Satellite Communications A Part 2

1
Satellite Communications APart 2

• Antenna Basics
• -Professor Barry G Evans-

2
Antenna Radiation Pattern

• Antenna radiation pattern in polar coordinates

3
Near Field Far Field Transition Region
4
Multimode Feed

• Computed isogain contours at 6 GHz
• Using multimode feed

5
Antenna Radiation Pattern
6
Passive Reflecting surface

• Radiating Source
• (Feedhorn)

Passive Reflecting Surface (Auxiliary or Sub)
Passive Reflecting Surface (Main)
7
Asymmetric (or offset)dual reflector systems

• (a) Cassegrain

(b) Gregorian
8
Dual-offset Gregorian antenna

• Dual offset Gregorian antenna
• for satellite communication services

9
Simple Satellite Antennas
Coverage patterns for ECS (Circular and Elliptic
Beams)

• 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.

10
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)
• This is only a simple equation for the antenna
  main beam, therefore we could find 4dB beamwidth
  relationship by putting
• and loss -4

11
Contoured beam coverage
Contoured beam coverage

• Contoured beam coverage of a Eurobeam zone
  satellite

12
INTELSAT V coverage diagrams
Shaped zone beams
Shaped hemi beams
13
4 GHz and 6 GHz antennason INTELSAT VI
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4 GHz and 6 GHz antennason INTELSAT VI (cont.)
15
Antenna Radiation Characteristics

• PT Total power supplied to the antenna
• PO Total power radiated by the antenna
• P(?,?) Radiated power in the angular director
  (?,?)

Antenna radiation pattern or polar diagram
Antenna gain function
Antenna directivity function
16
Antenna Gain
where, ? operating wavelength ? physical
aperture area of the antenna ? antenna
efficiency factor
For circular aperture antennas,
where, D circular aperture diameter
17
Antenna Efficiency
? antenna efficiency factor (less than or
equal to unity) 100 x ? 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
distributions in the antenna aperture ?S
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 of
subreflector, supports, etc. ?E MANUFACTURING
LOSSES includes losses due to profile errors,
misalignments, etc. ?L OHMIC LOSSES includes
losses in the primary feedchain
18
Typical efficiency factors for a large
Cassegrain antenna

• Gain of 30m Antenna at 4GHz.
• G 10 log 0.684 ((? x 30 x 4)/3) dBi
• 60.3 dBi

19
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

20
Polarisation of the electric field
Electric field vector
Direction of propagation
Locus of the tip of the electric field vector on
plane x,y during one period ( 1/frequency)
Radiating Antenna
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

21
Elliptical Polarisation

• x,y plane perpendicular to direction of
  propagation
• Elliptical Polarisation is characterised by-
• Axial ratio of the ellipse, Emax/Emin
• 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

22
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

23
Operation of polarizer
24
Polarisation

• LINEAR
• CIRCULAR
• Antennas can be
• Single polarised e.g.vertical linear at all
  frequencies
• Orthogonally polarised e.g.vertical linear at
  receive
• in receive and transmit bands frequencies
  horizontal linear at
• transmit frequencies
• Dual-Polarised e.g. vertical and horizontal
  linear at all frequencies

EUTELSAT
INTELSAT 11/14GHz
horizontal
vertical
INTELSAT 4/6GHz
left hand
right hand
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Definitions

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

26
Cross-polar discrimination
Theta (degs)
27
Axisymmetric Systems

• Linear Polarisation

28
Axisymmetric Systems (cont.)

• Circular Polarisations

29
Offset Systems

• Linear Polarisation

Co-polar Cross-polar
Plane perpendicular to offset
30
Offset Systems

• Circular Polarisations (RHCP)

No cross-polar generated
Plane perpendicular to offset
31
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.

32
Noise Temperature

• Components for total system noise temperature
• Antenna noise temperature
• Noise temperature due to feed system
• Receiver noise temperature

33
Antenna noise temperature

• Dependent on
• Antenna radiation pattern G(?,?)
• Antenna elevation ?0
• Brightness temperature which is a function of
  frequency

34
Antenna noise temperature
,brightness temperature function

• where, cos ? cos ?0 cos ? - sin ?0 sin ? cos ?
  
• ?0 antenna elevation angle

Typical brightness temperature function at 4GHz
35
Feed system andReceived noise temperature

• Dependent on
• feed loss
• ambient noise temperature
• If ambient noise temperature is 290K and feed
  loss is small (lt1dB) then feed system noise temp.
  is
• TP 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

36
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.

37
Sidelobe Specifications (cont.)
Decibels relative to isotropic

• Angle of axis (degrees)

38
Minimum Satellite Spacings
39
Sidelobe Specification
40
Antenna tracking techniques

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

41
Gain Loss

• Simple expression for antenna main beam pattern

Pointing loss
Antenna diameter 25m at 4GHz, ?H?67/D? , D? ?
333 ?if half power beamwidth, ?H0.2deg Pointing
error, ?P0.05deg
42
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

43
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.

44
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
45
Four-Horn Static split System
Sum ABCD ?AZ(AB)-(CD) ?EL(AC)-(BD)
Simple two-channel tracking feed a Modes in horn
apertures b Comparator bridge network
46
Four-Horn Static split System (cont.)
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

47
Monopulse TrackingStatic Split System
48
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.