Satellite Communications A Part 2

Antenna Basics -Professor Barry G Evans Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.1

Antenna Radiation Pattern

Antenna radiation pattern in polar coordinates SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.2

Near Field – Far Field Transition Region

λ 4 radians D Constant No phase difference between centre and edge ray Autumn2004 (c) University of Surrey parallel beam region  λ 4 Phase difference R  D 2 2 λ R  2 D 2 λ Near-Field Region SatCommsA - Part 2 - Antennas - B G Evans  λ 16 Phase difference Far-Field Region 2.3

Multimode Feed

Autumn2004 (c) University of Surrey Computed isogain contours at 6 GHz Using multimode feed SatCommsA - Part 2 - Antennas - B G Evans 2.4

Antenna Radiation Pattern

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.5

Passive Reflecting surface Autumn2004 (c) University of Surrey Radiating Source (Feedhorn) Passive Reflecting Surface (Auxiliary or ‘Sub’) SatCommsA - Part 2 - Antennas - B G Evans Passive Reflecting Surface (Main) 2.6

Asymmetric (or offset) dual reflector systems

(a) Cassegrain

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

(b) Gregorian

2.7

Dual-offset Gregorian antenna

Sub Reflector Feed Main reflector 5.5m diameter Antenna is shown At 30 °angle of elevation

Autumn2004 (c) University of Surrey Dual –offset Gregorian antenna for satellite communication services SatCommsA - Part 2 - Antennas - B G Evans 2.8

Simple Satellite Antennas

SPOT ATLANTIC SPOT EAST SPOT ATLANTIC EUROPEAN Coverage patterns for ECS (Circular and Elliptic Beams)

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Typical Example: Global Coverage Beam (17.0

° Beamwidth) General Requirement is to maximise edge of coverage gain. Occurs when the E.O.C. gain contour is approximately –4dB from the peak.

SatCommsA - Part 2 - Antennas - B G Evans 2.9

Relationship between coverage area and antenna diameter

• Circular Coverage area diameter = N degrees • Assume –4dB contour at E.O.C. area, then 4dB beamwidth (  4 ) of antenna should be,  4 = N Degrees • Relationship between 4dB and 3dB beamwidth • From tracking considerations we have Loss (dB) =   12  θ P θ H   2 dB • This is only a simple equation for the antenna main beam, therefore we could find 4dB beamwidth relationship by putting θ P  1 2 θ 4 and loss = -4   4    12  1 2 θ θ 4 H   2 dB  θ 4  4 3 θ H  1.15

θ H 2.10

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Contoured beam coverage

34.28

° 31.28

° reflector 48.61

° 1000m scale 0m feed cluster power divider Contoured beam coverage

Contoured beam coverage of a Eurobeam zone satellite SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.11

INTELSAT V coverage diagrams

Shaped zone beams

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Shaped hemi beams

SatCommsA - Part 2 - Antennas - B G Evans 2.12

4 GHz and 6 GHz antennas on INTELSAT VI

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.13

4 GHz and 6 GHz antennas on INTELSAT VI (cont.)

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.14

Antenna Radiation Characteristics

P T – Total power supplied to the antenna P O – Total power radiated by the antenna P(  ,  ) – Radiated power in the angular director (  ,  )  P O  2 0 π  π 0 P

 

sin θ dθ dφ Antenna radiation pattern or polar diagram E    10 log 10  P      dB Antenna gain function G  10 log 10  4 πP   P T  dB i Antenna directivity function D    10 log 10  4 πP   P O  dBi SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.15

Autumn2004 (c) University of Surrey

Antenna Gain

G  G  10 log 10  4 πP   G  10 log 10   4 λ 2 π .

η .

   dBi P T  dBi where,   = operating wavelength = physical aperture area of the antenna  = antenna efficiency factor For circular aperture antennas, G  10 log 10    η   πD λ 2    dBi where, D = circular aperture diameter SatCommsA - Part 2 - Antennas - B G Evans 2.16

Antenna Efficiency



100 x

 = antenna efficiency factor (less than or equal to unity) = antenna efficiency expressed as a percentage  =  I x  S x  B x  E x  L x … 

I

–

ILLUMINATION EFFICIENCY

accounts for the non-uniformity of the illumination and phase 

S

– distributions in the antenna aperture

SPILLOVER EFFICIENCY

ratio of the total power in the antenna aperture to the total power radiated by the primary feedhorn 

I

–

BLOCKAGE FACTOR

incomplete utilisation of the antenna aperture due to the blocking effects 

E

– of subreflector, supports, etc.

