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Introduction to Microwaves,Satellite commn,Radar systemsMicrowaves

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Introductory notes for undergraduate students of electronics

Introductory notes for undergraduate students of electronics

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  • 1. MICROWAVESINTRODUCTION Microwaves are electromagnetic waves with wavelengths ranging from aslong as one meter to as short as one millimeter, or , with frequencies between 500MHz and 100 GHz.[It can be even up to 300 G.Hz] . Electromagnetic waves longer than microwaves ((lower frequency) are called"Radio waves". Electromagnetic radiation with shorter wavelengths may be called"millimeter waves".MICROWAVES IN COMMUNICATIONS: Microwave communication is the transmission of signals via radio using a seriesof microwave towers. Microwave communication is known as a form of “line of sight”communication, because there must be nothing obstructing the transmission of databetween these towers for signals to be properly sent and received. The technology used for microwave communication was developed in theearly 1940’s by Western Union. The first microwave message was sent in 1945 fromtowers located in New York and Philadelphia. After this successful attempt, microwavecommunication became the most commonly used data transmission method fortelecommunications service providers. Microwave communication takes place bothanalog and digital formats. While digital is the most advanced form of microwavecommunication, both analog and digital methods gives certain benefits for the users.Analog microwave communication may be most economical for use when compared todigital communication. Digital microwave communication utilizes more advanced, morereliable technology. Typically, microwaves are used in television news to transmit a signal from aremote location to a television station. Most satellite communication systems operate inthe C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow largebandwidth while avoiding the crowded UHF frequencies and staying below theatmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for 1
  • 2. the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite.Military communications run primarily over X or Ku-band links. Radar uses microwave radiation to detect the range, speed, and othercharacteristics of remote objects. Most of the radio astronomy systems uses microwaves.MICROWAVE FREQUENCY BANDSThe various bands of the Microwave region are shown in the following table. S.No Type of Band Frequency Range1 L band 1 to 2 GHz2 S band 2 to 4 GHz3 C band 4 to 8 GHz4 X band 8 to 12 GHz5 Ku band 12 to 18 GHz6 K band 18 to 26.5 GHz7 Ka band 26.5 to 40 GHz8 Q band 33 to 50 GHz9 U band 40 to 60 GHz10 V band 50 to 75 GHz11 E band 60 to 90 GHz12 W band 7 5 to 110 GHz13 F band 90 to 140 GHz14 D band 110 to 170 GHzMICROWAVE PROPERTIES:The Microwaves behaves similar to light rays. They exhibit the following properties.(i)They can be focused with lenses made of wax or paraffin(ii) They can be refracted with prisms of wax or paraffin materials.(iii) They can be reflected from large, plane sheets of metal 2
  • 3. (iv) Microwaves can be diffracted by slits in metal surfaces and interferometers can be constructed for their use.(v) Microwaves can pass through dry wood whereas the light waves cannot pass through.(vi) Microwaves propagate in free space, in various materials, and in waveguides.(vii) Microwaves undergo polarization with paraffin crystals.(viii) Microwaves also exhibit total internal reflection.(ix) Microwave radiation (at 2450 MHz) is non-ionizing(x) Microwaves also cause heatingMICROWAVE GENERATION A klystron tube is a special type of vacuum tube invented in 1937 by the Varianbrothers. A klystron tube is used to produce microwave energy. In this application, itworks similar to an organ pipe. When the air in the organ tube vibrates, the organ tubeemits sound energy of a specific frequency that we hear as a single note. When theelectrons in the klystron tube vibrate, the klystron tube emits high frequency microwaveenergy that can be detected by a radar receiver.There are two types of klystrons tubes in use: (i) The floating drift and (ii) The ReflexKlystron.REFLEX KLYSTRON : Reflex klystrons were developed in 1940 by the Soviet engineers N. D.Deviatkov, E. N. Danil’tsev , and I. V. Piskunov, working as a group, and,independently, by the Soviet engineer V. F. Kovalenko. The Reflex Klystron is a single cavity variable frequency microwavegenerator oscillator. It has low power and low efficiency. The principle of the Reflexklystron is that , the electron beam, having passed through the resonator gap, arrives atthe decelerating field of the reflector, to be repelled by the field and pass through theresonator gap in the opposite direction .During the first transit through the gap, theultrahigh frequency electric field of the gap modulates the electron velocities. The secondtime, moving in the opposite direction, the electrons arrive at the gap grouped in bunches. 3
