Anteena Basics

8/3/2019 Anteena Basics

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ANTENNAS

Antennas form a essential part of any radio communication system.

Antenna is that part of a transmitting or receiving system which isdesigned to radiate or to receive electromagnetic waves.

An antenna can also be viewed as a transitional structure betweenfree-space and a transmission line (such as a coaxial line).

An important property of an antenna is the ability to focus and shapethe radiated power in space e.g.: it enhances the power in somewanted directions and suppresses the power in other directions.

Many different types and mechanical forms of antennas exist.

Each type is specifically designed for special purposes.


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ANTENNAS TYPES

In mobile communications two main categories of antennas used are

Omni directional antenna

These antennas are mostly used in rural areas.

In all horizontal direction these antennas radiate withequal power.

In the vertical plane these antennas radiate uniformlyacross all azimuth angles and have a main beam withupper and lower side lobes.


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ANTENNAS TYPES

Directional antenna

These antennas are mostly used in mobile cellular systems toget higher gain compared to omnidirectional antenna and tominimise interference effects in the network.

In the vertical plane these antennas radiate uniformly across all

azimuth angles and have a main beam with upper and lowerside lobes.

In these type of antennas, the radiation is directed at a specificangle instead of uniformly across all azimuth angles in case ofomni antennas.


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ANTENNA CHARACTERISTIC

Radiation Pattern

The main characteristics of antenna is the radiation pattern.

The antenna pattern is a graphical representation in three dimensions ofthe radiation of the antenna as a function of angular direction.

Antenna radiation performance is usually measured and recorded in two

orthogonal principal planes (E-Plane and H-plane or vertical andhorizontal planes).

The pattern of most base station antennas contains a main lobe andseveral minor lobes, termed side lobes.

A side lobe occurring in space in the direction opposite to the main lobe iscalled back lobe.


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ANTENNA CHARACTERISTICS

Radiation Pattern


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Antenna Gain

Antenna gain is a measure for antennas efficiency.

Gain is the ratio of the maximum radiation in a given direction to that of areference antenna for equal input power.

Generally the reference antenna is a isotropic antenna.

Gain is measured generally in decibels above isotropic(dBi) or decibelsabove a dipole(dBd).

An isotropic radiator is an ideal antenna which radiates power with unitgain uniformly in all directions. dBi = dBd + 2.15

Antenna gain depends on the mechanical size, the effective aperaturearea, the frequency band and the antenna configuration.

Antennas for GSM1800 can achieve some 5 to 6 dB more gain thanantennas for GSM900 while maintaining the same mechanical size.


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ANTENNA CHARACTERISTICSMain Lobe Axis

Power Beamwidth

Side Lobe

Back Lobe

First Null


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Front-to-back ratio

It is the ratio of the maximum directivity of an antenna to its directivity in aspecified rearward direction.

Generally antenna with a high front-to-back ratio should be used.

First Null Beamwidth

The first null beamwidth (FNBW) is the angular span between the firstpattern nulls adjacent to the main lobe.

This term describes the angular coverage of the downtilted cells.


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Antenna Lobes

Main lobe is the radiation lobe containing the direction of maximumradiation.

Side lobes

Half-power beamwidth

The half power beamwidth (HPBW) is the angle between the points onthe main lobe that are 3dB lower in gain compared to the maximum.

Narrow angles mean good focusing of radiated power.

Polarisation

Polarisation is the propagation of the electric field vector .

Antennas used in cellular communications are usually vertically polarisedor cross polarised.


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Frequency bandwidth

It is the range of frequencies within which the performance of theantenna, with respect to some characteristics, conforms to a specifiedstandard.

VSWR of an antenna is the main bandwidth limiting factor.

Antenna impedance

Maximum power coupling into the antennas can be achieved when theantenna impedance matches the cables impedance.

Typical value is 50 ohms.

Mechanical size

Mechanical size is related to achievable antenna gain.

Large antennas provide higher gains but also need care in deployementand apply high torque to the antenna mast.


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COUPLING BETWEEN

ANTENNAS

Antenna radiation pattern will become superimposed when the distancebetween the antennas becomes too small.

This means the other antenna will mutually influence the individualantenna patterns.

Generally 5 to 10 horizontal separation provides sufficient decoupling ofantenna patterns.

The vertical distance needed for decoupling is usually much smaller asthe vertical beamwidth is generally less.

A 1 separation in the vertical direction is sufficient in most cases.


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ANTENNA INSTALLATION

Antenna installation configurations depend on the operators preferences. It is important to keep sufficient decoupling distances between antennas.

If TX and RX direction use separated antennas, it is advisable to keep ahorizontal separation between the antennas in order to reduce the TXsignal power at the RX input stages.


