training course on weather radar systems - RTC, Regional Training ...
training course on weather radar systems - RTC, Regional Training ...
training course on weather radar systems - RTC, Regional Training ...
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TURKEY RADAR TRAINING 1.0 / ALANYA 2005TURKISH STATE METEOROLOGICAL SERVICE(TSMS)WORLD METEOROLOGICAL ORGANIZATION(WMO)COMMISSION FOR INSTRUMENTS AND METHODS OF OBSERVATIONS(CIMO)OPAG ON CAPACITY BUILDING (OPAG-CB)EXPERT TEAM ON TRAINING ACTIVITIES AND TRAINING MATERIALSTRAINING COURSE ONWEATHER RADAR SYSTEMSMODULE B: RADAR HARDWAREERCAN BÜYÜKBAŞ-Electr<strong>on</strong>ics EngineerOĞUZHAN ŞİRECİ -Electr<strong>on</strong>ics EngineerİSMAİL TEMİR -Mechanical EngineerELECTRONIC OBSERVING SYTEMS DIVISIONTURKISH STATE METEOROLOGICAL SERVICE12–16 SEPTEMBER 2005WMO RMTC-TURKEYALANYA FACILITIES, ANTALYA, TURKEY
MODULE B RADAR HARDWARERADAR HARDWARECONTENTS1. Operati<strong>on</strong> Principles of Weather Radar 31.1. General overview 31.2. Typical <strong>radar</strong> system applicati<strong>on</strong>s and block diagrams 62. Receiver 82.1. General overview 82.2. Oscillators 132.3. Low Noise Amplifier (LNA) 132.4. Duplexer&TR Limiter 142.5. Basic Characteristics of Receiver 162.5.1. Minimum Detectable Signal (MDS) 162.5.2. Dynamic Range 172.5.3. RX Gain 172.5.4. One dB (1 dB) Compressi<strong>on</strong> Point 183. Transmitter 193.1. General overview 193.2. Transmitter types 233.2.1. Magnetr<strong>on</strong> tubes 233.2.2. Klystr<strong>on</strong> tubes 243.2.3. Travelling Wave Tubes (TWTs) 253.2.4. Gyrotr<strong>on</strong>s 263.2.5. Solid State Power Amplifiers 273.3. Modulator 27TURKEY RADAR TRAINING 1.0 / ALANYA 20051
MODULE B RADAR HARDWARE4. Antenna 314.1. General overview 314.2. Antenna types 324.2.1. Phased array antenna 324.2.2. Parabolic reflectors 334.2.3. Offset feed reflectors 344.3. Basic Characteristics of Antenna 354.3.1. Antenna Gain 354.3.2. Antenna Pattern 364.4. Main parts of a <strong>radar</strong> antenna 384.4.1. Pedestal&reflector 394.4.2. Rotary joint 404.4.3. Slip ring 414.4.4. Waveguide and feed horn 414.4.5. Dual polarizati<strong>on</strong> units (in polarimetric <strong>radar</strong>s) 424.4.6. Antenna c<strong>on</strong>trol unit, servo motors, encoder 465. Radome 505.1. General overview 505.2. Radome types 516. Radar C<strong>on</strong>trol Processor(RCP) and Radar Signal Processor(RSP) 526.1. General overview 526.2. C<strong>on</strong>trol processor 526.3. Signal processor 527. References 568. Video Show (Applicable during <str<strong>on</strong>g>training</str<strong>on</strong>g> <str<strong>on</strong>g>course</str<strong>on</strong>g> <strong>on</strong>ly)TURKEY RADAR TRAINING 1.0 / ALANYA 20052
MODULE B RADAR HARDWARE1. OPERATION PRINCIPLE OF WEATHER RADAR1.1. General OverviewFigure-1.1Primarily, the <strong>radar</strong> c<strong>on</strong>sists of a transmitter to generate microwave signal, an antenna tosend the signal out to space and to receive energy scattered (echoes) by targets around, areceiver to detect and process the received signals by means of processors and a displayto graphically present the signal in usable form.In figure 1-1 of <strong>radar</strong> with klystr<strong>on</strong> transmitter, an RF carrier signal is generated in thereceiver and fed to the klystr<strong>on</strong> amplifier. The transmitter generates a microwave pulse bymeans of pulse-modulated amplificati<strong>on</strong> of the carrier signal by the klystr<strong>on</strong>. Themicrowave pulse is routed through the duplexer and radiated by the antenna. During thistransmit phase the receiver is blocked by the T/R limiter of the duplexer which preventsleakage from the circulator of the duplexer to the highly sensitive receiver input stages.The antenna emits the transmitter pulse in a symmetrical pencil beam. The atmospherearound the <strong>radar</strong> is scanned by moving the antenna in azimuth and elevati<strong>on</strong>.TURKEY RADAR TRAINING 1.0 / ALANYA 20053