  • 4. The ultrahigh frequency field in the gap retards these bunches and converts some of theirkinetic energy to the energy of ultrahigh-frequency oscillations. This is nothing but theMicrowave energy.Construction: The Reflex Klystron consists of electron gun, filament surrounded by acathode and a focusing electrode at cathode potential. The electron beam emitted fromthe cathode is accelerated by the Grid and passed through the anode cavity to the repellerspace between the anode cavity and repeller electrode as shown in figure.1.Working: The electron beam from the cathode is velocity modulated by the cavity gapvoltage.Due to this some of the electrons accelerates and enters the repeller space with agreater velocity than the velocity electrons with unchanged velocity.Some of theelectrons decelerates and enters the repeller space with less velocity.In the repeller regionall the electrons are bunched together and pass through the cavity gap for every one cycleas shown in figure 2. During the returning path the bunched electrons pass the gap duringthe negative cycle and deliver the kinetic energy to the electromagnetic energy of thefield in the anode cavity.The output is taken from the anode cavity. 4
  • 5. Reflex klystrons are the most widely used ultrahigh-frequency device. They aremanufactured for operation in the decimeter, centimeter, and millimeter wave bands.Their output power ranges from 5 mW to 5 W. The efficiency of the Reflex Klystronranges from 20% to 30%. Reflex klystrons are used as heterodynes in superheterodyneradio receivers, as driving oscillators in radio transmitters, as low-power oscillators inradar, in radio navigation.APPLICATIONS OF MICROWAVES :Microwaves find applications in various fields . They are(1) Microwaves are used in RADAR communications.(2) Microwave ovens are used for cooking the food at a very faster rate.(2.45G.Hz,600W)(3 ) Microwave heating is used in rubber, plastic, paper industries for drying and curing Products and food processing industries.(4) Microwaves can be used to transmit power over long distances(5) Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) Spectroscopy(6) Used in long distance communications like, Telephone networks, T.V Networks, Telemetry etc...(7) Microwaves are used in Microstrip and disk filters, delay lines, and phase shifters.(8) Microwaves are used in Mining industries ,for tunneling and breaking rocks etc..(9) Used in Bio-medical applications (Diathermy for localized superficial heating)(10) Microwaves are used in tumor detection based medical applications.(11) In Microwave tomography(12) In Microwave acoustic imaging.(13) In identifying the objects by non-contact method(14) Microwave radiometers are used to map atmospheric temperatures , moisture conditions. 5
  • 6. (15).Satellite and terrestrial communication links with very high capacities are possible.(16).Various molecular, atomic, and nuclear resonances occur at microwave frequencies, so, there are unique applications in the areas of basic science, remote-sensing, medical diagnostics and treatment. INTRODUCTION TO SATELLITE COMMUNICATIONINTRODUCTION : The first artificial satellite was placed in orbit by the Russians in 1957. Thatsatellite was called Sputnik and it is the beginning of an era. During the early 1960s,the Navy used the moon as a medium for passing messages between ships at sea andshore stations. This method of communications proved reliable when other methodsfailed. Communications via satellite is a natural outgrowth of modern technology and ofthe continuing demand for greater capacity and higher quality in communications. A Satellite is defined as a body that revolves around another larger body in apath called orbit. For example the moon is the natural satellite to the earth. SimilarlyEarth is the satellite to the Sun. A communication satellite is a microwave repeater stationthat is used for tele-communication, radio and television signals. There are nearly 750satellites in space which are mostly used for communication applications. A satellite communications system uses satellites to relay radio transmissionsbetween earth terminals. There are two types of communications satellites .One isACTIVE and the other is PASSIVE. A passive satellite only reflects received radiosignals back to earth.whereas an active satellite acts as a REPEATER ; it amplifiessignals received and then retransmits them back to earth. This increases signal strength atthe receiving terminal to a higher level than would be available from a passive satellite. Atypical operational link involves an active satellite and two or more earth terminals. Onestation transmits to the satellite on a frequency called the UP-LINK frequency. Thesatellite then amplifies the signal, converts it to the DOWN-LINK frequency, andtransmits it back to earth. The signal is next picked up by the receiving terminal. 6
