Satellite antennas

1. A Report on
Study of different types of Antenna Systems
used in satellite Communication Systems

Prepared by
• K.GAUTHAM REDDY - 2011A8PS364G



A Report prepared in partial fulfilment of the requirements of the course

EEE F472: SATELLITE COMMUNICATION
INSTRUCTOR: M.K.Deshmukh




Birla Institute of Technology and Science – Pilani
13/03/2014

2. INTRODUCTION
An antenna is defined by the IEEE as a “transmitting or receiving system that is
designed to radiate or receive electromagnetic waves” . An antenna can be any
shape or size. A list of some common types of antennas is wire, aperture,
microstrip , reflector, and arrays. Each antenna configuration has a radiation
pattern and design parameters, in addition to their benefits and drawbacks.
Satellite antenna design is a challenging task for design engineers because it
differs in many aspects from classical antenna design.Satellite antennas are
designed for point to multipoint (or vice versa) operation as opposed to point-to-
point operation for conventional ground station antennas. The design objective
for the ground station antennas is to achieve highest gain in the boresight
direction while for satellite antennas every point within the coverage area is
important. Satellite antenna parameters even at the edge of the coverage are
very important to be ensured.Monotonic (unshaped) beams are circular or
elliptical and follows natural roll off of an antenna aperture providing highest gain
at boresight of the coverage.Normally it does not fulfill the stringent
requirements imposed on satellite payload.To meet high performance of satellite
payload these days contoured beam (shaped) antennas are used.Satellite antenna
designer at least must be well conversant with payload performance
parameters,satellite platform,satellite launch vehicle and international
regulations.The choice of any antenna parameter is likely to affect several other
system parameters both inside and outside the antenna subsystem.Satellite
antenna designer comes across a variety of factors and feels ousted form heaven
of classical antenna design. At the same time,he finds himself confronted with
great concern to achieve more general system objectives.
For a design to be declared flight proven,it needs to undergo a myriad of critical
qualification tests and experimentation. Set of space standard procedures are
strictly followed at every stage including design, manufacturing, testing etc. of a
flight standard hardware. Satellite once launched is to operate for a certain
period usually 10-15 years and if a fault occurs, it has to be corrected by onboard
computer(s) or telecomm and otherwise it may lead to satellite’s malfunctioning.

3. Satellite systems needs to be very rugged and reliable so that satellite can operate
and provide the desired services satisfactorily throughout its life. In satellite
industry, best does not necessarily mean fit to requirements.
Communication Payload:
The communication payload can be broadly divided into two sections
1)Repeater section
2)Antenna section
The signal is received by the receiving antenna and is processed by the
repeater.The repeater can be transparent or regenerative.Mostly repeaters used
for public communication are transparent and regenerative repeaters are used
for military purposes.After processing the signal in repeater it is transmitted by
the transmitting antenna. Communication antennas used in geostationary
satellites are normally reflector antennas to overcome the high loss due to height
of GEO orbit .The receiving and transmitting antennas can be the same hardware
unit or different depending upon many factors that will be discussed in the
coming sections.To decide the configuration of satellite antennas in very
challenging task and is affected by many factors to be explored in the subsequent
sections.
Payload Performance Parameters:
In mission planning phase the service zones (called coverage regions or coverage
polygons hereafter) and performance parameters for all the coverage are
decided. The main performance parameters are operating frequency band(s),
antenna polarization(s),EIRP(Effective Isotropic Radiated Power), G/T(Figure of
merit) and XPD(Cross Polarization Discrimination).Different polygon can have
different performance parameter values depending upon many factors
considered in the planning phase based on need and market survey e.g. intended
application, predicted transponder usage, revenue generation and user
equipment’s size and cost. From EIRP value, it is to decide the gain of
transponders and the gain of antenna considering the feeder losses.The