MODULE B RADAR HARDWAREFigure-1.2After the transmit pulse is terminated, the T/R limiter extinguishes and thus c<strong>on</strong>nects thereceiver via the circulator to the antenna. The receive phase starts and the receiver isacquiring the signals scattered by the targets. This phase lasts until the next pulse istransmitted.Due to its high sensitivity and its large dynamic range, the receiver is capable of thedetecti<strong>on</strong> of far clear-air echoes as well as str<strong>on</strong>g signals from close thunderstorms.After receipt of the echoes, processing units performs the tasks to c<strong>on</strong>vert those signals torequired products to display <strong>on</strong> indicators/displays.Main units of a meteorological <strong>radar</strong> system with their basic functi<strong>on</strong>s are shown in the<strong>radar</strong> block diagram below. The units compose the <strong>radar</strong> system and their functi<strong>on</strong>s will beexplained in following chapters.TURKEY RADAR TRAINING 1.0 / ALANYA 20054
MODULE B RADAR HARDWAREFigure-1.3TURKEY RADAR TRAINING 1.0 / ALANYA 20055
MODULE B RADAR HARDWARE1.2. Typical Radar System Applicati<strong>on</strong>s and Block DiagramsSingle Magnetr<strong>on</strong> Radar SystemBlock diagram of the RX for single Magnetr<strong>on</strong><strong>radar</strong> system is shown in Fig. Transmitted BurstRF is fed into the RX, so that digital phasedetecti<strong>on</strong> can be made with Burst IF (downc<strong>on</strong>verted in frequency) in the Signal Processor.Dual Magnetr<strong>on</strong> Radar SystemTwo identical RXs for Magnetr<strong>on</strong> <strong>radar</strong> areused. Interswitching is performed throughWaveguide Switch c<strong>on</strong>trolled by <strong>radar</strong> software.Figure-1.4Figure-1.6Single Klystr<strong>on</strong> Radar SystemBlock diagram of the RX for single Klystr<strong>on</strong> <strong>radar</strong>system is shown in Fig. Exciter TX RF is generatedin the RX and fed to Transmitter to be amplified byKlystr<strong>on</strong>. Reference COHO (having same phase asTX RF) is output for digital phase detecti<strong>on</strong>processed in the Signal Processor.Dual Klystr<strong>on</strong> Radar SystemTwo identical RXs for Klystr<strong>on</strong> <strong>radar</strong> are used.Interswitching is performed through WaveguideSwitch c<strong>on</strong>trolled by <strong>radar</strong> softwareFigure-1.5Figure-1.7TURKEY RADAR TRAINING 1.0 / ALANYA 20056
MODULE B RADAR HARDWAREDual-Polarizati<strong>on</strong> Magnetr<strong>on</strong> Radar SystemSince observati<strong>on</strong> is performed in both horiz<strong>on</strong>taland vertical polarizati<strong>on</strong> simultaneously, tworecepti<strong>on</strong> circuits are composed in the RX.Higher phase stability is required for the RX usedin Dual-Polarizati<strong>on</strong> system since it should becapable to process Differential Phase betweenhoriz<strong>on</strong>tal and vertical channel for successfultargets. Thus, two STALOs (for eachhoriz<strong>on</strong>tal/vertical channel) are synchr<strong>on</strong>ized to thereference COHO. COHO is also fed to IF Digitizerso that the digitizing clock is madesynchr<strong>on</strong>izati<strong>on</strong>.Dual-Polarizati<strong>on</strong> Klystr<strong>on</strong> Radar SystemThe system c<strong>on</strong>figurati<strong>on</strong> is same as of Dual-Polarizati<strong>on</strong> magnetr<strong>on</strong> <strong>radar</strong> system except aKlystr<strong>on</strong> transmitter is used insteadFigure-1.9Figure-1.8TURKEY RADAR TRAINING 1.0 / ALANYA 20057
