antenna principle and model selection-47.pdf

WCDMA Antenna Principle and Model selection

ZTE University

TD&W&PCS BSS Course Team

Principles of Antenna (2)

When the conductive wire has alternating current, it can form radiation

of electromagnetic wave, with the radioactive capacity related to the

length and form of the conductive wire.

When the length of the conductive wire increases to a degree

comparable to wavelength, the current on the conductive wire sharply

increases, forming strong radiation. Generally the straight conductive

wire above that can form noticeable radiation is called dipole .

Principles of Antenna

Principles of Antenna (3)

A dipole with the two rods of the same length is called

symmetrical dipole, or 1/2 wavelength dipole. A single 1/2

wavelength symmetrical dipole can be used independently, or

multiple 1/2 wavelength symmetrical dipole can form an antenna

array.

Wavelength

1/2 Wavelength

1/4 Wavelength

1/4 Wavelength

1/2 Wavelength

dipole


Outer View of Antenna (1)

--- Outdoor NodeB patch directional antenna


For example, 1 symmetrical dipoleReceiving power: 1mW

Antenna array of 4 symmetrical dipolesReceiving power: 4 mW

GAIN= 10log(4mW/1mW) = 6dBd

The high gain of the patch antenna is formed by the antenna array of multiplebasic dipoles

Gain of Antenna


Gain of Antenna

The definition of the gain of an antenna is related to the 1/2 wavelength dipole or the omni radiator.

The omni radiator assumes that the radiation powers in all directions are equal. The gain of the antenna in a certain direction is a value of the field strength generated in this direction over the intensity by the omni radiator in this direction.

Generally the gain of the antenna has two units: dBd and dBi.

dBi indicates the field strength in the direction of the largest radiation of the antenna, compared with the reference value of the omni radiator.

The gain of the antenna compared with the 1/2 wavelength dipole is indicated with dBd.

0dBd=2.15 dBi


Difference of dBd and dBi

2.15dB

Pattern radiation of a single symmetrical dipole

A omni homogeneous radiator has the same

radiation in all directions

Gain of an antenna compared with a symmetrical dipole is indicated with “dBd”.Gain of an antenna compared with an omni homogeneous radiator is indicated with “dBi”.For example: 3dBd = 5.15dBi


Antenna Direction (1)

The antenna direction refers to the capability of radiating

electromagnetic wave in a certain direction.

For the receiving antenna, pattern means the receiving

capability of the wave promulgated from different

directions.

The characteristic curve of antenna direction is usually

indicated with pattern.

Pattern is employed to describe the capability of

transmitting/receiving electromagnetic wave in all

directions in the space.


Antenna Pattern (2)

Top view


120°(eg)

Peak

- 10dB point

- 10dB point

60° (eg) Peak

- 3dB point

- 3dB point

15° (eg) Peak

Peak - 3dB

Peak - 3dB

32° (eg) Peak

Peak - 10dB

Peak - 10dB

Vertical pattern

3dB beamwidth Horizontal pattern 10dB beamwidth

Beamwidth of Antenna


Horizontal Lobe 3dB Width


Directional antenna Omni antenna

Vertical Lobe 3dB Width


Work Frequency Range of Antenna

In disregard of transmitting or receiving antenna, it always works within a certain frequency range. With the considerations of out-of-band anti-interference capacity, the usual practice is to select the bandwidth of the antenna that just meets the frequency requirements.

At 850MHz, the 1/2 wavelength is best

At 890

MHz Antenna dipole

At 820

MHz


Polarization of Antenna

The field direction of the electromagnetic field of the antenna radiation is the polarization of the antenna. The NodeB antenna usually applies linear polarization. With the ground as reference plane, if the field vector is perpendicular to the ground, it is vertical polarization (VP); if the field vector is parallel to the ground, it is horizontal polarization (HP). In the case of a dual-polarization antenna, the +45° and -45° orthogonal dual-linear polarization are adopted.

Vertical polarization

Horizontal polarization

+ 45° tilted polarization - 45° tilted polarization


Dual-polarization Antenna

The dual-polarization consists of two antennae with orthogonal poles

within the same radome. The adoption of dual-polarization antenna can

sharply reduce the number of antennae, streamline the installation

engineering of antenna, lower cost, and save space in antenna

installation.

