Polarization and Transponders
in Satellite
College of Electronic Engineering
2015-2016
What is Polarization?
Polarization is the property of electromagnetic
waves that describes the direction of the
transverse electric field.
Since electromagnetic waves consist of an electric
and a magnetic field vibrating at right angles to
each other.
Types of Polarization
Linear Polarization (horizontal
or vertical):
◦ the two orthogonal components
of the electric field are in phase;
◦ The direction of the line in the
plane depends on the relative
amplitudes of the two
components.
Circular Polarization:
◦ The two components are exactly
90º out of phase and have exactly
the same amplitude.
Elliptical Polarization:
Linear Polarization Circular Polarization Elliptical Polarization ◦ All other cases.
Satellite Communications
Alternating vertical and
horizontal polarization is
widely used on satellite
communications.
This reduces interference
between programs on the
same frequency band
transmitted from adjacent
satellites (One uses vertical,
the next horizontal, and so on)
Allows for reduced angular Information Resources for Telecommunication Professionals
separation between the
satellites.
Polarization
In the far field zone of a transmitting antenna, the
radiated wave takes on the characteristics of a
transverse electromagnetic wave (TEM).
Far field zone: At distances greater than 2*D^2/ λ
from the antenna, where D is the larger linear
dimension of the antenna and λ is the wavelength.
Ex: For a parabolic antenna of 3 m diameter
transmitting a 6 GHz wave (λ = 5 cm), the far field zone
begins at approximately 360 m.
=2*3^2/0.05 360m
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Transverse electromagnetic wave (TEM): Both the
magnetic field H and the electric field E are
transverse to the direction of propagation (k) .
TEM wave is a plane at great distance from the
transmitting antenna.
Polarization (cont.)
The direction of the line traced out by the tip of the electric field
vector determine the polarization of the wave.
Electric and magnetic fields are varying as functions of time. The
magnetic field varies exactly in phase with the electric field, and
its amplitude is proportional to the electric field in this discussion.
Polarization: Linear, elliptical, and circular polarization.
Linear polarization: The tip of the E vector may be trace out a
straight line, in which case the polarization is referred to as linear.
Vertical polarization: the electric field was perpendicular to the
earth’s surface or parallel to the earth’s polar axis.
Horizontal polarization: the electric field was parallel to the
earth’s surface.
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Polarization of Satellite Signals
Satellite linear polarization:
The horizontal polarization is where the electric field vector
is parallel to the equatorial plane,
The vertical polarization is where the electric field vector is
parallel to the earth’s polar axis.
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Cross-Polarization Discrimination
The propagation path between a satellite and earth station
passes through the ionosphere, and possibly through layers of
ice crystals in the upper atmosphere and rain, all of which are
capable of altering the polarization of the wave being transmitted.
Depolarization: An orthogonal component may be generated
from the transmitted polarization, an effect referred to as
depolarization.
This can cause interference where orthogonal polarization is
used to provided isolation between signals, as in the case of
frequency reuse.
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Cross-Polarization Discrimination
(continued)
Two measures are in use to quantify the effects of polarization
interference:
Cross-Polarization Discrimination: The most widely used measure
is called Cross-Polarization Discrimination (XPD).
Polarization Isolation: The second measure is polarization isolation
(I) that is defined by the ratio of received co-polar power to received
cross-polar power and thus takes into account any additional
depolarization introduced by the receiving system.
When the transmitted signals have the same magnitudes and where the
receiving system introduces negligible depolarization, then I and XPD
give identical results.
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Depolarization
Ionospheric Depolarization:
One of the effects of the ionoshpere is to produce a
rotation of the polarization of a signal, an effect known
as Faraday rotation. (Michael Faraday, 1791-1867)
Faraday rotation is inversely proportional to frequency
squared and is not considered to be a serious problem
for frequency above about 10 GHz.
Maximum values for Faraday rotation are 9 degree at 4
GHz and 4 degree at 6 GHz. In order to counter the
depolarizing effects of Faraday rotation, circular
polarization may be used.
When a linear polarization is used, a 15 dB amount
added to the XPD, where is only 0.13 dB as with
circular polarization.
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Depolarization
(continued)
Rain Depolarization: The shape of small
raindrops is close to spherical, but large
drops are better modeled as oblate
spheroids .
There will be a difference therefore in the
attenuation, termed differential attenuation,
and phase shift, termed differential phase
shift.
Ice Depolarization: Value of 2 dB added to
the XPD are suggested for North America
and 4 to 5 dB for maritime regions.
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Homework (9)
5.1 Explain what is meant by a plane TEM wave.
5.15 Explain what is meant by vertical polarization of a satellite
signal.
5.16 Explain what is meant by horizontal polarization of a
satellite signal.
5.19 Explain what is meant by cross-polarization discrimination.
5.20 Explain the difference between cross-polarization
discrimination and polarization isolation.
5.22 Why is Faraday rotation of no concern with circular
polarized wave?
5.23 Explain how depolarization is caused by rain.
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Transponders in Satellite
In a communications satellite, the equipment which
provides the connecting link between the satellite’s
transmit and receive antennas is referred to as the
transponder.
transponder
Basic communication satellite components
Transponders in Satellite
A transponder is the series of interconnected units which
forms a single communications channel between the
receive and transmit antennas in a communications
satellite.
Mainly operating in C-band (6/4 GHz) and Ku-band
(14/12 GHz).