MODULE B RADAR HARDWARE2. RECEIVER2.1. General OverviewA pulsed <strong>radar</strong> sends out a high-power signal for a brief durati<strong>on</strong> (typically a fewmicrosec<strong>on</strong>ds) and then waits for the echoes of the signal from the targets around the <strong>radar</strong>to reach the antenna. After waiting for sufficient time (typically a few millisec<strong>on</strong>ds), thenext pulse is sent out. As the signal pulse travels at the speed of light, it does not take morethan a few millisec<strong>on</strong>ds for the signal to cover several hundred kilometres and get back tothe <strong>radar</strong> with a wealth of informati<strong>on</strong>. Modern <strong>radar</strong> receivers are mostly solid-state superheterodynetype, in which the received energy is mixed with a reference signal of differentfrequency for scaling it down in frequency for ease of processing. After down c<strong>on</strong>versi<strong>on</strong>,the informati<strong>on</strong> c<strong>on</strong>tained therein is filtered out and sent for further processing and display.Echoes of meteorological relevance span a wide range of the power spectrum say, from –110 dBm to about 0 dBm. A single receiver with such high dynamic range (the differencebetween the weakest and str<strong>on</strong>gest power that it can handle) was difficult to design. Tillrecently, two receivers, <strong>on</strong>e with logarithmic output to handle higher power and anotherwith linear output to handle weaker power were used. However, <strong>radar</strong>s with single linearreceiver and dynamic range higher than 100 dB have now become available in DigitalReceivers.TURKEY RADAR TRAINING 1.0 / ALANYA 20058
MODULE B RADAR HARDWARESome pictures of sub-comp<strong>on</strong>ents of receiversFigure-2.1TURKEY RADAR TRAINING 1.0 / ALANYA 20059
MODULE B RADAR HARDWAREThe receiver includes the RF fr<strong>on</strong>t end, the down c<strong>on</strong>versi<strong>on</strong> and up c<strong>on</strong>versi<strong>on</strong> chains, theSTALO and COHO secti<strong>on</strong>s as well as the IF secti<strong>on</strong> with LOG channel and coherent LINchannel in Analogue Receivers.The RF fr<strong>on</strong>t end is directly c<strong>on</strong>nected to the duplexer. It preamplifies the received scatteredsignals by means of a low-noise amplifier and down c<strong>on</strong>verts them by mixing them with theStable Local Oscillator (STALO) to the intermediate frequency (IF).The generati<strong>on</strong> of transmissi<strong>on</strong> frequency occurs through multiplicati<strong>on</strong>s, c<strong>on</strong>versi<strong>on</strong> andfiltering of quartz oscillators signals in order to obtain very low phase noise figure and hencehigh Doppler performances.The phase coherent COHO output signal serves as reference for the I/O phase demodulatorwhich generates the coherent I and Q video signals which are fed to the signal processor.In the IF secti<strong>on</strong> of an Analogue Receiver the signal is matched filtered and splitted into theLOG and LIN channel. The LOG channel comprises a successive detecti<strong>on</strong> logarithmic videoamplifier which compresses the signal and feeds it to the signal processor.Figure-2.2TURKEY RADAR TRAINING 1.0 / ALANYA 200510