V/H (vertical/horizontal) Tilt (+/- 45°)


Antenna Beam Downtilt

Applied to suppress coverage and reduce cross-

modulation

Two modes: Mechanical downtilt and electrical downtilt


Impact of Downtilt on Coverage


Beam Downtilt

The purpose of the downtilt technology is to tilt the main beam to

reduce the radiation level to the adjacent coverage cells. In the case,

though the frequency level at the edge of the cell is reduced, the

interference level is much lower than the frequency level.

No downtilt Electrical downtilt

Mechanical downtilt


F/R Ratio

In the antenna pattern, the ratio of max. value of front and back

lobes is called front/back ratio . The F/R ratio of the outdoor

NodeB antenna is preferably generally larger than 25dB.

Front powerRear power


Input Impedance of Antenna

The ratio of the signal voltage and the signal current of the

antenna and the feeder connection points, or the two ends of

the feeding points, is called impedance of antenna.

Input impedance has resistance component and reactance

component. For any antenna, we make adjustment through the

antenna impedance. Within the required work frequency range,

the real part of impedance is very small and imaginary part is

very close to 50 Ω, so that the antenna impedance is Zin = Rin

= 50 Ω. This is necessary to ensure the impedance of antenna

and that of feeder to be well matched.


VSWR

The generation of VSWR : As the incident wave power is transmitted

to the antenna input end and is not completely absorbed (radiation.

Reflection wave is generated and stacked to generate VSWR.

The value of VSWR is between 1 and infinite. VSWR is 1, indicating

full match. VSWR is infinite, indicating full reflection and full mismatch.

9.5 W80 ohms

50 ohmsForward: 10W

Backward: 0.5W


Reflection coefficient :

|Γ|=|(Za-Zo)/(Za+Zo)|，

Za: Input impedance，

Zo: Antenna standard input impedance ，

VSWR=(1+|Γ|)/(1-|Γ|).

RL=-20lg|Γ|,

eg:

if VSWR=1.5:1, then RL=-13.98dB.

VSWR

Side Lobe Suppression and Null Fill-in


Content


Model Selection of Antenna

Parameters Related to Antenna Model Selection

In selecting antennae, a large number of antennae is

involved.

Such parameters as radiation pattern, gain, horizontal

lobe width, vertical lobe width, and downtilt mode are

selected according to the terrain, ground objects, height

of NodeB, and coverage radius in the coverage.

The selection of other parameters is relatively simple and

done according to the designed system.


Polarization Mode (1)

NodeB antenna adopts linear polarization mode.

In particular, single-polarization antenna adopts vertical

linear polarization, whereas dual-polarization antenna

adopts ±45° dual-linear polarization.


Polarization Mode (2)

In downtown of cities, the number of

NodeB is large, and the coverage radius of

each NodeB is small. It is suggested to

adopt dual-polarization antenna.

In suburb and countryside, the number of

NodeB is small and the coverage radius is

large relatively. Space diversity can be

adopted to enhance the receiving effect of

the NodeB. The single-polarization

antenna can be adopted.


Antenna Radiation Direction

For an omni antenna, the radiation intensity in

all directions on the same horizontal plane is

equal in theory. It is applicable to the omni cell.

The selection of omni antenna to achieve

large-scope coverage in the countryside is

economical

The directional antenna enables the direction

of the radiation of the antenna in the horizontal

plane. It is applicable for the coverage of

sector cell. Directional antenna can be

selected for downtown and suburb of cities.


Horizontal Lobe Width

The horizontal lobe width of the omni antenna is all 360°.

In the cases of 20° and 30° widths, the gain is high. They are applied for the coverage of the narrow land stripes or express highways.

In the case of 65° width, it is applied mostly in the typical three-sector NodeB configuration in the densely populated city areas.

In the case of 95° width, it is applied mostly in the typical three-sector NodeB configuration in the suburban areas.

In the case of 105° width, it is applied mostly in the typical three-sector NodeB configuration in the sparsely populated areas.



