basic knowledge of antenna and antenna selection
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Basic Knowledge of Antenna
and Antenna Selection
Edition: V1.0
Released by:
GSM Network Planning & Optimization Dept.
Engineering Service Division
ZTE Corporation
This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.
This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.
Training Materials
for GSM Network Planning & Optimization
Version Introduction
Versio
n
Date Writer Assessor Amendment records
V1.0 2007-07-01 Wang JinFu Chen Ni None
V1.1 2009-02-16 Fei AiPing Chen Ni Document standardization
This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.
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Key words:
antenna basics、beam width、gain、tilt angle、circumstances、selection of antenna types
Abstract:
In this guidebook, theories of antenna、antenna parameters and related concepts and
definitions are mainly described. Besides, selection of antenna types under different
circumstances and antenna installation specifications are also introduced.
Abbreviation:
None
Reference:
<Antenna Rudiments> Internet
< Selection Flow of GSM Antenna Types > Guidebook for GSM Network Planning &
Optimization Dept., Engineering Service Division, ZTE Corporation
This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.
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Contents
1 Overview of Antenna.........................................................................................................................1
1.1 General Development of BTS Antenna Technology.......................................................................1
1.2 Theories of Antenna Radiation........................................................................................................3
1.2.1 Electromagnetic Wave Radiation of Electric Dipole.............................................................3
1.2.2 Half-wave Dipole..................................................................................................................3
1.3 Internal Structure & Types of Mobile Communication BTS Antenna............................................4
1.3.1 Plate-shaped Directional Antenna.........................................................................................4
1.3.2 Isotropic Dipole Antenna with Series Feed...........................................................................9
2 Concepts & Meanings of Antenna Parameters.............................................................................11
2.1 Antenna Gain................................................................................................................................11
2.2 Directional Diagram of Radiation.................................................................................................12
2.3 Beam Width..................................................................................................................................12
2.3.1 Horizontal Beam Width.......................................................................................................12
2.3.2 Vertical Beam Width...........................................................................................................14
2.4 Frequency Band............................................................................................................................15
2.5 Polarization Mode.........................................................................................................................15
2.6 Down-tilt Mode.............................................................................................................................16
2.7 Antenna Front-to-back Ratio.........................................................................................................18
2.8 Antenna Input Impedance( Zin)....................................................................................................18
2.9 Antenna VSWR.............................................................................................................................19
2.10 Side Lobe Suppression & Null Fill...............................................................................................19
2.11 Third-order Intermodulation.........................................................................................................20
2.12 Isolation between Ports.................................................................................................................21
2.13 Case...............................................................................................................................................21
3 Measurement of Key Antenna Indicators Knowledge point......................................................22
3.1 Measurement of Antenna Gain......................................................................................................22
3.2 Measurement of 3dB Beam Width & Front-to-back Ratio............................................................22
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3.3 Measurement of Antenna VSWR..................................................................................................23
3.4 Isolation Measurement of Dual-Polarized Antenna......................................................................24
3.5 Measurement of Intermodulation..................................................................................................24
4 Antenna Parameters........................................................................................................................26
4.1 Antenna Directional Angle............................................................................................................26
4.2 Antenna Height.............................................................................................................................27
4.3 Antenna Down-tilt........................................................................................................................29
5 Antenna Types..................................................................................................................................31
5.1 Antenna Types...............................................................................................................................31
6 Circumstances of Antenna Application..........................................................................................36
6.1 Dense Urban..................................................................................................................................37
6.2 Urban (Towns)..............................................................................................................................37
6.3 Suburb (Counties) & Villages.......................................................................................................38
6.4 Railways/ Highways (Roads)........................................................................................................38
6.5 Scenery Areas................................................................................................................................39
7 Selection of Antenna Type...............................................................................................................41
7.1 BTS antenna selection for urban...................................................................................................41
7.2 BTS antenna selection for suburb.................................................................................................41
7.3 BTS antenna selection for highway coverage...............................................................................42
7.4 BTS antenna selection for mountain area......................................................................................43
8 Installation & Modulation of Antenna...........................................................................................45
8.1 Antenna installation on Pole.........................................................................................................45
8.1.1 Installation pole must be straight.........................................................................................45
8.1.2 Lightening protection..........................................................................................................45
8.1.3 Diversity reception..............................................................................................................46
8.1.4 Antenna isolation.................................................................................................................47
8.2 Antenna Installation on Iron Tower..............................................................................................47
8.3 Summary.......................................................................................................................................48
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Diagram Contents
Picture 1-1 Diagram of Relation between Doublet Angle and Radiation Power of
Electromagnetic Wave.....................................................................................................................3
Picture 1-2 Half-wave Dipole...........................................................................................................4
Picture 1-3 Half-wave Folded Dipole.............................................................................................4
Picture 1-4 Diagram of Directional Plate-shaped Antenna Appearing............................6
Picture 1-5 Vertically-set Line Array Formed by Several Half-wave Dipole..................6
Picture 1-6 Apply horizontally oriented theory by adding baffle-board to one side
the line array (take the line array of double half-wave dipole with baffle-
board as an example )............................................................................................................7
Picture 1-7 Directional plate-shaped antenna formed with several half-wave dipole8
Picture 1-8 Directional plate-shaped antenna consisting of several microstrip
dipole............................................................................................................................................9
Picture 1-9 Structure of plate-shaped directional antenna array......................................................9
Picture 1-10 Structure of Isotropic Dipole Antenna with Series Feed.............................................10
Picture 2-1 Diagram of antenna gain in dBi and dBd...........................................................11
Picture 2-2 Field strength diagram of isotropic antenna and directional antenna. 12
Picture 2-3 Diagram of horizontal beam width (3dB) of BTS antenna...........................................13
Picture 2-4 Diagram of BTS antenna coverage.....................................................................14
Picture 2-5 Diagram of BTS antenna vertical with 3dB beam width..............................14
Picture 2-6 Common polarization mode of BTS antenna...................................................16
Picture 2-7 Diagram of dual-polarization BTS antenna......................................................16
Picture 2-8 Diagram of BTS antenna down-tilt.......................................................................17
Picture 2-9 Comparison on BTS antenna down-tilt modes................................................17
Picture 2-10 Diagram of comparison between BTS antenna with or without null fill effect................20
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Picture 3-1 Block diagram of measurement of antenna gain 、 half-power beam
width、front-to-back ratio、cross polarization discrimination.............................22
Picture 3-2 Block diagram of VSWR measurement...............................................................23
Picture 3-3 Block diagram of antenna isolation......................................................................24
Picture 3-4 Block diagram of antenna intermodulation measurement........................25
Picture 6-1 Dense urban..................................................................................................................37
Picture 6-2 Urban (towns).....................................................................................................................38
Picture 6-3 Suburb................................................................................................................................38
Picture 6-4 Railways & highways.........................................................................................................39
Picture 6-5 Scenery areas......................................................................................................................39
Picture 8-1 Solid diagram and vertical view of antenna.....................................................47
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Table Contents
Table 8-1 Horizontal diversity distance of antenna............................................................46
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1 Overview of Antenna
Knowledge Point
In this chapter, the radiation theory of BTS antenna and its structure types are
described. In addition, antenna technology and its development trend are introduced.