MANUFACTURING LOSSES



L

– includes losses due to profile errors, misalignments, etc.

OHMIC LOSSES

includes losses in the primary feedchain SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.17

Typical efficiency factors for a large Cassegrain antenna

FACTOR ILLUMINATION EFF.

SUBREFLECTOR S/O MAIN REFLECTOR S/O EFFICIENCY (%) 98.7

88.3

96.0

BLOCKAGE LOSSES MANUFACTURING LOSSES FEED OHMIC LOSSES 92.6

92.4

95.5

ANTENNA EFFICIENCY 68.4% Gain of 30m Antenna at 4GHz.

G = 10 log 0.684 ((  x 30 x 4)/3) dBi = 60.3 dBi SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey LOSS (dB) 0.06

0.54

0.18

0.33

0.34

0.2

1.65dB

2.18

Antenna half-power beamwidth (HPBW)

HPBW = Angular width between the two points in the antenna radiation pattern which are 3dB below the main beam peak HPBW = N  /D , degrees Where,  = operating wavelength D = circular aperture diameter N = beamwidth factor dependent on the aperture illumination distribution In general 58  N  75 uniform distribution tapered distribution SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.19

Polarisation of the electric field

Electric field vector Radiating Antenna Locus of the tip of the electric field vector on plane x,y during one period (= 1/frequency) x,y represents plane Perpendicular to direction of propagation

In the most general case the locus is an ellipse and the wave is said to be: ELLIPTICALLY POLARISED SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.20

Elliptical Polarisation

x,y plane perpendicular to direction of propagation Elliptical Polarisation is characterised by: 1.

2.

3.

Axial ratio of the ellipse, E max /E min Inclination angle of the ellipse,  Rotation sense of E as seen from the antenna looking in the direction of propagation Right Hand – Clockwise rotation Left Hand – Anti-clockwise rotation Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.21

Elliptical Polarisation (cont.)

Most antennas are either:

Linear polarised or circularly polarised

Both are particular cases of elliptical polarisation:-

Linear when the Axial ratio is infinite Circular when the Axial ratio is unity

Note that elliptical polarisation can be expressed as either the combination of two linear polarisations or the combination of two circular polarisation 2.22

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Operation of polarizer

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.23

Polarisation

LINEAR

vertical horizontal EUTELSAT INTELSAT 11/14GHz

CIRCULAR

INTELSAT 4/6GHz

Antennas can be: 1.

Single polarised 2.

3.

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right hand

Orthogonally polarised in receive and transmit bands Dual-Polarised

left hand

e.g.vertical linear at all frequencies e.g.vertical linear at receive frequencies horizontal linear at transmit frequencies e.g. vertical and horizontal linear at all frequencies SatCommsA - Part 2 - Antennas - B G Evans 2.24

Definitions

CO-POLAR – component of field parallel to the field of the reference source CROSS-POLAR – component in orthogonal direction

reference source copolar component cross-polar component vertical vertical horizontal

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left hand circular left hand circular

SatCommsA - Part 2 - Antennas - B G Evans

right hand circular

2.25

Cross-polar discrimination

Copolar Cross-polar Discrimination (XPD) Autumn2004 (c) University of Surrey Cross-polar Theta (degs) SatCommsA - Part 2 - Antennas - B G Evans 2.26

Axisymmetric Systems

• Linear Polarisation Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.27

Axisymmetric Systems (cont.)

• Circular Polarisations Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.28

Offset Systems

• Linear Polarisation Co-polar Cross-polar Autumn2004 (c) University of Surrey Plane perpendicular to offset SatCommsA - Part 2 - Antennas - B G Evans 2.29

Offset Systems

• Circular Polarisations (RHCP) No cross-polar generated Autumn2004 (c) University of Surrey Plane perpendicular to offset SatCommsA - Part 2 - Antennas - B G Evans 2.30

Reciprocity

• The principle of reciprocity is of fundamental importance in antenna theory.

• Implies that the performance characteristics of an antenna may be determined either by analysis or measurement with the antenna operating as a transmitter or with the antenna operating as a receiver.

• In practice: For analysis the antenna is generally assumed to be transmitting. For measurements the antenna is generally assumed to be received.

