muhamad asvial - universitas...
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MUHAMAD ASVIALCenter for Information and Communication Engineering Research (CICER)
Electrical Engineering Department, University of IndonesiaKampus UI Depok, 16424, Indonesia
[email protected]://www.ee.ui.ac.id/cicer
Antenna Basic 2
Lecture 7
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1
Elliptical Polarisation
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
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2
Polarisation
LINEAR
CIRCULAR
Antennas can be:1. Single polarised e.g.vertical linear at all frequencies
2. Orthogonally polarised e.g.vertical linear at receive
in receive and transmit band frequencies horizontal linear at
transmit frequencies
3. Dual-Polarised e.g. vertical and horizontal linear atall frequencies
vertical horizontal
right hand left hand
EUTELSATINTELSAT11/14GHz
INTELSAT4/6GHz
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3
Definitions
CO-POLAR – component of field parallel to the field of the reference
source
CROSS-POLAR – component in orthogonal direction
vertical horizontal
right handcircular
left handcircular
reference source
vertical
copolarcomponent
cross-polarcomponent
left handcircular
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4
Cross-polar discrimination
Copolar
Cross-polarDiscrimination(XPD)
Cross-polar
Am
plitu
de (d
B)
Theta (degs)
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5
Axis-symmetric Systems
• Linear Polarisation
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6
Axis-symmetric Systems (cont.)
• Circular Polarisations
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7
Offset Systems
• Linear PolarisationCo-polar
Cross-polar
Plane perpendicular to offset
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8
Offset Systems
• Circular PolarisationsNo cross-polar generated
Plane perpendicular to offset
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9
Noise Temperature
• Components for total system noise temperature:
– Antenna noise temperature
– Noise temperature due to feed
system
– Receiver noise temperature
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10
Antenna noise temperature
• Dependent on:– Antenna radiation pattern [G(,)]– Antenna elevation [0]
– Brightness temperature which is a
function of frequency
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11
Antenna noise temperature
θdφdθsinφθ,Gφθ,,θT'θTπ2
0
π
00π4
10A
)T(θφθ,,θT' *0
where, cos * = cos 0 cos - sin 0 sin cos 0 = antenna elevation angle
Typical brightness temperature function at 4GHz
,brightness temperature function
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12
Feed system andReceived 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 temperatureDependent on type of LNA and whether cooled or uncooled
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13
Side-lobe Specifications
• Transmit – Mandatory to avoid interference
into other systems
• Receive – Advisable to reduce interference
from other systems
• 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.
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14
Side-lobe Specifications (cont.)
Angle of axis (degrees)
Decib
els
rela
tive
to isotropic
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15
Minimum Satellite Spacings
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16
Side-lobe Specification
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17
Antenna tracking techniques
• Monopulse
– Static split
– Higher order modes
• Conical scan
• Step track
• Programmed track
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18
Gain Loss
Simple expression for antenna main beam pattern
dBθθ10log20θG
2
H
6.010
2
H
P6.010 θ
θ10log20(xdB)
dBθθ12Loss
2
H
P
Pointing loss
Antenna diameter 25m at 4GHz, H67/D , D 333
if half power beamwidth, H=0.2deg
Pointing error, P=0.05deg dB2.005.012GainLoss
2
dB57.0
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19
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
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20
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 m
ade.
– 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.
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21
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.
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22
Four-Horn Static split System
Sum = A+B+C+D
AZ=(A+B)-(C+D)
EL=(A+C)-(B+D)
A
B
A
B
A
B
C
D
C
D
C
D
Sumchannel
Azimuthdifference
Elevationdifference
Aaz Ae
Simple two-channel tracking feed
a Modes in horn apertures
b Comparator bridge network
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23
Four-Horn Static split System (cont.)
Normal sum type pattern
Difference pattern has null on boresight
Satellite should be steered to be in null
Sum and difference channel radiation patterns
a Feed illumination patterns
b Reflector for field patterns
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24
Monopulse Tracking Static – Split System
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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.