1.1 General Development of BTS Antenna Technology
In the cellular mobile communication system, antenna is the transducer between the
signals of communication equipment circuit and air radiated electromagnetic wave; in
other words, antennas convert electromagnetic waves into electrical currents and vice
versa. It is the connector in wireless air communication.
Cellular communication system requires reliable communications between BTS and
MS, it has specific requirements on antenna system. The radio signal power sent out
from the transmitter will be delivered to antenna through antenna feed (cable), then it
will be sent out in the form of electromagnetic wave. When the electromagnetic wave
arrives at the receiving site, it is received by the antenna at the site (just a fraction of
the power is received), then it is delivered to the radio receiver through antenna feed. It
is obvious that antenna plays a very important role in sending and receiving
electromagnetic wave. Telecommunication won’t be possible without antenna’s
participation. Telecommunication performance is largely affected by antenna
gain 、 antenna coverage direction 、 beam width 、 usable driving power 、 antenna
configuration and its polarization mode, etc..
Technology & market status of Chinese antenna suppliers
As shown in some related materials, in the fields of high-tech commercial BTS
antenna 、 smart antenna and Bluetooth antenna, which are used in mobile
communications、spread spectrum communications and microwave communications,
Chinese antenna suppliers only take 20% market share of the whole; and compared
with the famous foreign suppliers, their production scale and strength are far left
behind. Generally speaking, Chinese antenna suppliers feature in large amount、small
scale and weak strength. According to incomplete statistical survey, until the first half
year of 2002 there were more than 100 manufacturers providing communication
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antennas, and most of them were medium/small-sized. In terms of annual production
and sales volume, only a few of them have reached above 30 millions (RMB) with
more than 200 employees, which included Xi'an Haitian Antenna Technologies Co.,
Ltd., MOBI Antenna Technologies (SHENZHEN) co., Ltd., Kenbotong
Communication Ltd., Guangdong Shenglu Telecommunication Tech Co.,Ltd., and
Tongyu Communication Equipment Co., Ltd., etc..
Advantages of foreign antenna manufacturers
Most of foreign antenna manufacturers possess solid financial strength 、 high brand
fame and abundant talents and technologies. Several among them have a history of
over half a century, and world famous brands, with which their annual sales volumes
exceed USD 2 billions; while the annual sales volume of the largest Chinese domestic
antenna manufacturer is just over RMB 100 millions, which is just a small fraction of
that of the foreign giants. What’s more, after China entered WTO, famous foreign
antenna manufacturers, leading Allen Telecom Inc., Andrew company and Katherine
company, have started to invest in building factories in China, which has made a great
impact on China’s national antenna manufacturers.
Development trend of antenna industry
In 1897, Marconi created antenna and for the first time realized radio communication.
The history of antenna is just about a century, but because of its important application
in military, it has been highly valued and emphasized. Due to the development in the
past half century, the hardware technology of antenna is now relatively mature. Now
antenna design is developing towards wide band、multi-function and high density of
integration. Various kinds of antenna technologies like dual-polarization 、adjustable
electrical down-tilt and multi-frequency band multiplexing are gradually being
launched into commercial operation; great advances have been made in smart antenna
technology too.
After twenty years development, the technology gap between homemade antennas and
the famous foreign brands is being narrowed; some skills used in homemade antennas
are even equally matched with the advanced foreign skills. In China, the brand fame
and credit worthiness have been increased gradually. While the gaps in capital and
talents are still wide. Homemade antennas feature in moderate prices, good services,
and fulfilling the needs of communication construction.
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1.2 Theories of Antenna Radiation
Antennas must be able to efficiently convert the circuit signals in the BTS equipment to
electromagnetic waves in the free airspace and vice versa. The efficiency and ability of
antenna electromagnetic radiation is a very professional microwave technology. Here
introduced are some radiation theories of antenna element, which are most frequently
used currently.
1.2.1 Electromagnetic Wave Radiation of Electric Dipole
The conductor whose length is much shorter than the wavelength is referred to electric
doublet. When there is alternating current running in the wire, electromagnetic wave
radiation may happen, the power of which is affected by the length and shape of the
wire. As shown in Picture 1-1: if two wires are too close to each other, electromagnetic
wave will be limited inbetween them, and its power is very weak; if the two wires are
stretched to a certain angle, the Electromagnetic field will spread in the space around,
and the radiation power will increase.
We must note that when the wire length( L) is far shorter than the wave length (λ), the
radiation power is very weak; when wire length is increased to more or less the wave
length, the alternating current in the wire will strengthen accordingly, thus the radiation
will be more powerful.
Picture 1-1 Diagram of Relation between Doublet Angle and Radiation Power of
Electromagnetic Wave
1.2.2 Half-wave Dipole
Half-wave dipole is the most widely used type of antenna so far. A single half-wave
doublet can function independently or be used as paraboloid antenna feed; several half-
wave doublets can be adopted to form an antenna array.
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Dipole with two arms of the same length is called balanced dipole. When the length of
each arm is 1/4 wave length, and full length of two arms is 1/2 wave length, the dipole
is referred to half-wave doublet. See picture 1-2.
Picture 1-2 Half-wave Dipole
There is another kind of half-wave dipole, which can be obtained by forming a full-
wave doublet into a slender rectangular with the two ends of the doublet overlapped.
The slender rectangular is called folded dipole, the length of which is also 1/2 wave
length, so it is also referred as half-wave folded dipole.
Picture 1-3 Half-wave Folded Dipole
1.3 Internal Structure & Types of Mobile Communication BTS Antenna
1.3.1 Plate-shaped Directional Antenna
Directional plate-shaped antenna is the most widely used BTS antenna and is of great
importance. This kind of antenna features in high gain、good sector directional pattern、This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.
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small back beam 、 reliable sealability, convenient control over depression angle of
vertical pattern、and long performance life.
See Picture 1-4 for antenna appearing.
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Picture 1-4 Diagram of Directional Plate-shaped Antenna Appearing
1.3.1.1 Formation of High Gain
Picture 1-5 Vertically-set Line Array Formed by Several Half-wave Dipole
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Picture 1-6 Apply horizontally oriented theory by adding baffle-board to one side
the line array (take the line array of double half-wave dipole with baffle-board
as an example )
Currently, almost all antenna manufacturers adopt plate-shaped dipole array structure
in designing BTS directional antennas. Two types of dipole are adopted.
1.3.1.2 Balanced Dipole
It is standard half-wave balanced dipole (add one additional dipole to lower the
dipole’s ground clearance, and reduce the thickness of antenna).
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Picture 1-7 Directional plate-shaped antenna formed with several half-wave dipole
1.3.1.3 Microstrip Dipole
It is a variant of half-wave dipole. It uses transmission line of 1/4 wave length to form
radiation.