Downtown of cities

Suggestions for different scenarios:


Downtown of cities

The S111 NodeB in downtown of cities generally adopts

antennae with 65° horizontal lob width and 7° to 10°

vertical lobe width, with the gain of the antennae ranging

within 15 to 18 dBi. For the S110 or directional single-

sector station, the antennae with 65 °, 90 ° or wider

horizontal lobe width. The selection is based on the actual

situation. The selection of vertical lobe and gain is the

same as the S111 station. For omni station, antennae with

small gain and electronic downtilt are selected.



Suburb and countryside


Suburb and countryside

Directional antennae adopt antennae with 90° horizontal

lobe width and 5° to 7° vertical lobe width, with the gain

ranging within 15 to 18 dBi. Omni antennae adopts

antennae with 5° to 7° vertical lobe width, with the gain

ranging within 9 to 12 dBi.



Water surface


Water surface (large lake and sea surface), gobi, and desert

Directional antenna: If the coverage is relatively open and wide,

antennae with 90° or 105° horizontal lobe width and 5° to 7° , with

the gain ranging within 14 to 18dBi ,vertical lobe width can be

selected. If the coverage distance is long but the width is narrow

(e.g., lake and terrain factors), the 65° narrow beamwidth antenna

can be selected.

Omni antenna: The antennae with 5° to 7° vertical lobe width and

gain ranging within 9 to 12dBi can be selected



单扇区功分定向

道路或河流

道路或河流

双扇区定向或单扇区功分定向

道路或河流

全向扇区

天线基本原理

Narrow land strips


Narrow land strips (such as highway and railway)

Antennae for highway and railway are selected according to

the coverage line distance and shape of the highway and

railway concerned.

If the line is relatively straightforward, high-gain antennae

with 20° to 30° horizontal lobe width and 5° to 7° vertical lobe

width can be selected.

If the line is a curve in a large amplitude, antennae with 65°,

90°, or even larger horizontal lobe width, and with 5° to 7°

vertical lobe width can be selected.



天线基本原理

Complicated terrain with

a large fall


Areas of a complicated terrain with a large fall

In the actual networking planning, there may be a scenario that

features a large fall. In that case, antennae with 10° to 18°

vertical lobe width can be selected according to the actual

situation. In another case, the area that needs a large coverage

is higher than the mount height of the antennae. The antennae

with 18° to 30° vertical lobe width can be selected according to

the actual situation.


Downtilt Mode of Antennae (1)

In the case of mechanical downtilt antenna, it is tilted when they are installed. The price is low. It is mostly applied in the scenario with the downtilt angle smaller than 10°.

In the case of electrical downtilt antenna, the price is relatively high, yet with a larger downtilt range (larger than 10°. When the downtilt angle is wide, the antenna pattern shows no obvious distortion, and the back lobe of the antenna will also be downtilted at the same time.

In particular, the fixed electrical downtilt antenna with a small angle plus the mechanical downtilt scheme has advantages in performance and cost, which is the mainstream option of downtilt.

Antenna downtilt modes fall into mechanical downtilt and electrical downtilt. Electrical downtilt can be further divided into fixed electrical downtilt and adjustable electrical downtilt.


Downtilt Mode of Antennae (2)

The application of electrical downtilt antenna includes the following scenarios:

In the case of city sites with specially small coverage radius, large downtiltangle is needed to reduce the interference with the adjacent cells. In the case of high sites, to reduce the interference with the adjacent cells and the problem of “light shadow”, it is better to select the first upper side lobe suppression and the first null fill-in, with large-angle electrical downtiltor adjustable electrical downtilt antenna.In the case of sites higher than the surroundings (e.g., mountain top and riverside), electrical downtilt antenna can be selected. Omni antenna cannot be mechanically downtilted. High omni NodeB should select the electrical downtilt antenna with different angles according to the different situations.


Down tilt angle of Antennae (2)

α = arctg(H/L) – γe

In rural area

α: mechanical down tilt ; H: effective height;

L: cell radius; β :Vertical Lobe 3dB Width ;

γe: electrical down tilt;

antenna principle and model selection-47.pdf

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