l7pre antenna basics 2010v1

8/7/2019 L7pre Antenna Basics 2010v1

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UEET4563Mobile and SatelliteMobile and Satellite

CommunicationsCommunications

Pre L7 – AntennaBasics



Antenna basics Antenna basics• Antenna pattern

•Gain

• Polarisation

satellite antennas

patch

2UTAR 2010

multiband 3G ceilingmounted dipole

Wi-Fi dipole

monopole



Antenna pattern Antenna pattern• Field strength measured at any point from an antenna

depends on the distance and direction – normally at farfield.

• If the distance is fixed, one can plot field strengthdistribution versus the direction at which measurement istaken.

• This plot is known as the antenna pattern.

• An antenna has the same attern at an iven fre uenc-30

-25

-20

-15

c r o s s - p o l a r i s a t i o n p a t t e r n [ d B ]

-30

-25

-20

-15

-10

-5

0

d c o - p

o l a r i s a t i o n p a t t e r n [ d B ]

Co-polarCross-polar

-150 -100 -50 0 50 100 150

-150

-100

-50

0

50

100

150

Azimuth [degree]

P h a s e r e s p o n s e [ d

e g r e e ]

Co-polarCross-polar

whether it transmits or receives – reciprocity.

3UTAR 2010

monopole

dipole

-150 -100 -50 0 50 100 150-40

-35

Azimuth [degree]

N o r m a l i s e

-150 -100 -50 0 50 100 150-45

-40

-35

N o r m

a l i s



Isotropic antennaIsotropic antenna• Isotropic radiator is the simplest antenna mathematically

• Radiates all the power supplied to it, equally in all directions (in 3D)

• Theoretical only, cannot be constructed

• Useful as a reference – other antennas are often compared with it

4UTAR 2010

perfectspherepattern



Half Half--wave dipole antennawave dipole antenna• Simplest practical antenna

• Actual length is typically about 95% of a half wavelength in free space

5UTAR 2010

perfect donutshape pattern

perfect donutshape pattern

perfectazimuthpattern

perfectelevationpattern



Antenna gain, dBi Antenna gain, dBi

• P = power flux density (PFD) of an antenna

• P 0 = PFD of isotropic antenna

( ) ( )( )

( )( )dBilog10

0

0

=

=

φ φ

φ φ

φ

PP

PP

G

•PFD is radiated power passing through a given area [W/m 2]

• dBi means decibels with respect to an isotropic radiator

• If is not specified, take the maximum gain value

6UTAR 2010

P 0 (φ )

P( φ )

φ



Antenna gain, dBd Antenna gain, dBd

• dBd means decibels with respect to an ideal half-wave dipole antenna

• P d = PFD of ideal half-wave dipole antenna

( ) ( )( )

( )( )dBdlog10=

=

φ φ

φ φ

φ

d

d

PP

PP

G

•If is not specified, take the maximum gain value

• Gain [dBi] = Gain [dBd] + 2.14

7UTAR 2010

P d (φ )

P( φ )φ



Effective Isotropic Radiated Power Effective Isotropic Radiated Power (EIRP)(EIRP)• EIRP is the amount of power that would have to be applied to an isotropic radiator to give the same power

density at a given point

8UTAR 2010

[ ]dBinEIRP

orEIRP

t t

t t

GP

GP

+=

=

P t

samefielddensity



Effective aperture gainEffective aperture gainη = efficiency of antenna design, less than or equal 1 (since real antenna is not perfect)

A =antenna aperture

Aeff = Effective aperture

A A A λ

π G eff eff η =

=2max

4

• As a receiver, antenna aperture can be visualised as the area of a circle constructed broadsideto incoming radiation where all radiation passing within the circle is delivered by the antennato a matched load

• For an antenna with a circular aperture or reflector of diameter D and geometric surface A:

9UTAR 2010

4

2

π D A =

( ) [ ]dBilog10 maxdBimax,

2

max

GG

λπ DηG

=

=Gain increases by 6 dB whenantenna diameter is doubled, orfrequency is doubled.



• Angular beamwidth is the angle defined by the directions corresponding to a givenfallout with respect to the maximum value

• Have to specify beamwidth with respect to x-dB• Normally it is with respect to 3 dB

– half power beamwidth

• A roximation for arabolic3 dB

beamdiwdth

BeamwidthBeamwidth

boresight direction

reflector antenna: main lobe

side lobe

10UTAR 2010

[ ]

2

3dBmax

3dB

70

degree70

≈

≈

φ π η

λ φ

G

D

with max. gain



PolarisationPolarisation• Orientation of E-field at a point in space over one period of the oscillation

• Normally perpendicular to direction of travel of wave

• Linear polarisation: – vertical

– horizontal

• Non linear olarisation:

linear polarisation

– circular – elliptical

11UTAR 2010

horizontal

vertical



Circular polarisationCircular polarisation• Circularly polarised wave consists of two perpendicular electromagnetic plane

waves of equal amplitude and 90° difference in phase

• If the wave is approaching an observer, and its E-field appears to be rotatingcounterclockwise (as seen from observer), the wave is right circular polarised

• The E-field makes 1 complete revolution in 1 wavelength

12UTAR 2010

circular polarisation

right

left



Elliptical polarisationElliptical polarisation• Elliptically polarised wave consists of two perpendicular waves of unequal

amplitude which differ in phase by 90°

• If the wave is approaching an observer, and its E-field appears to be rotatingcounterclockwise (as seen from observer), the wave is right elliptic polarised

right

13UTAR 2010

elliptical polarisation

left



Cross polarisationCross polarisation• Tx and Rx should remain in same polarisation for reception of strongest signal – co-polarisation – component of field parallel to the field of the reference source

• Cross-polarisation is radiation orthogonal to the desired polarisation – e.g., the cross-polarisation of a vertically polarised antenna is the horizontally polarised field

• Cross polarisation discrimination (XPD) & Cross polarisation isolation (XPI)

Tx RxTx Rx

14UTAR 2010

vertical

horizontala x

a ca

vertical

horizontal

b x

b bc

vertical

horizontala x

ac

a

vertical

=

=

=

x

c

x

c

x

c

ab

ba

aa

log20XPIorlog20XPI

log20XPD



Satellite antennasSatellite antennas• 4 main types of antennas:

– wire (monopole or dipole)

– horn – reflector

– phased array

• Wire antannas are used to provide communications for the TTC&M systems –omnidirectional coverage.

• Horn antennas are used at microwave frequencies when relatively wide beams are requiredfor large coverage.

• Reflector and phased antennas are used to provide high gain (>23 dB) or narrowerbeamwidth (<10°).

• Phased array antennas are used to shape the beampattern to desired shape – scanning

beams.

15UTAR 2010

η of circular parabolic reflectorantenna is usually at 0.55



Antennas coverage zone Antennas coverage zone• If 2 earth stations are 300 km apart, and the satellite is in GSO, the stations’ angular

separation at the satellite is about 0.5°

• For φ 3dB = 0.5°, D/ λ = 150 – antenna aperture diameter of 11.3 m @ 4 GHz.

– aperture size of 1.5 m wide @ 20 GHz

– note: the launching spacecraft should not be more than 3.5 m wide in diameter; antenna reflectorsare not folded for launch – require accurate surface

• Phased array feed is used to create many 0.5° beams which can be clustered to serve thecoverage zone of the satellite

• To provide separate beam for each earth station would require one antenna feed per station – e.g. multiple feed antenna with single reflector

16UTAR 2010

global beam spot beams multiple spot beams/ scanning beams

orthogonallypolarised beam

l7pre antenna basics 2010v1

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