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Picture 1-8 Directional plate-shaped antenna consisting of several microstrip
dipole
1.3.1.4 Dipole Array Structure of BTS Antenna
Picture 1-9 Structure of plate-shaped directional antenna array
1.3.2 Isotropic Dipole Antenna with Series Feed
Isotropic antenna adopts several half-wave dipoles with series feed to create and
increase radiation gain.
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Picture 1-10 Structure of Isotropic Dipole Antenna with Series Feed
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2 Concepts & Meanings of Antenna Parameters
Knowledge point
This chapter mainly introduces the concepts of BTS antenna technology parameters
and their significant application in network planning.
2.1 Antenna Gain
Gain as a parameter measures the directionality of a given antenna. The gain of an
antenna is a passive phenomenon - power is not added by the antenna, but simply
redistributed to provide more radiated power in a certain direction than would be
transmitted by an isotropic antenna. If an antenna has a greater than one gain in some
directions, it must have a less than one gain in other directions since energy is
conserved by the antenna. Sometimes, the half-wave dipole is taken as a reference
instead of the isotropic radiator. The gain is then given in dBd (decibels over dipole).
Antenna gain is usually expressed in dBd or dBi. dBi represents a reference value( of
the antenna field strength in the direction with highest radiation power) relative to
isotropic radiator (see Picture 2-11); antenna gain of half-wave dipole is expressed in
dBd. There is a fixed difference between the two (see Picture 2-11), that is 0dBd equals
to 2.15dBi.
Picture 2-11 Diagram of antenna gain in dBi and dBd
Currently, antennas with gains ranging from 0dBi to 20dBi are used in both domestic
and foreign BTSs. Gain of antenna used for indoor micro cellular coverage usually
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ranges from 0 to 8 dBi; that of antenna on outdoor BTS is from 9dBi (isotropic
antenna gain) to 18dBi (directional antenna gain).
Antennas with around 20dBi gain and relatively narrow beam width are usually used in
the coverage on the vast but less populated highways.
2.2 Directional Diagram of Radiation
Directional diagram of BTS antenna radiation falls into two types: isotropic radiation
diagram and directional radiation diagram. As shown in Picture 2-2, the two on the left
of the diagram are the horizontal sectional drawing and solid radiation drawing of
isotropic antenna; on the right are the horizontal sectional drawing and solid radiation
drawing of directional antenna. The radiation strength of isotropic antenna in all
directions of the same horizontal plane is theoretically the same. It applies to isotropic
cells. The red part in Picture 2-2 is the metal baffle-board in the radome of directional
antenna, which enables the antenna to radiate into different directions on the horizontal
plane. It applies to cover fan-shaped cells.
Picture 2-12 Field strength diagram of isotropic antenna and directional antenna
2.3 Beam Width
2.3.1 Horizontal Beam Width
The horizontal beam width of isotropic antenna is 360 (see the right one in Picture 2-
3), while the common horizontal beam width (3dB) of directional antenna includes:
20、30、65、90、105、120、180 (see the left one in Picture 2-3).
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Picture 2-13 Diagram of horizontal beam width (3dB) of BTS antenna
The gain of antenna with 20or 30 horizontal beam width is higher, which is usually
used in covering strip areas and highways; antenna with 65 beam width is used in the
coverage for the typical three-sector sites in dense urban area; antenna with 90 beam
width is used in the coverage of the three-sector of typical site in suburb area; antenna
with 105 beam width is used in the coverage of the three-sector of typical site in the
vast and less populated area. See Picture 2-14.
Antenna with 120、180beam width is used to cover the special-shaped sectors with
extremely wide angles.
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Picture 2-14 Diagram of BTS antenna coverage
2.3.2 Vertical Beam Width
Picture 2-15 Diagram of BTS antenna vertical with 3dB beam width
The 3dB vertical beam width is closely related to the antenna gain and the 3dB
horizontal beam width. The 3dB vertical beam width is about 10. Generally, in the
antenna with same gain and same antenna designing skills, the wider the horizontal
beam width is, the narrower the vertical beam width will be.
The narrower 3dB vertical beam width will create larger shade area, as shown in
Picture 2-15. Of the two sets of antennas with the same height and without down-tilt,
the shadow area created by the wider vertical beam width is OX’’ (red area), which is
smaller than that created by the narrower vertical beam width OX (blue area).
When selecting antenna types, with the same antenna gain, we’d better select
those with wider 3dB vertical beam width, so as to ensure the coverage of the
serving area and reduce shadow area.
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2.4 Frequency Band
For different BTSs, the frequency band of antenna selected should include those
required.
For GSM900 system, double-frequency antennas for 890-960MHz、870-960MHz、807-
960 MHz and 890-1880 MHz are all applicable.
For CDMA800 system,use 824-896MHZ antenna.
For CDMA1900 system, use 1850-1990MHZ antenna.
In order to reduce the out-of-band interference signal, band width of the selected
antenna may just satisfy the frequency band’s requirement.
2.5 Polarization Mode
BTS antenna usually adopts linear polarization mode, as shown in Picture 2-16. Uni-
polarization antenna often adopts vertical linear polarization; dual-polarized antenna
often adopts 45bilinear polarization. As one dual-polarization antenna is made up of
two orthogonal antennas in one radome (Picture 2-17), the adoption of dual-
polarization antenna can help reduce number of antennas, simplify installation
procedures, reduce cost and occupied space.
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Picture 2-16 Common polarization mode of BTS antenna
Picture 2-17 Diagram of dual-polarization BTS antenna
2.6 Down-tilt Mode
In order to expand the coverage on areas nearby the BTS, and reduce shadow area and
interference to the adjacent BTSs, antenna shall not be installed too high, and in the
mean time down-tilt mode shall be adopted. As shown in Picture 2-18, the shadow
areas created by the lower antenna (yellow) and the down-tilt antenna (green) are OX’’
and OX’, which are both smaller than that created by the high antenna (blue) without
down-tilt OX.
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Picture 2-18 Diagram of BTS antenna down-tilt
There are several modes of antenna down-tilt: mechanical down-tilt, settled electrical
down-tilt, adjustable electrical down-tilt, remote-adjustable electrical down-tilt. As for
mechanical, it is just set slant during installation; it is often used in antenna with down-
tilt angle within 10. When the down-tilt is further expanded, the right ahead of the
coverage will appear sunken, and the two edges appear squashed, the directional
diagram becomes deformed, and in the mean time interference to the adjacent BTSs
becomes stronger, as shown in Picture 2-19. Another defect of mechanical down-tilt is
that the back lobe of antenna will upward, which can result in interference to the
adjacent sectors and call drop to MS on upper layer of adjacent cells.
Picture 2-19 Comparison on BTS antenna down-tilt modes
The angle of electrical down-tilt antenna is relatively wide (can be wider than 10); the
directional diagram is not apparently out of shape; the back lobe declines in the mean
time; there is no interference to MS on adjacent high buildings.