  • 7. For covering the majority portion of the earth a minimum of three satellites arerequired.KEPLER’S lAWS : In the early 1600s, Johannes Kepler proposed three laws of planetary motion.These Kepler’s laws are found to be very useful in understanding not only the planetarymotion but the satellite motion also.The satellites also obey the Kepler’s laws.Keplers three laws can be described as follows :(i)The Law of EllipsesThe path of the planets about the sun is elliptical in shape, with the center of the sunbeing located at one of its foci.(ii) The Law of Equal AreasAn imaginary line drawn from the center of the sun to the center of the planet will sweepout equal areas in equal intervals of time.(iii)The Law of Harmonies The ratio of the squares of the periods of any two planets is equal to the ratio of thecubes of their average distances from the sun.GEO-STATIONARY ORBIT : A geostationary orbit or Geostationary Earth Orbit (GEO) is a circulargeosynchronous orbit directly above the Earths equator (0° latitude), with a period equalto the Earths rotational period and an orbital eccentricity of approximately zero. Anobject in a geostationary orbit appears motionless, at a fixed position in the sky, toground observers.So,the relative velocity between the Earth and the Geostationary orbit is zero. Communications satellites and weather satellites are placed in geostationaryorbits, so that the satellite antennas that communicate with them do not have to move totrack them, but can be pointed permanently at the position in the sky where they stay.Due to the constant 0° latitude and circularity of geostationary orbits, satellites in GEOdiffer in location by longitude only. 7
  • 8. Geostationary orbits are useful because they cause a satellite to appear stationarywith respect to a fixed point on the rotating Earth, allowing a fixed antenna to maintain alink with the satellite. The height of a Geostationary satellite from the surface of the earth is 35,786kilometres or nearly 36,000 km.TRANSPONDERS A transponder is an automatic electronic control device that receives, cross-examines, amplifies and retransmits the received signalon a different frequency. It ismainly used in wireless communication. The word ‘Transponder’ is a combination oftwo words; transmitter and responder.A communications satellite’s channels are alsocalled transponders, because each is a separate transceiver or repeater. A transponder works by receiving a signal on a component called “interrogator”since it effectively inquires for information, then automatically transmitting a radio wavesignal at a predestined frequency. In order to broadcast a signal on a dissimilar frequencythan the one received, a special component called the “frequency converter” is provided.By receiving and transmitting on dissimilar frequencies, the interrogator and transpondersignals can be sensed concurrently. Transponders are basically of two types; active transponders and passivetransponders. An active transponder includes its very own power supply and constantlyemit radio signals which are tracked and monitored. These can also be automatic deviceswhich strengthen the received signals and relay them to another location. A passive transponder does not include its own power source. The passivetransponder collects power from a close by electric or magnetic field offered by a reader.The reader cross-examines the neighboring field for transponders that may be in itsproximity and stimulates enough power into the transponder’s electronic circuitry that thetransponder becomes active and retransmits to the reader its identification ID as well asany added information required.Block Diagram of the Transponder : A transponder is not a single unit. It consists of a Diplexor,band passfilter,wide-band receiver, power amplifiers, Input De-Mux and output Mux etc.A 8
