Satellite antennas


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Satellite antennas

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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) 5) Communication 6)