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2.7 Antenna Front-to-back Ratio
The indicator of antenna front-to-back ratio is related to the electric dimension of
antenna baffle-board. Larger electric dimension can provide better front-to-back ratio
index. For example, the horizontal dimension of antenna with horizontal 3dB beam
width and 65angle is larger than that of antenna with horizontal 3dB beam width and
90 angle, thus the former one excels the later one. The front-to-back ratio of outdoor
BTS antenna should be larger than 25dB. Since micro cellular antenna is relatively
smaller in dimension, its front-to-back ratio index should be smaller than 25dB.
2.8 Antenna Input Impedance( Zin)
Definition: antenna input impedance is the ratio of input-end signal voltage to the
signal current. It consists of resistive component (Rin) and reactive component (Xin),
Zin= Rin + j Xin. The existence of Xin will reduce the signal power that antenna
extracts from the feed, so we shall make Xin=0 as possible, that is we shall make
antenna input impedance a pure resistance. As a matter of fact, even if it is a well-
designed and well-modulated antenna, its input impedance still consists of Xin of small
value.
Input impedance is related to the structure、dimension and operating wavelength of the
antenna. Half-wave dipole is the most important basic antenna, its input impedance is
Zin = 73.1+j 42.5 (Ω). When its wavelength is shortened by (3~5)%, the Xin
in it can be eliminated, and the antenna input impedance can be pure resistance; its
input impedance now is Zin = 73.1(Ω), (characteristic impedance 73.1Ω). Precisely
speaking, antenna input impedance of pure resistance is only considered in terms of dot
frequency.
The input impedance of half-wave folded dipole is four times that of half-wave dipole,
that is Zin = 280 (Ω) (characteristic impedance 300Ω).
However, within the required operating frequency range, through modulation of
antenna impedance, we can make the imaginary part of the input impedance very small
and real part very close to 50Ω, thus the antenna input impedance can be Zin = Rin =
50Ω, which is a must for good impedance match between antenna and the feed.
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2.9 Antenna VSWR
Antenna VSWR is the indicator for matching degree between antenna feed and BTS.
Definition of VSWR:
Umax——the anti-node voltage on feeder
Umin——the wave node voltage on feeder
Because the input wave can not be absorbed completely when it’s delivered to the
input end B of antenna, and reflection wave occurs and overlays, VSWR will form.
The larger VSWR is, the larger the reflection will be, and the worse the matching will
be.
What bad effects can be resulted from bad VSWR? What is the acceptable VSWR? An
appropriate VSWR shall be the balance between the quantity of energy lost and the
generating cost.
(1) VSWR>1, it means that some of the power input into the antenna is reflected,
and the radiation power is reduced;
(2) The feed loss is increased. Loss rate of 7/8"cable is 4dB/100m, which is the result
based on the condition of VSWR=1 (perfectly-matched); energy loss is increased
due to the reflection of power, thus the input power from feed to antenna is
reduced.
2.10Side Lobe Suppression & Null Fill
As antennas are usually installed on high places of metal towers or buildings, side
lobes on vertical plane (especially the first side lobe) shall be suppressed as much
as possible, so as to reduce energy loss. In the mean time, the compensation to the
downward side lobe null on vertical plane shall be strengthened to make the null
depth of the sector pattern shallow, so as to improve the coverage of adjacent area
of BTS and reduce shadow area and blind spots. Picture 2-20 shows the contrast
between BTS antenna with null fill and that without null fill, the horizontal
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ordinate showing the distance to the BTS, the vertical ordinate showing the
ground signal strength.
Picture 2-20 Diagram of comparison between BTS antenna with or without null fill effect
Null fill = (vertical first lower null fill value/ radiation direction maximum)%
= 20log (vertical first lower null fill value/ radiation direction maximum)dB
Strictly speaking, antennas without features of side lobe suppression and null fill shall
not be used, so as to ensure good coverage over the serving area.
2.11 Third-order Intermodulation
The third-order intermodulation indicator of most foreign antennas can reach -
150dBC@243dBm, while that of the common antennas is just -130 d B C@2 43d B m
. The difference is related to the design of antenna and the selection of connector.
Because the strength of received signal is much weaker than that of transmission signal
at BTS, once the intermodulation product of transmission signal of multi-carrier falls
into the receiving frequency band, the BTS will not be able to function normally.
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2.12Isolation between Ports
When multiport antenna is used, the isolation between ports shall be more than 30dB.
For example, isolation between the two different polarized ports of dual-polarization
antenna, isolation between the two ports with different frequency bands of outdoor
double-frequency antenna, isolation among the four ports of double-frequency and
dual-polarization antenna, all should be more than 30dB.
2.13Case
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3 Measurement of Key Antenna Indicators Knowledge point
This chapter mainly introduces the measurement method, measurement theory and
measurement environment of key indicators of BTS antenna.
3.1 Measurement of Antenna Gain
(1) Choose standard plain antenna measurement field. Indstall antenna and connect
instruments and meters according to Picture 3-21;
Picture 3-21 Block diagram of measurement of antenna gain、half-power beam
width、front-to-back ratio、cross polarization discrimination
(2) Direct datum gain antenna to the direction of central lobe of the source antenna,
then take down the receiving level of datum gain antenna P1(dBm);
(3) Direct the measured antenna to the direction of central lobe of the source antenna,
take down the receiving level of datum gain antenna P2(dBm);
(4) Calculate and get the gain of the measured antenna: G=(gain of datum antenna
G0)+(P2-P1);
(5) In order to show the gain feature of antenna in the given frequency band, at least
measure three frequency points in high /medium/low frequency band.
3.2 Measurement of 3dB Beam Width & Front-to-back Ratio
(1) Choose standard plain antenna measurement field. Install antenna and connect
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instruments and meters according to Picture 3-21;
(2) Direct the measured antenna to the direction of central lobe of the source antenna,
then start the rotating floor to make the measured antenna rotate horizontally, and take
down the receiving level of the measured antenna according to the function of angles of
rotation, then from the function curve get the half-wave beam width and front-to-back
ratio of the measured antenna;
(3) Keep the measured antenna aflat with its top pointing at the direction of central
lobe of source antenna, then start the rotating floor to make the measured antenna
rotate horizontally, and take down the receiving level of the measured antenna
according to the function of angles of rotation, then get the vertical half-wave beam
width of the measured antenna from the function curve;
(4) In order to show the gain feature of antenna in the given frequency band, at least
measure three frequency points in high /medium/low frequency band.
3.3 Measurement of Antenna VSWR
(1) Choose standard plain antenna measurement field or chamber without echo to
install antenna according to Picture 3-22;
Picture 3-22 Block diagram of VSWR measurement
(2) At the calibration port, fine-tune the meters and instruments with a short circuit
device or a open circuit instead of the measured antenna;
(3) Connect the calibration port with the measured antenna, read the VSWR of the
measured antenna.