4. transponder power is a strong function of availability and choice of technology for
high power devices like TWTAs (Travelling Wave Tube Amplifiers) and SSPAs(Solid
State Power Amplifiers).The satellite thermal design is a strong function of heat
generated by high power devices. Higher the operating power of a device,higher
is the probability of its failure and higher risks to the mission.To satisfy the
system’s reliability criterions, the design becomes more complex when dealing
with high power devices.So to make the design simpler and more reliable, it is
tried at best that the power of the transponder is kept minimum and high gain is
achieved using larger antennas to meet the EIRP requirements while satisfying the
mass, launch vehicle fairing accommodation and other related system constraints.
Gain distribution between antenna and transponder may require several
iterations and tradeoffs.
While considering the G/T parameter, satellite antenna designer need to know
the coverage region characteristics to find the antenna noise temperature. If
satellite antenna is looking towards ocean,it will have less noise temperature,on
the other hand,if antenna is looking at the hot earth its noise temperature will be
high which will finally affect the G/T of satellite.Normally worst case is considered
and antenna noise temperature is taken to be 290K.We also need to know the
noise generated by satellite receiver to find the system noise temperature. It is
tried to get a receiver with lower noise but low noise receivers are costly. Once
system noise temperature is known, the focus is shifted to receive antenna gain
and corresponding its size. An optimum antenna size is selected bearing in mind
performance required constraints.
Reciprocity Theorem for Antennas
The reciprocity theorem for antennas states that if a current I is induced in an
antenna B, operated in the receive mode, by an emf applied at the terminals of
antenna A operated in the transmit mode, then the same emf applied to the
terminals of B will induce the same current at the terminals of A.
A number of important consequences result from the reciprocity theorem. All
practical antennas have directional patterns, that is they transmit more energy in
some directions than others, and they receive more energy when pointing in

5. some directions than others. The reciprocity theorem requires that the directional
pattern for an antenna operating in the transmit mode is the same as that when
operating in the receive mode.
Another important consequence of the reciprocity theorem is that the antenna
impedance is the same for both mode of operation
Brief explaination of some fundamental parameters of antennas:
1)Beamwidth
The beamwidth of an antenna is described as the angles created by comparing
thehalf-power points (3 dB) on the main radiation lobe to its maximum power
point. In an example, the beam angle is 300, which is the sum of the two angles
created at the pointswhere thefield strengthdrops to 0. 0’ field strength is
measured in u/V/m) of the maximum voltage at thecenter of the lobe.(These
points are known as the half-power points.)
2) Polarization
Polarization of an antenna refers to the direction in space of the E field
(electricvector) portion of the electromagnetic wave being radiated by the
transmitting system.Low-frequency antennas are usually vertically polarized
because of ground effect (reflectedwaves, etc.) and physical Construction
methods. High-frequency antennas are generallyhorizontally polarized.
3) Bandwidth
Antenna bandwidth is “the range of frequencies within which the performance of
the antenna, with respect to some characteristic, conforms to a specified
standard” . The bandwidth can be viewed as the frequencies left and right of the
center frequency (usually the resonant frequency) in which the antenna
performance meets the specified values. The impedance bandwidth of an
antenna is commonly agreed upon as the power delivered to the antenna greater
than or equal to 90% of the available power . Another way to interpret the
antenna bandwidth is in terms of the reflection coefficient Γ. Γ is usually plotted in
as the power reflection coefficient by using Equation

6. Z0 is the line impedance and is typical equal to the generator resistance, usually
50Ω. Za is the antenna radiation resistance. When the power reflection coefficient
is -10dB, it represents 90% of the available power to the antenna is being sent to
antenna.
4) Directivity and Gain
Directivity is defined as “the ratio of radiation intensity, in a given direction, to the
radiation intensity that would be obtained if the power accepted by the antenna
where radiating isotropic ally” . In other words it’s the ratio of the radiation
intensity of an antenna to one that radiates equally in all direction. This is similar
to that of antenna gain but antenna gain takes into account the efficiency of the
antenna while directivity is the losses gain of an antenna. Directivity can be
calculated using the Poynting Vector, P, which tells you the average real power
per unit area radiated by an antenna in free space . The equation for the
directivity of an antenna is given by Equation

Pa is the total power radiated by the antenna and r is the distance between the
two antennas. The antenna gain takes into account loss so the gain of an antenna
will always be less than the directivity. Knowing the directivity of the antenna, the
total power radiated by the antenna, and the received power which takes into
account loss, you can calculate the antenna gain using Equation