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ANTENNA DOWNTILTING

Network planners often have the problem that the base station antennaprovides an overcoverage.

If the overlapping area between two cells is too large, increased switchingbetween the base station (handover) occurs.

There may even be interference of a neighbouring cell with the same

frequency.

If hopping is used in the network, then limiting the overlap is required toreduce the overall hit rate.

In general, the vertical pattern of an antenna radiates the main energy

towards the horizon. Only that part of the energy which is radiated below the horizon can be

used for the coverage of the sector.

Downtilting the antenna limits the range by reducing the field strength in

the horizon.


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ANTENNA DOWNTILTING

Antenna downtilting is the downward tilt of the vertical pattern towards theground by a fixed angle measured w.r.t the horizon.

Downtilting of the antenna changes the position of the half-powerbeamwidth and the first null relative to the horizon.

Normally the maximum gain is at 0(parallel to the horizon) and never

intersects the horizon.

A small downtilt places the beams maximum at the cell edge

With appropriate downtilt, the received signal strength within the cellimproves due to the placement of the main lobe within the cell radius and

falls off in regions approaching the cell boundary and towards the reusecell.

There are two methods of downtilting

Mechanical downtilting

Electrical downtilting.


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MECHANICAL DOWNTILTING

Mechanical downtilting consists of physically rotating an antenna

downward about an axis from its vertical position.

In a mechanical downtilt as the front lobe moves downward the back lobemoves upwards.

This is one of the potential drawback as compared to the electrical

downtilt because coverage behind the antenna can be negatively affectedas the back lobe rises above the horizon.

Additionally , mechanical downtilt does not change the gain of theantenna at +/- 90deg from antenna horizon.

As the antenna is given downtilt, the footprint starts changing with a notchbeing formed in the front while it spreads on the sides.

After 10 degrees downtilt the notch effect is quiet visible and the spreadon the sides are high. This may lead to inteference on the sides.


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MECHANICAL DOWNTILTINGVertical antenna pattern at 0

Vertical antenna pattern at 15 downtilt

Backlobe shoots over the horizon


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ELECTRICAL DOWNTILT Electrical downtilt uses a phase taper in the antenna array to angle the

pattern downwards. This allows the the antenna to be mounted vertically.

Electrical downtilt is the only practical way to achieve patterndowntilting with omnidirectional antennas.

Electrical downtilt affects both front and back lobes. If the front lobe is downtilted the back lobe is also downtilted by equal

amount.

Electrical downtilting also reduces the gain equally at all angles on thehorizon. The that adjusted downtilt angle is constant over the wholeazimuth range.

Variable electrical downtilt antennas are very costly.


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ELECTRICAL DOWNTILT


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ELECTRICAL DOWNTILTHorizontal and vertical pattern for allgon 7144 antenna

Horizontal Beamwidth = 90

Vertical Beamwidth = 16

Electrical Downtilt = 16


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OBSTACLE REQUIREMENT

Nearby obstacles are those reflecting or shadowing materials that canobstruct the radio beam both in horizontal and vertical planes.

When mounting the antenna on a roof top, the dominating obstacle inthe vertical plane is the roof edge itself and in the horizontal plane,obstacles further away like surrounding buildings, can act as reflecting

or shadowing material. The antenna beam will be distorted if the antenna is too close to the

roof. Hence the antenna must be mounted at a minimum height abovethe rooftop or other obstacles.

If antennas are wall mounted, a safety margin of 15 degrees betweenthe reflecting surface and the 3-dB lobe should be kept.


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OBSTACLE REQUIREMENT

Main RadiationDirection

Half PowerBeamwidth

Safety Margin15 Degrees

Building


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OPTIMAL DOWNTILT Although the use of downtilt can be a effective tool for controlling

interference, there is a optimum amount by which the antenna can be

downtilted whereby both the coverage losses and the interference atthe reuse cell can be kept at a minimum.

downtilt angle (D)

3 dB Beamwidth

Main lobe

Height (H)

Cellmax


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OPTIMAL DOWNTILT

The figure shows a cells coverage area.

The primary illumination area is the area on the ground that receives thesignal contained within the 3dB vertical beamwidth of the antenna.

The distance from the base station to the outer limit of the illuminationarea is denoted by Cellmax.

It should be noted that the cellmax can be different from the cellboundary area which is customer defined.

Ideally in a well planned network Cellmax should always be less thanthe co-channel reuse distance to minimise interference.

We now derive the relation between height (H), downtilt angle (D), 3dBvertical beamwidth and Cellmax.

As shown in the schematic is the angle between the upper limit of the3dB beamwidth and the horizon.