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3.4 Isolation Measurement of Dual-Polarized Antenna
(1) Choose standard plain antenna measurement field or chamber without echo to
install antenna according to Picture 3-23;
Picture 3-23 Block diagram of antenna isolation
(2) Short-circuit the two feeds to the measured dual-polarized antenna, reset the
meters to 0dB;
(3) Connect the meter feeds with the measured antenna, read the worst isolation of the
measured dual-polarized antenna.
3.5 Measurement of Intermodulation
(1) Choose standard plain antenna measurement field or chamber without echo to
install antenna according to Picture 3-24;
Picture 3-24 Block diagram of antenna intermodulation measurement
(2) In the operating frequency band, choose two appropriate frequencies f1h and f2h ,
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make the intermodulation product f3=2f2-f1 (or f3=2f1-f2), which is also required to
be within the operating frequency band;
(3) F1 and f2 input 20W individual tone power to the antenna simultaneously;
(4) With f3 receiver, read the level of three-order intermodulation product.
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4 Antenna Parameters
Knowledge point
This chapter mainly introduces the engineering parameters of antenna in network
planning and their influences on coverage, and how to improve network performance
with changes in engineering parameters.
4.1 Antenna Directional Angle
The electromagnetic field of antenna radiation distributes in compliance with the
angular coordinates in a fixed distance, the graph of which is called directional diagram
(directional pattern). The diagram expressed with field strength is called field strength
pattern. The diagram expressed with power density is called power pattern. The
diagram expressed with phase is called phase pattern.
Antenna directional diagram is a solid figure in space, but it is usually expressed with
the pattern formed with two orthogonal principal planes, which is called plane pattern (
or vertical pattern or horizontal pattern). As for horizontal pattern, it is classified into
isotropic antenna pattern and directional antenna pattern. Directional antenna pattern
consists of many shapes, such as heart-shaped and 8-shaped, etc..
Antenna has directional property due to oscillator array and changes in oscillator feed
phase, which is theoretically similar to optical interference effect. Therefore, energy in
some directions is increased, while in other directions is reduced, that is how antenna
lobes (beams) and null come into being. The lobe with strongest energy is called
principal lobe; the upper/lower lobe with second strongest engergy is called first side
lobe; make analogy like this. As for directional antenna, it has back lobe except for
those lobes above.
Adjustment of antenna directional angles is of great importance to the performance of
mobile communication network. On one hand, precise directional angles can ensure the
real BTS coverage meeting our expectations and thus ensure the operation quality of
the whole network; on the other hand, adjustment of directional angles according to
call traffic volume and actual network conditions can further optimize the existing
networks.
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Based on ideal cellular communication model, directional BTS is usually divided into
three cells in the current GSM system (mainly with ERICSSON equipment):
Cell A: azimuth angle 0°, antenna pointing to due north;
Cell B: azimuth angle 120°, antenna pointing to southeast;
Cell C: azimuth angle 240, antenna pointing to southwest.
In GSM network construction and planning, we install and adjust antenna azimuth
angle strictly according to the rules above, which is one of the installation
specifications. Deviation in the setting of azimuth angle can result in unreasonable
discrepancy between the real coverage and the designed coverage and some
unexpected co-channel or adjacent channel interference.
In the real GSM network, specific landforms like high buildings 、mountains and water
face, etc. can cause signal refraction and reflection, which can result in big discrepancy
between the real coverage and the ideal model, thus some areas enjoy stronger signals,
while some suffer from weaker ones. In this case, we shall appropriately adjust the
antenna azimuth angle according to the real network situation, so as to ensure the
signal strength in the weak areas and achieve the goal of network optimization.
Besides, the discrepancy in real population density of different areas can cause call
traffic unbalance in cells under the antennas’ coverage. Again we can adjust the
antenna azimuth angle to make traffic balanced. Certainly, we usually don’t
recommend adjustment in antenna azimuth angle, because interference in system may
be resulted to some degree. While in some special circumstances, like some emergent
meetings or large-scale public activities, the traffic concentrates in come cells, we can
make adjustment in antenna azimuth angle for the occasion, in order to balance the
traffic and optimize the network. What’s more, as for the signal blind zone or weak
zone in suburb, we can also achieve network optimization through adjustment in
antenna azimuth angle; but remember to test the signal in around area with field
strength test vehicles, so as to ensure the network operation quality.
4.2 Antenna Height
The signal power received at the receiver is related to many factors, which can be
concluded into two types:
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Parameters at the transmitting terminal and the receiving terminal;
Interference from landforms and ground objects.
Parameters at transmitting and receiving terminals include: transmitting
power、antenna gain、feed loss、antenna height、operating frequency, and the distance
between transmitting terminal and receiving terminal. Interference from landforms and
ground objects is caused by landform fluctuations and ground objects’ screening of
signals. All transmission models are related to the height of antennas, therefore antenna
height has great influence on path loss.
The coverage distance between transmitting terminal and receiving terminal can
approximately be expressed with the following formula:
receiving power
transmitting power
height of receiving antenna
height of transmitting antenna
gain of receiving antenna
gain of transmitting antenna
path-loss improvement factor
When parameters at transmitter and receiver are fixed, the coverage distance is in direct
proportion to the height and gain of antenna.
Due to fewer base stations at the beginning period of GSM network construction, BTS
antennas were usually installed relatively higher. As mobile communication has
developed swiftly in recent years, number of BTS has increased dramatically; there is a
site every 500m in urban areas. In this case, we must reduce the BTS original coverage
and lower antenna height, or network quality will be severely influenced. Main related
influences fall into three aspects as shown bellow:
(1) Unbalanced traffic. Over high antenna will result in over large BTS coverage and
too much traffic in one BTS. While the traffic in adjacent BTS is less due to its small
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coverage and being covered by the BTS with too large coverage, thus it can not
function well and unbalanced traffic will be caused.
(2) Interference within system. Over high antenna can cause cross-BTS interference
(mainly including co-channel interference and adjacent channel interference) 、 call
drop 、 cross talk and too much stray noise, thus the quality of the whole radio
communication network will decrease.
(3) Island effect. It is about BTS coverage problem. When the BTS is covering
special landforms like vast water face or mountainous areas, though the original
coverage distance remains the same, an “exclave area” will appear in the far distance,
because of the reflection from water face or mountains; while the adjacent BTSs
having handover relation with the BTS can not be covered due to obstruction of the
landforms. Therefore, handover relation between the “exclave area” and the adjacent
BTSs does not exist, and the “exclave area” becomes an island. When a MS uses the
signal in the “exclave area”, it can easily suffer from call drop due to lack of handover
relation.
4.3 Antenna Down-tilt
Make the principal lobe of antenna lean in a certain angle through antenna down-tilt, to
reduce the power level to the adjacent BTS, which is to reduce interference.
As a matter of fact, the value of antenna down-tilt is directly related to the parameters
of antenna height 、coverage semi-diameter 、vertical beam and electrical down-tilt.
