1. Satellite Communications A Part 2
Antenna Basics
-Professor Barry G Evans-
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

3. Near Field – Far Field Transition Region
Far-Field Region
Constant
No phase difference
between centre and edge ray
Near-Field Region
Approximate Beam Edge
parallel
beam
region
Phase difference
radians D
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

5. Antenna Radiation Pattern
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

7. Asymmetric (or offset) dual reflector systems
(a) Cassegrain
(b) Gregorian
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

8. Dual-offset Gregorian antenna
Antenna is shown
At 30°angle of
elevation
Main reflector
5.5m diameter
Feed
Sub
Reflector
Dual –offset Gregorian antenna
for satellite communication services
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

9. Simple Satellite Antennas
SPOT EAST
EUROPEAN
SPOT ATLANTIC
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.
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

11. Contoured beam coverage
reflector
1000m 0m
scale
feed cluster
power divider
34.28°
31.28°
48.61°
Contoured beam coverage
Contoured beam coverage of a Eurobeam zone satellite
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

12. INTELSAT V coverage diagrams
Shaped zone beams
Shaped hemi beams
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

13. 4 GHz and 6 GHz antennas on INTELSAT VI
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

14. 4 GHz and 6 GHz antennas on INTELSAT VI (cont.)
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

16. SatCommsA - Part 2 - Antennas - B G Evans
Antenna Gain
where,  = operating wavelength
 = physical aperture area of the antenna
 = antenna efficiency factor
For circular aperture antennas,
where, D = circular aperture diameter
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

18. Typical efficiency factors for a large Cassegrain antenna
LOSS (dB)
ILLUMINATION EFF.
98.7
0.06
SUBREFLECTOR S/O
88.3
0.54
MAIN REFLECTOR S/O
96.0
0.18
BLOCKAGE LOSSES
92.6
0.33
MANUFACTURING LOSSES
92.4
0.34
FEED OHMIC LOSSES
95.5
0.2
ANTENNA EFFICIENCY
68.4%
1.65dB
Gain of 30m Antenna at 4GHz.
G = 10 log (( x 30 x 4)/3) dBi
= 60.3 dBi
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

20. Polarisation of the 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
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

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

23. Operation of polarizer
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

24. SatCommsA - Part 2 - Antennas - B G Evans
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
vertical
horizontal
INTELSAT
4/6GHz
right hand
left hand
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

25. SatCommsA - Part 2 - Antennas - B G Evans
Definitions
CO-POLAR – component of field parallel to the field of the reference source
CROSS-POLAR – component in orthogonal direction
vertical
horizontal
right hand
circular
left hand
reference
source
copolar
component
cross-polar
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

26. Cross-polar discrimination
Copolar
Cross-polar
Discrimination
(XPD)
Amplitude (dB)
Theta (degs)
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

27. SatCommsA - Part 2 - Antennas - B G Evans
Axisymmetric Systems
Linear Polarisation
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

28. Axisymmetric Systems (cont.)
Circular Polarisations
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

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

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

32. 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
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

33. Antenna noise temperature
Dependent on:
Antenna radiation pattern [G(,)]
Antenna elevation [0]
Brightness temperature which is a function of frequency
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

35. 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:
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
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

37. Sidelobe Specifications (cont.)
Decibels relative to isotropic
Angle of axis (degrees)
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

38. Minimum Satellite Spacings
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

39. Sidelobe Specification
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

40. Antenna tracking techniques
Monopulse
Static split
Higher order modes
Conical scan
Step track
Programmed track
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

41. SatCommsA - Part 2 - Antennas - B G Evans
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, H=0.2deg
Pointing error, P=0.05deg
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

43. 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.
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

44. 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
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

45. Four-Horn Static split SystemB C D
Sum
channel
Azimuth
difference
Elevation 
Aaz Ae
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
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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

47. Monopulse Tracking Static – Split System
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans

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