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OPTIMAL DOWNTILT

tan ( ) = Cellmax / H

= D - 0.5 * 3dB vertical beamwidth

Cellmax = H * tan (D - 0.5 * 3dB vertical beamwidth)

For the Cellmax to be a positive quantity , downtilt angle must be morethan half of the 3dB vertical beamwidth.

When the downtilt angle is less than half of the 3dB beamwidth, part ofthe signal from the main beam shoots over the horizon .

The signal directed towards or above the horizon can potentially causeinterference at the reuse sites.


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DIVERSITY ANTENNA

SYSTEMS


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Diversity Antenna System

NEED OF DIVERSITY

Building

Building

Building


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Diversity Antenna SystemsNEED OF DIVERSITY

In a typical cellular radio environment, the communication between thecell site and mobile is not by a direct radio path but via many paths.

The direct path between the transmitter and the receiver is obstructedby buildings and other objects.

Hence the signal that arrives at the receiver is either by reflection from

the flat sides of buildings or by diffraction around man made or naturalobstructions.

When various incoming radiowaves arrive at the receiver antenna,they combine constructively or destructively, which leads to a rapidvariation in signal strength.

The signal fluctuations are known as multipathfading.


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Diversity Antenna SystemsMultipath Propagation

Multipath propagation causes large and rapid fluctuations in a signal These fluctuations are not the same as the propagation path loss.

Multipath causes three major things

Rapid changes in signal strength over a short distance or time.

Random frequency modulation due to Doppler Shifts on differentmultipath signals.

Time dispersion caused by multipath delays

These are called fading effects

Multipath propagation results in small-scale fading.


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Diversity Antenna SystemsDIVERSITY TECHNIQUE

Diversity techniques have been recognised as an effective meanswhich enhances the immunity of the communication system to themultipath fading. GSM therefore extensively adopts diversitytechniques that include

Diversity techniquesInterleaving

In time domain

Frequency Hopping

In Frequency domain

Spatial diversity

In spatial domain

Polarisation diversity

In polarisation domain


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Diversity Antenna SystemsCONCEPT OF DIVERSITY ANTENNA SYSTEMS

Spatial and polarisation diversity techniques are realised throughantenna systems.

A diversity antenna system provides a number of receiving branchesor ports from which the diversified signals are derived and fed to areceiver. The receiver then combines the incoming signals from the

branches to produce a combined signal with improved quality interms of signal strength or signal-to-noise ratio (S/N).

The performance of a diversity antenna system primarily relies onthe branch correlation and signal level difference between branches.


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Diversity Antenna System

Transmissionmedia 1

Transmission

Tmedia 2 Peak

Fade

ReceiverInformation

CONCEPT OF DIVERSITY ANTENNA SYSTEMS


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Diversity Antenna SystemsCORRELATION BETWEEN BRANCHES

The branch correlation coefficient (r) represents the degree ofsimilarity between the signals from two different receiving branches.

The correlation coefficient ranges from 0 to 1.

r=1 means the signals from two different branches behave exactlythe same. In this case, the signals are coherent.

r=0 means the signals from two different branches behavecompletely different. In this case, the signals are uncorrelated.

To achieve the best performance, a diversity antenna system isrequired to provide uncorrelated signals.

For r=1, the diversity antenna becomes ineffective in combating themultipath fading.

In reality, however, it is not always practical to have a diversityantenna system which guarantees r=0. Extensive research in thisfield has revealed that a diversity antenna system can perform

satisfactorily provided that r 0.7.


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Diversity Antenna Systems

Time

SignalStrengt

Combined signalSignal 1Signal 2

Combining

Combined signal

fed to receiver Signal 2

Signal 1


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Diversity Antenna SystemsSIGNAL LEVEL DIFFERENCE

The second key parameter for a good diversity antenna system isthe mean signal level difference.

The difference is a statistical parameter which indicates the balanceof the signal strengths from the two receiving branches.

In a real system, the statistical balance can be verified by comparing

the mean values of the two signals measured over a lengthy period.

If the ratio betn the median values is 0dB, the two receiving branchesare statistically balanced.

The performance of the diversity system will deteriorate while theratio increases or decreases from 0dB.


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Diversity Antenna Systems

Signal level difference

Signalstrength

Time

SIGNAL LEVEL DIFFERENCE


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Diversity Antenna SystemsSPATIAL DIVERSITY ANTENNA SYSTEMS

The spatial diversity antenna system is constructed by physicallyseparating two receiving base station antennas.

Once they are separated far enough, both antennas receiveindependent fading signals. As a result, the signals captured by theantennas are most likely uncorrelated.

The further apart are the antennas, the more likely that the signalsare uncorrelated.