When the coverage semi-diameter is fixed, the higher the antenna is, the larger down-
tilt is needed; if the antenna height is fixed, the smaller the coverage semi-diameter is,
the larger the down-tilt should be.
In urban area where the scattering of BTSs is dense, it’s quite easy that interference
between BTSs occur. In order to make most of the energy be radiated within the
coverage and reduce interference from adjacent cells, when setting the initial down-tilt,
we should make the half-power points on the principal lobe aim at the coverage rim.
The calculation formula is as follows:
α= arctg(2H/L)×(180/π)+(β/2)–γe
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In areas like suburb、villages、highways and sea face, in order to get coverage as far as
possible, we can narrow the down-tilt, and make the maximum gain point aim at the
coverage rim. The formula of down-tilt is shown bellow:
α= arctg(H/L) ×(180/π)+(β/2)–γe
In the above formulas,
α is the initial mechanical tilt of the antenna, expressed in degree;
H represents the effective height of the BTS, which is the difference between the fixed
location of antenna and the average height of the area covered, expressed in meter;
L represents the distance from the BTS antenna to the sector’s rim needed to be
covered, expressed in meter;
β represents vertical beam width of the antenna, expressed in degree;
γe represents down-tilt of the antenna, expressed in degree.
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5 Antenna Types
Knowledge point
In this chapter, we’ll get to know the classification methods of antenna, the basic
classification of BTS antenna; and through examples of typical antennas, get to know
the value range of antenna and the exterior appearance of different types of antennas.
5.1 Antenna Types
There are various kinds of antennas, so as to meet requirements of different
frequencies、different applications、different situations. For so many kinds of antennas,
there are a variety of classification methods:
Classification by use: communication antenna, TV antenna, radar antenna, etc.;
Classification by operation frequency band: short wave antenna, ultra-short wave
antenna, microwave antenna, etc. ;
Classification by appearance: line-shaped antenna, plane-shaped antenna, etc. ;
Classification by directivity: isotropic antenna, directional antenna, etc..
Since there is no big difference in operating frequency、antenna gain and front-to-back
ratio of different antennas used in mobile communication systems, we’ll analyze and
compare them in terms of down-tilt’s influence on antenna pattern and radio network.
Isotropic antenna
Isotropic antenna radiates in all directions (360°) of the horizontal pattern, which
means it radiates without particular directivity. As for the beam with fixed width on
vertical pattern, normally the smaller the beam width is, the higher the antenna gain
will be. This kind of antenna is often used in BTS at suburb with large-cell model,
since it features in larger coverage.
Directional antenna
Directional antenna radiates within some certain angle, which means it radiates with
particular directivity. As for the beam with fixed width on vertical pattern, normally the
smaller the beam width is, the higher the antenna gain will be. This kind of antenna is
often used in BTS at urban area with small-cell model, since it features in smaller
coverage, denser subscriber intensity and higher frequency utility ratio.
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We set up different types of BTSs according to requirements of network construction;
and different types of BTSs use different types of antennas according to their needs.
The basis for using different antennas is the technical parameters mentioned above.
Isotropic BTS adopts isotropic antenna with same antenna gain in all horizontal
directions; and directional BTS adopts directional antenna with changing antenna gain
in a particular horizontal direction. Usually antenna with horizontal beam width B=65°
is used in urban area; and antenna with horizontal beam width B=65°、90° or 120° is
often used in suburb (based on the BTS type and the local landforms); while in
villages, the application of isotropic antenna with large coverage is the most economic.
Mechanically-adjustable antenna
The so called mechanically-adjustable antenna is a kind of antenna whose down-tilt can
be adjusted mechanically.
After mechanically-adjustable antenna is vertically installed, position of the bracket at
the back of it can be adjusted, if there is need in network optimization. During the
adjustment of bracket, there is obvious change in the coverage distance of the principal
lobe, but the amplitudes of vertical component and horizontal component remain the
same, thus the antenna pattern is easy to be out of shape. Practice has proved that the
best down-tilt range is 1°-5°. When the down-tilt changes from 5° to 10°, the antenna
pattern deforms a little bit. When it changes from 10° to 15°, the antenna pattern
deforms to a larger extent. When it is larger than 15°, there is big change on the
antenna pattern, which changes from a pear-shaped one to a spindle-shaped one; now
the coverage distance of principal lobe is obviously shortened, but not all parts of the
antenna pattern are within the BTS sector, which means that signal from this BTS can
be received in the adjacent BTS sector, thus serious interference in the system will be
caused.
Besides, if it’s needed to adjust the down-tilt of this kind of antenna in the daily
maintenance, the whole system shall be shut down. We can not keep monitoring
network indicators while adjusting the down-tilt. It is very troublesome to adjust the
down-tilt, because it needs maintenance staff to climb to where the antenna is located.
The down-tilt angle is a theoretical value obtained from the calculation of computer
analog analysis software, which is a little deviated from the actual best down-tilt. The
stepping degree for adjusting down-tilt is 1°, and the third-order intermodulation
indicator is -120dBc.
Electrical antenna
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The so called electrical antenna is a kind of antenna, which features in electrical down-
tilt.
The theory of electrical down-tilt is to change the amplitudes of vertical component
and horizontal component and the field strength of compound component, so as to
make vertical pattern of the antenna decline. Because the antenna field strength
increases and decreases at the same time in different directions, the antenna pattern
won’t change too much after down-tilt adjustment, the coverage distance of principal
lobe will be reduced, and in the mean time the antenna pattern will reduce its coverage
over the serving sector without producing any interference. Practice has proved that
when the down-tilt of electrical antenna changes from 1° to 5°, the antenna pattern is
roughly the same as that of mechanically-adjustable antenna. When the down-tilt
changes from 5° to 10°, the antenna pattern is improved a little compared with that of
mechanically-adjustable antenna. When down-tilt changes within 10° and 15°, the
antenna pattern changes greatly. When the down-tilt is larger than 15°, the antenna
pattern is obviously different from that of mechanically-adjustable antenna, its shape
isn’t changed much, but the coverage distance of principal lobe is obviously reduced,
and the whole antenna pattern is within the BTS sector; we can increase down-tilt to
reduce the sector coverage without any interference. This is the expected ideal antenna
pattern. It’s proved that adoption of electrical antenna can reduce call loss and
interference.
What’s more, electrical antenna allows down-tilt adjustment without shutting down the
system, which keeps the monitoring on adjustment effects. The stepping precision is
relatively higher (0.1°), thus we can make fine adjustment on the network. The third-
order intermodulation indicator for electrical antenna is -150dBc; there is a difference
of 30dBc compared with that of mechanically-adjustable antenna. This is in favor of
eliminating adjacent-channel interference and stray interference.
