse power width is about 90º.

Horizontal beamwidth, beam skew and pattern consistency determine the direction of the coverage area performance is good or bad.


Multipath reflection propagation:

P ~ 1 / R ^ n

n = 2 ~ 4


± 60º Level Design:

------------------

Urban n = 3 ~ 3.5

9 ~ 10.5dB down

Country: n = 2

6 dB decrease

 

Vertical beam width and electrical dip accuracy

Determine the network coverage area distance performance is good or bad.

Observe the vertical plane of the figure below. The beam should be properly tilted so that the most downward angle is such that the maximum radiation is directed to the edge of the target service area in the figure. If the tilt is too much (yellow), the coverage level at the far end of the service area will drop sharply. If the tilt is too small, coverage outside the service area will cause the same-frequency interference problem.

 

Electric dip angle

The angle between the maximum radiation and the antenna normal.

 

Before and after than

Inhibit co-channel interference or pilot pollution an important indicator.

Often only the horizontal and vertical orientation of the pattern needs to be examined, and the worst value in the range of ± 30 ° in the backward direction should be specified.

The worse before and after than the indicator, the greater the backward radiation, and the more likely it is to interfere with the covered cell behind the antenna.

Before and after the vertical plane pattern will be examined in special applications, such as the high-rise buildings facing away from the base station.


Antenna gain

Refers to the ratio of the radiated power flux density of an antenna in a given direction to the maximum radiated power flux density of a reference antenna (usually using an ideal point source) at the same input power.

 

Antenna gain, pattern and antenna size

Antenna gain is used to measure the antenna to send and receive signals in a particular direction of ability, it is one of the important parameters to choose the base station antenna.

The higher the antenna gain, the better directivity, the more concentrated the energy, the narrower the lobe.

The higher the gain, the longer the antenna length.

 

Antenna gain several points:

1) Antenna is a passive device, can not generate energy. Antenna gain is simply the ability to focus energy efficiently in a specific direction or to receive electromagnetic waves.

2) Antenna gain generated by the superposition of the oscillator. The higher the gain, the longer the antenna length.

3) The higher the antenna gain, the better the directivity, the more concentrated the energy, the narrower the lobe.


Gain coverage of distance indicators, a reasonable choice of gain! ! !

Increasing the gain of the antenna increases the coverage distance but at the same time narrows the beam width, resulting in poor coverage uniformity. Antenna gain selection should be based on the beam and the target area for the premise, in order to increase the gain and too much pressure to narrow the vertical beam width is not desirable, only through the optimization program to achieve rapid decline in the service area outside the level of depression, Reduce the level of cross-polarization, using low loss, no surface wave parasitic radiation, low VSWR feed network and other means to improve the antenna gain is correct.

 

Cross-polarization ratio

Polarization diversity effect of the indicators

In order to obtain a good uplink diversity gain, dual polarized antennas are required to have good orthogonality 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 is significant (cross-polarization ratio) should be larger in the direction of maximum radiation 15dB, within ± 60º should be greater than 10dB, the minimum threshold should be greater than 7dB, as shown. In this way, it can be considered that the signals received by both polarizations are irrelevant to each other.

 

Sidelobe suppression

Auxiliary index to suppress co-channel interference or pilot pollution

For urban buildings dense application scenarios, on the one hand because of the communication capacity requirements of shrinking the honeycomb, on the other hand due to building blockage and multipath reflections, it is difficult to achieve large-distance coverage. Generally, a low gain antenna with a gain of 13-15dBi is used, and a micro-bevel coverage is used with a large downtilt. Therefore, it is highly probable that the first and second side lobes on the upper side of the main beam point to the same frequency cell in front, which requires that when designing an antenna , Trying to suppress the upper side lobe, thereby reducing interference.


Fill under zero

In some special scenes limited to reduce blind spots auxiliary indicators

In the antenna design, the appropriate filling of the next zero, it may reduce the call drop rate. However, zero filling should be more than adequate, when the zero filling requirements are higher, the larger gain loss, outweigh the benefits. For low-gain antenna, due to lobe wider, the application is usually more down tilt angle, the next sidelobe does not participate in the cover, do not need to zero fill.

The multipath effect, resulting in close zero effect is not obvious or disappear.


Roundness of the pattern

An indicator to evaluate the uniform coverage of an omnidirectional antenna

Just examine the circularity of the horizontal plane. Evaluation example: The indicator is ± 1dB, all the frequencies need to be better than the indicator.


Voltage standing wave ratio

VSWR: The ratio of the maximum voltage to the minimum voltage on the transmission line.

When the antenna port is not reflected, it is an ideal match with a standing wave ratio of 1; when the antenna port is totally reflected, the standing wave ratio is infinite.

 

Voltage standing wave ratio is the basic index of high efficiency radiation of antenna.

In the whole band VSWR study, take the maximum as an indicator.

Evaluation example: target of 1.5, all frequencies need to be better than the indicator.


Isolation

Refers to the proportion of another polarization signal received by one polarization.

Generally refers to the polarization of two polarized direct isolation.

 

Third order intermodulation

Make sure the transmitted interference from the antenna does not affect the sensitivity of the receiver

PIM3 in the whole frequency range, take the maximum as an indicator.

It can reflect the comprehensive level of vendor antenna products through interchanging indicators, especially the quality control ability of material production and assembly processes.

 

Intermodulation interference necessary conditions: a strong enough intermodulation signal level + can fall into the system to receive the band


The main parameters of the antenna unit of measurement

Unit of measure description

 

1) dB

Relative value, characterizes the relative relationship between two quantities, such as the power of A than the power of B big or small

How many dB, the 10log (A power value / B power value) calculation.

For example: A power value is 2W, B power value is 1W, that is, A times more than B, converted to dB units:

10log (2W / 1W) ≈3dB


2) dBm

The amount that characterizes the absolute value of power can also be considered as a ratio based on a power of 1 mW calculated as: 10 log (power value / 1 mw).

Example: The power value is 10w, which is converted into 10log (10w / 1mw) = 40dBm in dBm.


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 fixed reference bases: the reference for dBi is the omnidirectional ideal point source, and the reference for dBd is the half-wave oscillator.

Example: 0dBd = 2.15dBi.


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