7. In other words the gain is the efficiency multiplied by the directivity of the
antenna, the maximum possible gain of an antenna.
5) Radiation Efficiency
Radiation efficiency is the “ratio of total power radiate by an antenna to the net
power accepted by the antenna from the connected transmitter. ” Only 50% of
the power supplied through the TX network is used to transmit. In the best case
scenario, the maximum power accepted by the transmitting antenna is 50% of the
total power supplied and occurs when the generator impedance and the antenna
are matched, usually to 50Ω. The efficiency of an antenna is given by Equation
RL is your loss resistance which corresponds to the loss of your antenna and Rr is
the radiation resistance. In practice, you want your radiation resistance to be big
and the loss resistance to be as small as possible.

6)EIRP
Equivalent isotropic radiated power is the power radiated equally in all directions
that would produce a power flux density equivalent to the power flux density of
the actual antenna.
7)Figure of merit
It is the ratio of the antenna gain of the receiver G and the system temperature
T.The ratio G/T is important because it is an invariant that is independent of the
reference point where it is calculated,even though the gain and the system
temperature individually are different at different points.

8. Types of antenna systems:
1)Wire antennas
Wire antennas are used primarily at VHF and UHF to provide communications for
the TTC&M systems. They are positioned with great care on the body of the
satellite in an attempt to provide omnidirectional coverage. Most satellites
measure only a few wavelengths at VHF frequencies, which makes it difficult to
get the required antenna patterns, and there tend to be some orientations of the
satellite in which the sensitivity of the TTC&M system is reduced by nulls in the
antenna pattern.
An antenna pattern is a plot of the field strength in the far field of the
antenna when the antenna is driven by a transmitter. It is usually measured in
decibels (dB) because of low field strength.
2) Apereture antennas
An aperture antenna is an antenna that contains an opening in which
electromagnetic waves are transmitted or received through . Aperture antennas
can be many different shapes. Popular configurations of an aperture antenna are
waveguides and horns . Aperture antennas are used widely in aircrafts because
the can be covered with a dielectric. This dielectric protects the antenna from the
environments that an aircraft is exposed to. A waveguide is an antenna that
guides an electromagnetic wave. It consists of a conductive wall that is hollow in
the inside for the wave to travel. A horn antenna is “an antenna consisting of a
waveguide section in which the cross-sectional area increases towards an open
end which is the aperture” . A typical horn antenna is in Figure

9. There are three types of horn antennas: 1) E-plane sectoral horn, 2) H-plane
sectoralhorn, and 3) pyramidal horn. H-Plane Sectoral horn has a wider width to
of the aperture while E-Plane Sectoral horn has a wider height. The pyramidal
horn has approximately equal width and height.
Horns have very little loss, so the directivity of a horn is roughly equal to its gain.
The gain G of a pyramidal horn antenna (the ratio of the radiated power intensity
along its beam axis to the intensity of an isotropic antenna with the same input
power) is

Where A is the area of the aperture, d is the aperture diameter of a conical horn,
λ is the wavelength, eA is a dimensionless parameter between 0 and 1 called the
aperture efficiency.

3) Reflector antennas
A parabolic antenna is a high-gain reflector antenna used for radio, television
anddata communications, and also for radiolocation (RADAR), on
theUHFandSHFparts of theelectromagnetic spectrum. The relatively short

10. wavelength of electromagnetic (radio) energyat these frequencies allows
reasonably sized reflectors to exhibit the very desirable highlydirectional response
for both receiving and transmitting.With the advent of TVRO and DBS satellite
television, the parabolic antenna becamea ubiquitous feature of urban, suburban,
and even rural, landscapes. Extensive terrestrialmicrowave links, such as those
between cellphone base stations, and wireless WAN/LANapplications have also
proliferated this antenna type. Earlier applications included ground- based and
airborne radar and radio astronomy. The largest "dish" antenna in the world is
theArecibo Observatory's radio telescope at Arecibo, Puerto Rico, but, for beam-
steeringreasons, it is actually a spherical, rather than parabolic, reflector.