Dual-polarized antenna
Dual-polarized antenna uses new technology. It combines two sets of antennas with
mutually orthogonal polarization directions (+45° and -45°) and works in the duplex
model of transmitting and receiving signals in the mean time. Therefore, its outstanding
advantage is that it saves antennas for directional BTS. Normally the directional BTS
(three-sector) of GSM digital mobile communication network needs to use 9 antennas;
each of its sectors uses 3 (for space diversity, one for transmitting, two for receiving
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signals). If dual-polarized antenna is adopted, each sector needs only one antenna. In
the mean time, the orthogonality of ±45° polarization can ensure that the isolation
between the two sets of antennas (+45° and -45°) meets the requirement from
intermodulation ( ≥ 30dB ) , therefore the space interval between dual-polarized
antenna is just 20-30cm. Besides, dual-polarized antenna also possesses advantages
like reducing call loss and interference and improving the whole network quality,
which are the same as those of electrical antenna. What’s more, there is no specific
requirement for installing dual-polarized antenna and no need to acquire land for
building antenna tower. What’s needed is just a metal pole with 20cm diameter, and
then fix the antenna on the pole in the corresponding coverage direction. In this way,
basic construction cost is saved, and layout of BTS is more reasonable, and it’s much
easier to selection BTS location.
As for the selection of antenna type, we should select the antenna which meets the
needs of the local mobile network according to specific situations like network
coverage, call traffic volume, interference and network service quality.
--- in dense BTS area with high traffic volume, use dual-polarized antenna and
electrical antenna;
--- in suburb area with fewer BTS and low traffic, where larger coverage is required,
use the traditional mechanically-adjustable antenna.
Currently in area with dense traffic, the network call loss is high and interference is
big. One important reason is that the down-tilt of mechanically-adjustable antenna is
too large, which causes serious deformation of antenna pattern. In order to solve the
off-capacity problem we must shorten the distance between BTSs and enlarge down-
tilt. While if mechanically-adjustable antenna is used, antenna pattern starts to deform
when the down-tilt is larger than 5°; when it’s larger than 10°, antenna pattern will be
seriously out of shape. Therefore, the problems can not be solved by using
mechanically-adjustable antenna. It is recommended to use electrical antenna or dual-
polarized antenna instead of mechanically-adjustable antenna in dense traffic area. The
replaced mechanically-adjustable antennas can be used in areas with less traffic, like
villages and suburb.
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6 Circumstances of Antenna Application
Knowledge point
In this chapter, several real scene pictures are shown to illustrate antenna coverage
area classification and related radio environment.
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6.1 Dense Urban
Picture 6-25 Dense urban
6.2 Urban (Towns)
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Picture 6-26 Urban (towns)
6.3 Suburb (Counties) & Villages
Picture 6-27 Suburb
6.4 Railways/ Highways (Roads)
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Picture 6-28 Railways & highways
6.5 Scenery Areas
Picture 6-29 Scenery areas
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7 Selection of Antenna Type
Knowledge point
Combining with the real scene pictures, this chapter introduces classification of
coverage area and the selection of corresponding antenna.
7.1 BTS antenna selection for urban
Application environment feature: In urban area BTSs are densely distributed. The
coverage of a single BTS shall be small in order to avoid overshooting and reduce
inter-BTS interference. And in the mean time increase frequency multiplexing
Principles for antenna selection:
(1) Selection of polarization model: because it’s difficult to obtain BTS location in
urban area and there is space limit for antenna installation, it is recommended to use
dual-polarized antenna;
(2) Selection of antenna pattern: improving frequency multiplexing shall be mainly
considered in urban area, so directional antenna is the best option;
(3) Selection of half-power beam width: in order to better control the cell coverage
and suppress interference, half-power beam width shall be within 60~65°;
(4) Selection of antenna gain: since large coverage is not needed in urban area, it is
recommended to use antenna with medium gain (15-18dBi). The microcells used for
filling blind zones can use antennas with lower gain;
(5) Selection of down-tilt: in urban area, antenna tilt needs to be adjusted frequently,
and some antenna tilts are required to be set larger. Since mechanically-adjustable
antenna is good for controlling interference, it is suggested that antenna with preset
down-tilt be adopted. Therefore, antenna with fixed electrical down-tilt is
recommended; or electrical antenna when related conditions are satisfied.
7.2 BTS antenna selection for suburb
Application environment feature: in suburb or villages, BTSs are sparsely distributed
and call traffic volume is small, thus large coverage is requested. There is only one
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BTS in some places and large coverage is of great importance. In this case, we need to
take into consideration around areas of the BTS which need to be covered, when we
make antenna selection.
Principles for antenna selection:
(1) Selection of antenna pattern: if the BTS is required to cover the around area
without particular directivity, and call traffic is scattered, it is suggested that isotropic
antenna be used. We should note that coverage of isotropic antenna is not as far as that
of directional antenna due to its smaller antenna gain; and pay attention to the tower
body’s influence on coverage; and the antenna shall be kept vertical to ground. If
farther coverage distance is required by customers, then directional antenna shall be
adopted; normally, horizontal half-power directional antennas with beam width of 90 °,
105 °, 120 ° are recommended;
(2) Selection of antenna gain: based on coverage requirement, it is recommended to
adopt directional antenna with higher gain ( 16-18dBi ) or isotropic antenna with
gain of 9-11dBi in suburb area or villages;
(3) Selection of down-tilt: adjustment on antenna down-tilt is seldom needed in
suburb area, and requirement on adjustment range is not high, it is suggested that
mechanically-adjustable antenna be used; meanwhile, if antenna height is above 50
meters and there is request for coverage of proximal area, we can use antenna with
null-fill antenna to avoid blind zone under the tower.
7.3 BTS antenna selection for highway coverage
Application environment feature: on highways, call traffic volume is low and
subscribers move fast, so coverage is the key issue. Usually, the antenna needs to
provide zonary coverage, so directional antenna is recommended to be used. Isotropic
antenna can be adopted where the highway passes through towns or scenery areas. Vast
coverage is also requested, so we need to select antenna type based on BTS location and
BTS type. Conditions of highways can be quite different. There are plain and straight
ones, like express highway, railway, national highway and provincial highway, etc.. It is
recommended to set up BTSs at the side of highway and adopt S1/1/1 or S1/1 BTS type,
and equip the BTSs with directional antennas of high gain to meet coverage
requirement. There are also winding highways, like roads in mountainous areas and
small towns. In these circumstances, we should set up BTSs at high places so as to
cover the villages around.
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When selecting antenna at the initial planning stage, we should try to select antenna
with high gain and vast coverage.
Principles for antenna selection:
(1) Selection of antenna pattern: for BTSs to cover areas along railways and
highways, we can adopt directional antenna with narrow beam width and high antenna
gain. Besides, we can flexibly choose antenna type according to factors like landforms
and turnings on railways and highways;
(2) Selection of antenna gain: we can choose directional antenna with 17dBi-22dBi
gain; as for isotropic antenna, 11dBi;
(3) Selection of down-tilt: usually there is no need to set down-tilt for coverage of
highways, so it is recommended to use mechanically-adjustable antenna which is less
expensive. If antenna height is above 50 meters and there is request for coverage of
proximal area, we can use antenna with null fill (>15%) to avoid blind zone under
the tower.