2.31

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Noise Temperature • Components for total system noise temperature:

– Antenna noise temperature – Noise temperature due to feed system – Receiver noise temperature 2.32

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Antenna noise temperature • Dependent on:

– Antenna radiation pattern [G(  ,  )] – Antenna elevation [  0 ] – Brightness temperature which is a function of frequency 2.33

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Antenna noise temperature

T A

 

0  4 1 π T'  θ 0 , θ, φ   0 2 π  π 0 T' T(θ *



θ ) 0 , θ, φ sin θ d θdφ ,brightness temperature function where, cos  * = cos  0  0 cos  = antenna elevation angle - sin  0 sin  cos  Autumn2004 (c) University of Surrey Typical brightness temperature function at 4GHz SatCommsA - Part 2 - Antennas - B G Evans 2.34

Feed system and Received noise temperature

• Dependent on – feed loss – ambient noise temperature • If ambient noise temperature is 290K and feed loss is small (<1dB) then feed system noise temp. is: T P = 66.7x(loss in dB) K i.e. 6.7 K for each 0.1dB loss in feedchain • Received noise temperature Dependent on type of LNA and whether cooled or uncooled 2.35

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Sidelobe Specifications

• Transmit – Mandatory to avoid interference into other systems • Receive – Advisable to reduce interference from other systems • CCIR has recommendations for sidelobe levels which are used by operators, such as INTELSAT and EUTELSAT, as specifications.

• For antenna diameters greater than 150  , the sidelobe specification is independent of the size of antenna.

• Some specifications allow a percentage of sidelobes to be above template.

SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.36

Sidelobe Specifications (cont.)

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Angle of axis (degrees)

SatCommsA - Part 2 - Antennas - B G Evans 2.37

Minimum Satellite Spacings

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.38

Sidelobe Specification

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.39

Antenna tracking techniques

• Monopulse – Static split – Higher order modes • Conical scan • Step track • Programmed track 2.40

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Gain Loss

Simple expression for antenna main beam pattern G  20 log 10    10  0 .

6   θ θ H   2    dB Pointing loss Loss  (xdB) 12   θ P θ H    2 20 log 10    10  0 .

6   dB θ P θ H   2    Antenna diameter 25m at 4GHz,  H  67/D   if half power beamwidth,  H =0.2deg

Pointing error,  P =0.05deg

, D   333 GainLoss    12  0 .

0 .

05 7 5 0 .

2 dB 2 dB SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.41

Programmed Track

• A predetermined movement for the antenna is programmed into the memory of the controller. This updates the position of the antenna in a particular time interval.

• Precise satellite bearing relative to antenna needs to be known 2.42

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Step Track

• Sometimes referred to as hill-climbing • Antenna is moved predetermined distance in one direction.

– if satellite signal increases, a further similar move is made.

– if satellite signal decreases, a similar more is made in opposite direction.

• Some level of intelligence can be introduced • Fairly cheap to include but continuous movement of complete antenna is wearing driving motors.

2.43

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Conical Scan

• Mechanical steering concept • Antenna main beam is offset from mechanical boresight by tilt of feed or subreflector • Feed system is rotated (at high speed) such that antenna main beam performs a conical scan • Modulates the received satellite system if it is offset from the antenna boresight • Disadvantage is that it requires moving mechanical parts.

e.g. Goonhilly2 antenna feed rotates at 1000 rpm.

Antenna main beam Offset from boresight Boresight axis and axis of rotation

SatCommsA - Part 2 - Antennas - B G Evans Autumn2004 (c) University of Surrey 2.44

Four-Horn Static split System

A Sum channel C A Azimuth difference C A Elevation difference C B D B D B D



A az A e

Sum = A+B+C+D  AZ =(A+B)-(C+D)  EL =(A+C)-(B+D) Simple two-channel tracking feed a Modes in horn apertures b Comparator bridge network 2.45

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans

Four-Horn Static split System (cont.)

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Sum and difference channel radiation patterns a Feed illumination patterns b Reflector for field patterns Normal sum type pattern Difference pattern has null on boresight Satellite should be steered to be in null

SatCommsA - Part 2 - Antennas - B G Evans 2.46

Monopulse Tracking Static – Split System

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans 2.47

Multimode Tracking System

• Single feedhorn provides both communication channel and tracking information.

• Higher order modes are employed which have no field component (a null) in the boresight direction.

• As for static split system, the tracking accuracy is dependent on the slope of the null.

• Again error signals in azimuth and elevation are determined.

2.48

Autumn2004 (c) University of Surrey SatCommsA - Part 2 - Antennas - B G Evans