antenna array basics and analysis NPTEL ppt

antenna array basics and analysis NPTEL

1. ❑ANTENNA ARRAYS
❑MULTIPLICATIONS OF PATTERNS

2. Inthepointtopointcommunication,itisdesiredtohavemostoftheenergyradiatedinone
particulardirection.Thatmeansitisdesiredtohavegreaterdirectivityinadesireddirection.
Enlargingthedimensionsofsingleelementantennaoftenleadstomoredirective
characteristics.Butitnotpracticaltoincreaseantennasizetoalargevalueforhigh
directivity.Anotherwaytoenlargethedimensionsoftheantenna,withoutnecessarily
increasingthesizeisoftheindividualelements,istoformanassemblyofradiating
elementscalledantennaarray.
Antennaarrayistheradiatingsysteminwhichseveralantennasare
spacedproperlysoastogetgreaterfieldstrengthatafardistancefrom
theradiatingsystembycombiningradiationsatpointfromallthe
antennasinthesystem.
Theindividualantennaisgenerallycalledelementofanantennaarray.

3. Features of Antenna Arrays
➢Thetotalfieldproducedbytheantennaarrayatafardistanceisthevectorsumofthe
fieldsproducedbytheindividualantennasofthearray.
➢Theantennaarrayissaidtolineariftheelementsoftheantennaarrayareequally
spacedalongastraightline.
➢Thelinearantennaarrayissaidtobeuniformlineararrayifalltheelementsarefed
withacurrentofequalmagnitudewithprogressiveuniformphaseshiftalongtheline.
➢Anantennaarray’sradiationpattern,calledthearraypattern,canbechangedupon
excitingitselementswithdifferentcurrents(bothcurrentmagnitudesandcurrent
phases).
➢Thedesiredarraypatterncanbeobtainedfromanarray,withoutchangingitsphysical
dimensions.[Unlikeasingleantennawhoseradiationpatternisfixedforparticular
dimension]

4. Element Shapes and Configurations of Antenna Arrays
➢Antennaarraysusedforpersonal,commercial,andmilitaryapplicationsutilizedifferent
elementsincludingdipoles,loops,apertures,microstrips,patches(circular,rectangular)
horns,reflectorsetc.
➢Theantennasmaybeusedinvariousconfigurationssuchas
▪straightline(lineararray)
▪Circle(circulararray)
▪Rectangle(planararray)
➢Variousformsoftheantennaarrayareusedasradiatingsystems.
▪BroadsideArray
▪EndfireArray
▪CollinearArray
▪ParasiticArray

5. Antenna arrays are being increasingly used in wireless communications.
Advantages of using antenna arrays are:
1.They can provide a high gain (array gain) by using simple antenna
elements.
2.They provide a diversity gain in multipath signal reception.
3.They enable array signal processing features such as spatial filtering,
interference suppression, gain enhancement, target tracking, etc.
4.They can provide the capability of a steerable beam as in smart antennas
offering possibilities to control electronically
–Direction of maximum radiation
–Directions (positions) of nulls
–Beam-width
–Directivity
–Levels of sidelobes
Advantages of Antenna Arrays

6. Broadside Array
Thebroadsidearrayisthearrayofantennasinwhichalltheelementsare
placedparalleltoeachotherandthedirectionofmaximumradiationis
alwaysperpendiculartotheplanecontainingelements(axisofantenna
array).
Allelementsarefedwithcurrentsofequalmagnitudeandsamephase.

7. Broadside Array of Isotropic Point Sources
Twopointsourcesarewith
equalamplitudeandphase.
Pathdifference=dcosϕ
No.ofwavelengths
=
1wavelengthλ2π
Phaseangle
ψ=2π()
ψ=βdcosϕ
(β=2π/λ)

8. Radiation Pattern for Broadside Array
➢Field pattern for two point
source with spacing d= λ/2
and fed with currents equal
in magnitude and phase
➢The field pattern obtained is
bidirectional and it is a
figure of eight.
➢If this pattern is rotated by
360°about axis, it will
represent three dimensional
doughnut shaped space
pattern.

9. End Fire Array
Intheendfirearraynumberofidenticalantennasarespacedequally
alongalineinsimilararrangementasinbroadsidearray.Thedirectionof
themaximumradiationisalongtheaxisofarray.
Alltheantennaelementsarefedindividuallywithcurrentsofequal
magnitudesbuttheirphasesvaryprogressivelyalongtheline.

10. Radiation Pattern for End Fire Array
➢Field pattern for two point source with spacing d= λ/2 and fed with currents
equal in magnitude and opposite in phase (out of phase by 180
o
)
➢The field pattern obtained is horizontal figure of eight.
➢The maxima is along the axis of the array or along the line joining two point
sources.

11. Collinear Array
Inthecollineararray,theantennasare
arrangedco-axiallyi.e.theantennasare
arrangedendtoendalongasingleline.The
directionofmaximumradiationis
perpendiculartotheaxisofarray.
Theindividualelementsinthecollineararray
arefedwithcurrentsequalinmagnitudeand
phasesimilartothebroadsidearray.
Vertical
Horizontal

12. Multiplicationof Pattern
Itisamethodusedtodrawthepatternsofcomplicatedarraysrapidly,almost
byinspection.Theconceptofarrayisextendedtonon-isotropicsources.
Definition:Whenanantennawithaknownradiationpatternisduplicatedina
regulararray,thetotalfar-fieldradiationpattern|E|ofthearray(arraypattern)
willconsistoftheoriginalradiationpatternofasingleelementmultiplyingwith
themagnitudeofthearrayfactor|AF|.
KeyPoints:Arrayfactor
➢Array factor is basically the pattern of array of isotropic point sources.
➢Dependent on the geometry of the array.
➢It varies with the number of elements and their arrangement, relative
magnitudes, relative phases and element spacing
➢The array factor does not depend on the directional characteristic of the
array elements.

13. Multiplication of Pattern
The method is valid to any number of identical elements which need not have
equal magnitudes, phase and spacing between them.
Here 4 element array of equispacedidentical antennas are considered. All the
antennas are fed individually with currents of equal magnitudes, same phase.
The radiation pattern of the antennas (1) and
(2) together spaced at d= λ/2 and as well for
antennas (3) and (4) spaced at d= λ/2

14. Replacing array by two single antennas placed at λ
The resultant radiation pattern of four element array
can be obtained as the Multiplication of Pattern