The types of the configuration used in GSM networks are:

horizontal separation

vertical separation composite separation.


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Diversity Antenna SystemsTYPICAL SPATIAL ANTENNA DIVERSITY CONFIGURATIONS

Horizontal Separation Vertical Separation


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Diversity Antenna SystemsBranch correlation The physical limitation of the supporting structure should also be

considered while selecting the spatial diversity antenna configuration.For example, if a wide framework is not permitted on top of amounting tower, vertical separation is a alternative to be considered.

To achieve the required correlation coefficient (r 0.7) differentconfigurations require different separations.

The separation indicated in Table below shows that low values ofcorrelation are more easily obtained with horizontal rather thanvertical separation.

That is why most of the diversity antenna systems in GSM networksuse horizontal separation.

CRITERIA FOR SELECTING TYPE OF SPATIAL SEPARATION

d/ 900MHZ 1800MHZ d/ 900MHZ 1800MHZ

Separation 10 3.3m 1.7m 17 5.7m 2.8m

Horizontal Separation Vertical Separation


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Diversity Antenna SystemsCRITERIA FOR SELECTION OF SPATIAL SEPARATION

Signal level difference A system using horizontally separated diversity antennas has a

symmetrical configuration and is therefore able to provide balancedsignal strengths.

A system using vertically separated antennas needs large separation

to meet the required correlation. The consequence is that the two antennas have different antenna

height gains, which may result in imbalance between the two signalstrengths.


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Diversity Antenna SystemsCRITERIA FOR SELECTION OF SPATIAL SEPARATION

Angular dependence

Angular dependence reflects the dependence of the performance ofa diversity antenna system on the angular position of a mobilerelative to the boresight of the antenna.

Horizontally separated antenna system has high dependence on themobiles angular position.

The effective separation reduces as the mobile moves away from theantenna boresight.

As the mobile is 90 off the antenna boresight, the effective

separation becomes zero.

In such a case, the signals from two antennas are very likelycoherent which will then lead to a deterioration of the diversityperformance.


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Diversity Antenna SystemsANGULAR DEPENDANCE

Most of the GSM cell sites are 3 sectored cell sites.

The maximum angular offset is therefore approximately 60.

Simulation shows that the performance of a horizontally separatedantenna system experiences noticeable deterioration only when theangular offset exceeds 70 .

Separation

Reduced

Separation

Zero

Separation

View from boresight View from 45 deg off boresight View from 90 deg off boresight


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Diversity Antenna SystemsPROS AND CONS OF HORIZONTAL CONFIGURATION

Advantages

Easier to achieve low values of correlation and balance between thesignals. Hence widely used.

Disadvantages

High angular dependence. The impact is however marginal forsectorised applications.

Require sizeable headframe on the supporting structure.


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Diversity Antenna SystemsPROS AND CONS OF VERTICAL CONFIGURATION

Advantages

Slim supporting structure.

Angular independence

Disadvantages Require large separation for low values of correlation.

May cause imbalance between the two diversity branches.

Generally not used.


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Diversity Antenna SystemsTHREE ANTENNA SPATIAL CONFIGURATION

10 Separation

Receive 1 Transmit Receive 2


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Diversity Antenna SystemsPOLARISATION DIVERSITY ANTENNA SYSTEMS

A single (say vertical) polarised electromagnetic wave is converted toa wave with two orthogonal polarised fields while it is propagatingthrough scattering environment.

It has also been found that the two fields exhibit some extent ofdecorrelation.


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Diversity Antenna SystemsDUAL POLARISED ANTENNAS

A dual-polarisation antenna consists of two sets of radiating elementswhich radiate or, in reciprocal, receive two orthogonal polarisedfields.

The antenna has two input connectors which separately connects toeach set of the elements.

The antenna has therefore the ability to simultaneously transmit andreceive two orthogonally polarised fields.

H / VSlant 45


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Diversity Antenna SystemsADVANTAGES OF DUAL POLARISED ANTENNAS

The best advantage of using the dual polarisation antenna is thereduction in the number of antennas per sector.

Reduced size of the headframe of the supporting structure

Reduced windload and weight.

Reduced difficulty in site acquisition and installation.

Cost saving

Requiring slim tower

Requiring less installation time.

Cost of one dual polarisation antenna is generally lower than that

of two Single polarised antennas


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Diversity Antenna SystemsDUAL POLARISED ANTENNA CONFIGURATIONS

DUALPOLEANTENNA

T R

TX RX RX

DUALPOLEANTENNA

SINGLEPOL

EANTENNA

RX RX

TX

DUALPOLEANTENNA

T TR R

TX RX TX RX

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Anteena Basics