Directivity of the reflector antenna

The true gain of a reflector antenna takes into account radiation, aperture taper,
spillover, and achievement losses. Taking those factors into consideration, the
gain of a reflector antenna can be found using Equation

11. 4) Antenna Array
An antenna array is “an antenna comprised of a number of identical radiating
elements in a regular arrangement and excited to obtain a prescribed radiation
pattern [5].” Antenna arrays can be divided into two divisions: scanning and non-
scanning antenna arrays. Scanning arrays are able to move their main beam
electronically, usually by changing the phase of the elements. Non-scanning array
can only change their main beam lobe by moving the antenna orientation and are
used commonly to for directional radiation applications. A common type of
scanning array is the phased array. Non-scanning array examples are an array of
dipoles, or microstrip antenna.
Phased Array:Phased array antennas are an antenna that has multiple radiating
elements each connecting to a phase shifter. The phase shifting allows the
radiation pattern to be “steered” towards a certain direction.The lobe is increased
upward as a result of the phase shift demonstrating by changing the phase one
can steer the beam in desired location without actually moving the element.
5)Helix antenna
A helix antenna is defined as an antenna whose configuration relates to a helix.
The helix antenna is relatively light weight because it is constructed using a metal
conductor wire, a centersupport the helix structure, and is usually attached to a
grounThe lossless gain of a Helix Antenna is given by

12. Comparison of parameters of Horn and reflector antennas:
1)Parameters of horn antenna:
1) Bandwidth of horn antennas is typically of the order of 10:1, and can be up to
20:1(for example allowing it to operate from 1 GHz to 20 GHz)
2) Horns are widely used as antennas at UHF and microwave frequencies
3) The gain of horn antennas ranges up to 25 dBi, with 10 - 20 dBi being typical.
4) Horns have very little loss, so the directivity of a horn is roughly equal to its
gain.
5)For horn antennas as the gain increases beamwidth decreases.
Parameters of parabolic reflector antennas:
1) Parabolic antennas are used in the high frequency part of the radio spectrum,
at UHF and microwave (SHF) frequencies.
2) Approximate maximum gain of 140,000 times or about 50 dBi (decibels above
the isotropic level).
3) The aperture efficiency of typical parabolic antennas is 0.55 to 0.70 which
determines the directivity.
4) Bandwidth approximately equal to 100MHz
5)For parabolic antennas, the HPBWθ is given by

13. here k is a factor which varies slightly depending on the shape of the reflector and
the feed illumination pattern.
Comparision of Parameters of some of the antennas:

Conclusions:
An antenna is a structure—generally metallic and sometimes very complex
designedto provide an efficient coupling between space and the output of a
transmitter or the input to areceiver. Like a transmission line, an antenna is a
device with distributed constants, so thatcurrent, voltage and impedance all vary
from one point to the next one along it. This factor must be taken into account
when considering important antenna properties such asimpedance, gain and
shape of radiation pattern.
The gain of an antenna is determined by the intended area of coverage. The gain
at a given wavelength is achieved by appropriately choosing the size of the
antenna. The gain may also be expressed in terms of the half power beamwidth
The use of a directional antenna allows the electromagnetic waves to be focus
towards a specific section. This allows wireless connections at greater distance
from the router than a traditional omnidirectional antenna. In addition, higher

14. data rate can be achieved at greater distances. This antenna would be beneficial
in both rural and urban environments.
Reflector antennas are generally used to produce narrow beam for geostationary
satellites and earth stations. The efficiency of the antenna is optimized by the
method of illumination and choice of edge taper. Phased array antennas are used
on many LEO and MEO satellites. New technologies include large, unfurlable
antennas for producing small spot beams from geostationary orbit and shaped
reflectors for creating a shaped beam with only a single feed.
Thus antennas have become the very important thing for the Scienceand research
purposes.
References:
1) Ghulam Ahmad and S.A. Mohsin,”Modern Communication Satellite
Antenna Technology”
2) Christos G.Christodoulou and Parveen F.Wahid,”Fundamentals of
Antennas: Concepts and Applications”
3) Dennis Roddy,”Satellite communication”
4) https://www.wikipedia.org/
5) http://www.scribd.com/doc/12835003/Antennas-In-Satellite-
Communication
6) http://www.geosats.com/antennas.html