(4) Selection of front-to-back ratio: since most subscribers within antenna coverage
on highway move fast, the front-to-back ratio shall not be too large, so as to ensure
smooth handovers.
7.4 BTS antenna selection for mountain area
Application environment feature: in remote hills or mountainous area, the attenuation of
radio wave is relatively large due to obstruction of mountains, and coverage in these
areas is a difficult issue. Normally we use large coverage, because just a small number
of subscribers scatter within the large coverage semi-diameter and call traffic volume is
low. BTSs should be set up at the appropriate locations on top of mountains, hill-side, or
mountain foot. We need to select BTS location, BTS type and antenna type based on
different subscriber dispersion and landforms. Here listed are the common situations of
BTS construction: BTS construction in mountainous basin, BTS construction on high
mountains, BTS construction on hillside, BTS construction in common mountainous
area, etc..
Principles for antenna selection:
(1) Selection of antenna pattern: make selection based on BTS location, BTS type and
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request for coverage of around area; both directional antenna and isotropic antenna are
optional. As for BTSs on mountains, if the area to be covered is relatively low, we
should choose antenna pattern with larger vertical half-power angle to meet the
coverage needs in horizontal direction;
(2) Selection of antenna gain: use antennas with medium gain according to coverage
distance required; isotropic antenna (9-11dBi), directional antenna (15-18dBi);
(3) Selection of down-tilt: when we set up BTS on mountains and mountain foot area
needs to be covered, choose antenna with null fill or preset down-tilt. Amplitude of the
preset down-tilt is decided based on the relative height between the BTS and the area to
be covered. The larger the relative height is, the bigger the antenna preset down-tilt
should be.
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8 Installation & Modulation of Antenna
Knowledge point
This chapter mainly introduces the methods and essentials for installation and
adjustment of BTS antenna.
8.1 Antenna installation on Pole
8.1.1 Installation pole must be straight
Crooked pole or mishandled installation may cause the installation pole to incline,
which can affect accuracy of down-tilt and the receiving effect of isotropic antenna.
Therefore, we must first make sure installation pole is straight; a plumb bob can be
sued to check the straightness, so that we can ensure the isotropic antenna is vertical to
the ground after installation. Directional antenna’s down-tilt must be measured with tilt
meter. Take into consideration the inclination and curve of the pole when set
mechanical down-tilt.
During network planning and optimization, whether the installation pole is straight can
greatly influence the network performance. However, we often neglect to check the
straightness of the pole.
8.1.2 Lightening protection
Lightening protection must be properly handled during antenna installation, in order to
avoid lightening disturbance to BTS, and especially to antenna system on high
mountains, so as to ensure the safety of BTS structures, working staff, and equipment
inside BTS and its normal operation.
A complete lightening protection device must include:
1. lightening arrester: for controlling lightening strike point and prevent dangers to
equipment;
2. good grounding structure and earth resistance;
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3. well-designed down lead;
4. good equipotential bonding to avoid high voltage counterpunch;
5. ability to avoid high voltage surge caused by lightening.
Radio antenna should be installed within angle of protection of lightening rod (45°).
Lightening rod and down lead should be connected by welding. Material of down lead
should be galvanized flat steel (40mm×4mm). The distance between the joint of down
lead to ground net and the joint of ground lead to ground net shall be no less than 10m.
8.1.3 Diversity reception
In mobile communication, multipath transmission can cause signal fast fading, and
amplitude of level fading can reach 30dB, which means 20 times in a minute. Antenna
diversity reception skill can greatly reduce attenuation of receiving signal and improve
link quality. The principle for fixing antenna spacing interval is to ensure that different
antenna branches irrelevant to each other. Use the cross correlation coefficient of
branch signals to measure signal’s independence. Correlation coefficient of receiving
signal shall be smaller than 0.7.
Diversity distance of uni-polarized antenna
The horizontal space diversity distance for uni-polarized antenna is 20 , vertical space
diversity distance is about 15 . When BTS antenna spacing interval is fixed, increase
in antenna height can help reduce the relativity between antennas’s receiving signal.
Gain of horizontal space diversity is about 3~5dB, and gain of vertical space diversity
is about 2 ~ 4dB. Performance of horizontal space diversity is better than that of
vertical space diversity.
In actual project implementation, the horizontal diversity distance between two uni-
polarized antennas of the same sector shall be at least 10 .
Table 8-1 Horizontal diversity distance of antenna
Operating
frequency
Horizontal space diversity
distance
Vertical space diversity
distance
Min. Recommended Min. Recommended
450M 6.7m 13m 10m
800M 3.6m 7m 5.4m
1.9G 1.6m 3.m 2.4m
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2G 1.5m 3m 2.3m
8.1.4 Antenna isolation
Antenna isolation in the same system means the isolation distance between antennas of
different sectors of the same system shall be larger than 0.6m. In actual project
implementation, install antenna pole arms on the main pole bracket. The antenna is
installed on the installation pole. See Picture 8-30.
Picture 8-30 Solid diagram and vertical view of antenna
8.2 Antenna Installation on Iron Tower
In actual project implementation, install antenna on the pole arms, which are at least
1m away from the mail pole bracket. Vertical distance between antennas on different
installation poles is longer than 1m.
Generally, when installing antenna on metal towers, we shall bear in mind the
following issues:
(1) Installation of directional antenna on tower side: in order to reduce tower
antenna’s influence on antenna pattern, we should know that when the distance
between antenna center and the tower isλ/4 or 3λ/4, the antenna can get maximum
directivity;
(2) Installation of isotropic antenna on tower side: in order to reduce tower antenna’s
influence on antenna pattern, tower antenna can not be reverberator of the antenna.
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Therefore, antenna shall be installed on edges or corners, and the distance between
antenna and all parts of the tower shall be longer than λ.
(3) Multi-antennas sharing tower: try as much as possible to reduce coupling effect
and cross influence between antennas of multi-bands during transmitting and receiving
signals. Try to increase isolation between different antennas; the best way is to increase
distance between them. When multi-antennas share a tower, vertical installation shall
be adopted.
8.3 Summary
The distance from antenna to tower platform: 1M;
Spacing interval between diversity reception antennas of the same cell: >3M,
Horizontal spacing interval between isotropic antennas: >4M,
Horizontal pacing interval between directional antennas:>2.5M,
Vertical spacing interval between antennas on different platforms: >1M,
Transmitting and receiving antennas shall not be installed upside down unless there is
specific direction.
Antenna shall be within the range of lightening protection.
Antenna bearing: for directional antenna, the first sector to direction of north by east
60°, the second sector to direction of due south, the third sector to north by west 60°.
Antenna down-tilt: make sure that the actual down-tilt meets the requirement of SE
planning; error difference shall be less than 2°.
Antenna perpendicularity: make sure it’s no bigger than 2°, except for BTS with
antenna down-tilt.
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