  • 9. Diplexor is used to allow simultaneous transmission and reception.The Diplexor is a twoway microwave gate that permits the received carrier signals from the antenna andtransmitted carrier signals to the antenna. A basic band width of 500 M.Hz is available atC – band frequencies with an input link frequency range of 5.925 to 6.425 G.Hz .Thesefrequencies are passed through a wide-band ,Band-pass filter(BPF) to limit the noise andinterference.After this passed on to a wide band receiver which provides a frequencydown conversion common to all channels. The wide band receiver also provides lownoise amplification needed at the input to maintain a satisfactory signal to noiseratio.The output frequency range is 3.7 to 4.2 G.Hz which is the down link frequencyband. An input demultiplexer following the wideband receiver is an arrangement ofMicrowave circulators and filters that separates the 500 M.Hz band into the separatetransponder channel bandwidth cahnnels. Following the demultiplexer ,power amplifiersare provided for the individual transponder channels which the power levels up to thoserequired for retransmission on the downlink. 9
  • 10. INTRODUCTION TO RADAR SYSTEMSRADAR FUNDAMENTALS : The term RADAR is an acronym for, Radio Detection, And Ranging.It refers to electronic equipment that detects the presence, direction, height,and distance of objects or targets by using reflected electromagnetic energy. TheRADAR works on the simple principle that “ Radio waves are sent towards an object( target)and the reflected wave (Echo) is received and analysed to get the informationabout the target. The frequency of electromagnetic energy used for radar isunaffected by darkness and weather. This permits radar systems to determine theposition of ships, planes, and land masses that are invisible to the naked eyebecause of distance, dark-ness, or weather. Most of the present day radars usewavelengths between 1 mm to 1m. Broadly speaking there are two types of Radarsystems.(i) Pulsed Radar System and (ii) CW Doppler Radar system. Any radar system has several subsystems that perform standard functions. Atypical radar system consists of (i) SYNCHRONIZER (ii) TRANSMITTER, (iii) DUPLEXER, (iv) RECEIVER each connected to a directional antenna. The synchronizer is also known as s the "heart" of the radar system because it controlsand provides timing for the operation of the entire system. The specific function of thesynchronizer is to produce TRIGGER PULSES that start the transmitter, indicator sweepcircuits, and ranging circuits.The TRANSMITTER produces the short duration high-power RF pulses of energy thatare radiated into space by the antenna towards Target.DUPLEXER Whenever a single antenna is used for both transmitting and receiving, problemsarise. Switching the antenna between the transmit and receive modes gives problems.The simplest solution is to use a switch to transfer the antenna connection from thereceiver to the transmitter during the transmitted pulse and back to the receiver during thereturn (echo) pulse. No practical mechanical switches are available that can open and 10
  • 11. close in a few microseconds. Therefore, ELECTRONIC SWITCHES must be used.Switching systems of this type are called DUPLEXERS.RECEIVER. The energy reflected from a target to the antenna in a radar system is a verysmall fraction of the original transmitted energy. The echoes return as pulses of RFenergy of the same nature as those sent out by the transmitter. However, the power of areturn pulse is measured in fractions of microwatts instead of in kilowatts, and thevoltage arriving at the antenna is in the range of microvolts instead of kilovolts. The radarreceiver collects those pulses and after analyzing the data gives the information likerange,direction and velocity etc.. of the target. Very often the receiving antenna is sameas that of transmitting antenna.Block diagram of the RADARFREQUENCIES USED IN RADAR : The frequencies lying above UHF and the microwave ranges are used inRADAR systems.The various frequency ranges and the maximum available peak powerand the frequency band name are given in the table 1.below.From the table it is clear that 11
  • 12. the frequencies ranging from 300M.Hz to 170G.Hz are used in RADAR systems.For thevarious ranges of frequencies different band names are given.S.No Band Name Frequency- Range G.Hz Maximum peak power MW 1 UHF 0.3-1.0 5.0 2 L 1.0 - 1.5 30.0 3 S 1.5-3.9 25.0 4 C 3.9-8.0 15.0 5 X 8.0-12.5 10.0 6 Ku 12.5-18.0 2.0 7 K 18.0-26.5 0.6 8 Ka 26.5-40.0 0.25 9 V 40.0-80.0 0.12 10 N 80.0-170 0.01Each frequency band has its own particular characteristics that make it better for certainapplications than for others.With a suitably large antenna, UHF is a good frequency for reliable long rangesurveillance radar, especially for extraterrestrial targets such as spacecraft and ballisticmissiles. L band is the preferred frequency band for land based long-range airsurveillance radars. S band is the