Transponders in Satellite
Atypical transponder bandwidth is 36 MHz, and
allowing for a 4 MHz guardband between
transponders, 12 such transponders can be
accommodated in the 500-MHz bandwidth.
Transponders in Satellite
1- Two Stations on Earth want to communicate through
radio broadcast but are too far away to use conventional
means.
2- Two stations can use a
satellite as a relay station
for communication.
Transponders in Satellite
3- One Earth Station sends
a transmission to the
satellite. This is called a
Uplink.
4- The satellite Transponder
converts the signal and
sends it down to the
second earth station.
This is called a Downlink.
Transponders in Satellite
1. A transmitter-receiver system taken together is known
as Transponder.
2. Works LIKE a repeater in the sky.
3. Transponder bandwidth depends up on
a. nature of signal.
b. multiple access technique.
Functioning of a Transponders
1. Transponder is also principally used as a re-transmitter due to
the fact that it receives a definite signal from a specific source,
then it amplifies (magnifies) the signal before sending it to a
predefined location.
2. A transponder functions by receiving a signal, called an
"interrogator" because it is effectively "asking" for information,
then automatically conveying a radio wave at a predetermined
frequency.
3. In order to broadcast a signal on a different frequency than the
one received, a frequency converter is built in.
4. By receiving and transmitting on different frequencies, the
interrogator and transponder signals can be detected
simultaneously.
TYPES OF Transponders
Basic types of transponders are:
A. Conventional transparent ( non-regenerative satellite)
B. Processing (regenerative satellite)
Processing Conventional Transparent
Conventional Transparent Transponder
Transponder which translates the frequency of
the received signals and amplifies them but
applies no other deliberate processing before
retransmission.
Conventional Transparent Transponder
Only amplification and re-transmission of
signal is done.
Features of Conventional Transparent Transponder
It’s most common satellite transponder type used.
It capture signal from ground stations.
Capture as little interference and noise.
Amplify received carriers signal.
Works as a frequency converter.
Provide necessary power for transmission.
Radiate carriers in designated frequency band and
with assigned polarization.
Conventional Transparent Transponder
Advantages
• Relatively simple structure.
• Reliable system.
• Independent of modulation and coding
• Multiple purpose (TV, data, telephony,..)
Disadvantages
• Not only signal is amplified noise and interference
from ground is amplified as well
• C/N Ratio (stands for Carrier to Noise Ratio) at earth
station receiver is composed of C/N of uplink and C/N
of downlink
• longer propagation delay
Regenerative Transponder
A satellite that provides on-board demodulation and
remodulation of the information bearing signal is
referred to as an on-board processing (OBP) satellite.
The OBP satellite, also called a regenerative satellite
or a smart satellite
REGENERATIVE TRANSPONDER
Regenerative Transponder
Amplification and Frequency Translation along with
Signal Processing.
The uplink signal at fup is demodulated to baseband,
fbaseband.
The baseband signal is available for processing on-
board, including reformatting and error-correction.
A regenerative repeater for digital signals
Regenerative Transponder
The baseband information is then remodulated to
the downlink carrier at fdwn, possibly in a different
modulation format to the uplink and, after final
amplification, transmitted to the ground.
The demodulation/remodulation process removes
uplink noise and interference from the downlink,
while allowing additional on-board processing to
be accomplished.
A regenerative repeater for digital signals
Regenerative Transponder
ADVANTAGES
More signal regeneration so separation of up- and
downlink.
Because of on-board switching resulting in lower
complexity and cost of ground stations.
Digital signals easier to handle.
Different input/output formats.
DISADVANTAGES
More complexity on board resulting in:
– high reliability required
– potential single-point of failure in the sky
Classification based Frequency Conversion
Single Conversion Transponder
Double Conversion Transponder
Broadband Multiple-Channel Transponder
Multi-Channel Receiver Transponder
Single Conversion Transponder
Simplified single-conversion transponder for 6/4 GHz band
Double Conversion Transponder
Simplified double-conversion transponder (bent pipe) for 14/11 GHz
band
Broadband Multiple-Channel Transponder
Broadband multiple-channel repeater
Multi-Channel Receiver Transponder
Multi-channel receiver transponder
TRANSPONDER APPLICATIONS
Daily life applications of transponders
Transponders have large number of
applications in various fields:
a) Satellite communication
b) Aviation
c) Marine
d) Automotive
e) Road
f) Motorsport
g) Underwater,.. etc.
TRANSPONDER APPLICATIONS
They are also used in simple day-to-day tasks such
as opening a car’s door wirelessly.
Transponders are also used to compute distance
by evaluating the elapsed time between the
transferring of a signal and acknowledgment of
the transponder’s signal.
For example, sonar transponders are used to locate undersea
places, estimate depth, and trace locations.
HW (10)
7.11. Briefly describe the equipment sections making up a
transponder channel.
7.12. Draw to scale the uplink and downlink channeling schemes
for a 500-MHz-bandwidth C-band satellite, accommodating the
full complement of 36-MHz-bandwidth transponders. Assume the
use of 4-MHz guardbands.
7.13. Explain what is meant by frequency reuse, and describe
briefly two methods by which this can be achieved.
7.14. Explain what is meant by redundant receiver in connection
with communication satellites.
7.15. Describe the function of the input demultiplexer used
aboard a communications satellite.
7.16. Describe briefly the most common type of high-power
amplifying device used aboard a communications satellite.