preferred frequency band for long-range weatherradars that must make accurate estimates of rainfall rate. It is also a good frequency formedium-range air surveillance applications such as the airport surveillance radar.C-band frequency has been used for multifunction phased array air defense radars andfor medium-range weather radars.RADAR –RANGE EQUATION The Radar range equation is used to calculate the maximum range at which aRadar can detect a target.. To determine the maximum range of a Radar ,it is necessaryto determine the power of the received echoes, and to compare it with the minimumpower that the receiver can handle satisfactorily. If the peak value of transmitted pulsepower is Pt ,the power density at a distance r from the antenna is given by P = Pt / 4πr2 --------------------------(1)If Ap is the maximum power gain of the antenna usedfor transmission,the power densityat the target is given by 12
  • 13. P = Ap .Pt / 4πr2 (2)The power intercepted by the target depends on its Radar cross section or effectivearea..If this area is S ,the power hitting the target will be P = PS = Ap .Pt S / 4πr2 (3)Since the direction of the antenna id omnidirectional, the power density of its radiationat the receiving antenna will be P1 = P / 4πr2or P1 = Ap .Pt S / ( 4πr2 )2 (4)Similar to target, the receiving antenna also intercepts a part of the radiated power,whichis proportional to the cross-sectional area of the receiving antenna..But here we considerthe capture area of the receiving antenna..So,the received power is Pr = P1 A0 = Ap .Pt S A0 / ( 4πr2 )2 (5)Here the A0 is the capture area of the receiving antenna.Suppose the same antenna is used for both reception and transmission ,the maximumpower gain is given by Ap = 4π A0 / λ2 (6)Substituting (6) in the above equation (5) we get Pr = [4π A0 / λ2 ] Pt S A0 / ( 4πr2 )2 Pr = [4π A0 / λ2 ] Pt SA0 /16π2 r4 Pr = Pt SA02 /4π r4 λ2 (7)The maximum range r max is obtained when the received power is equal to the minimumreceivable power of the receiver, Pmin .Substituting this value in equation (7) and makingr as the Rmax ,we get Pmin = Pt SA02 /4π R4max λ2 13
  • 14. So, R4max = [Pt SA02 /4π Pmin λ2 ] Or Rmax = [Pt SA02 /4π Pmin λ2 ]1/4 (8)Substituting the value A0 = Apλ2 /4π in the above equation, we get Rmax = [Pt S λ2A2p /( 4π)3 .Pmin ]1/4 (9)Equations (8) and ( 9) are the two forms of the Radar-Range equations.As we haveconsidered all the ideal conditions in the above derivation ,the actual value will be lessthan the value given by the Radar –range equation.FACTORS INFLUENCING THE MAXIMUM RANGE Radar performance is affected by many factors. These conclusions can be madeform Radar-range equation.1. The maximum range of the Radar is proportional to the fourth root of the peaktransmitted pulse power. i.e for doubling the maximum range ,peak power must beincreased sixteen fold.2.A decrease in the minimum receivable power will increase the maximum range.3.Maximum range is proportional to the square root of the capture area of the antenna ordirectly proportional to its diameter if the wavelength is kept constant4. Atmospheric conditions also affect the performance of the Radar. For example,temperature inversion, moisture lapse, water droplets, and dust particles decrease theaccuracy of the Radar.5.The maximum range depends on the curvature of the earth.6.Noise also affects the performance of the RADAR. With increase of Noise in the medium ,there is a possibility of decrease in the maximum range of the Radar.APPLICATIONS OF RADAR Radar find wide spread applications in the different fields like Navigation, Overthe sea, on the ground and in space also. The applications can be classified into threegroups. 14
  • 15. (i) General applications(ii) Defence or military applications(iii) Scientific applicationsGeneral Applications1. Navigational aids using RADAR2.Weather forecasting3.Tracking the space craftsMilitary and defence applications4. Aiming at the enemy targets5.Detecting and obstructing the selected objects during nights6.Searching and aiming the submarines7. Assisting the fighter aircrafts8.In providing the proper guidance to missiliesScientific applications9. Study of planets and terrestrial space10. Applications in microwave spectroscopy.11.Tracking and guiding the space probes.LIMITATIONS :1. The CW Doppler Radar has a limitation in the maximum transmitted power .So it has a limitation on the maximum range.2. The presence of large number of Targets affects the performance of the CW Radar3. The Doppler Radar is incapable of indicating the range of the Target,it can only show only its velocity,ELECTROMAGNETIC SPECTRUM- MICROWAVE BANDS 15
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