MODULE B RADAR HARDWAREDigital receiver provides better performance (wide dynamic range, improved phase stability,etc) and higher system availability. The digital approach replaces virtually all of the traditi<strong>on</strong>alIF receiver comp<strong>on</strong>ents with flexible software-c<strong>on</strong>trolled modules that can be easily optimizedto the operati<strong>on</strong>al requirements.The functi<strong>on</strong> of the RX in a Digital Receiver is to amplify the detected RF signal by Low NoiseAmplifier (LNA), to c<strong>on</strong>vert to IF signal with a Stable Local Oscillator (STALO), and to send itto the IF Digitizer for Digital Phase Detecti<strong>on</strong>.For the RX used in a Klystr<strong>on</strong> system, there is an another functi<strong>on</strong> that to send the exciter TXRF signal to the Transmitter (TX), so that the TX transmits coherent RF which the phase isalways identical. The Exciter TX RF is generated from the COHO, and is Pulse/Phasemodulated and frequency up-c<strong>on</strong>verted with the STALO.For a Magnetr<strong>on</strong> system, the RX has the functi<strong>on</strong> to down-c<strong>on</strong>vert the Burst RF (transmittedpulse) frequency, so that the <strong>radar</strong> signal processor can perform the burst pulse analysis toobtain the amplitude, frequency and phase of the transmitted pulse.Figure-2.3A basic block diagram of a receiver for a klystr<strong>on</strong> <strong>radar</strong> system is given above.TURKEY RADAR TRAINING 1.0 / ALANYA 200511
MODULE B RADAR HARDWAREA basic block diagram of a receiver for a magnetr<strong>on</strong> <strong>radar</strong> system is given belowFigure-2.4Figure-2.5Inside of a Receiver Hardware (IF and RF Secti<strong>on</strong>s)Sec<strong>on</strong>d trip echoes can be a serious problem for applicati<strong>on</strong>s that require operati<strong>on</strong> at a highPRF. Sec<strong>on</strong>d trip echoes can appear separately or can be overlaid <strong>on</strong> first trip echoes. In theDoppler mode out-of-trip echo dealiasing and recovery is achieved by a pulse-to-pulse phaseTURKEY RADAR TRAINING 1.0 / ALANYA 200512
MODULE B RADAR HARDWAREshifting of the TX signal. The COHO is opti<strong>on</strong>ally interpulse phase-shift keyed to provide ameans for the <strong>radar</strong> signal processor to distinguish between different transmit pulses forsec<strong>on</strong>d-trip signal recovery purposes.2.2. OscillatorsIt is very important that the TX/ RX IF difference frequency and phase relati<strong>on</strong>ship bemaintained within such <strong>systems</strong>. Where that is achieved the system is referred to as beingCoherent. Coherent <strong>systems</strong> employs a coherent oscillator (COHO) and a series of stages thatrelate TX and LO frequencies. A stable local oscillator (STALO) is also used that determinesthe IF frequencies and, therefore, helps to maintain coherence.Lets look at coherent <strong>on</strong> receive. To be coherent <strong>on</strong> receive <strong>on</strong>e must have a COHerentOscillator (COHO). The COHO c<strong>on</strong>tains a stable phase and frequency relative to themagnetr<strong>on</strong>‘s output. The COHO signal is used to detect the I and Q elements of the receivedtargets for both <strong>weather</strong> and clutter relative to Transmitted Frequency.So, a permanent, clean RF carrier signal is generated by means of stabilized quartz oscillators,which are upc<strong>on</strong>verted by mixing and multiplicati<strong>on</strong> processes to form the signal amplified bythe transmitter and to serve as local oscillators for the receiver down-c<strong>on</strong>versi<strong>on</strong> chain.2.3. Low Noise Amplifier (LNA)LNA is a module to amplify weak signal received and sent from Antenna system with LowNoise Amplifier device (LNA). The LNA is a sensitive device which needs a protective circuitto prevent excess RF input which may leak from Transmitter. A TR-Limiter is normally usedfor the protecti<strong>on</strong>. The RF Input terminal shall be c<strong>on</strong>nected to the TR-Limiter output, directly.TURKEY RADAR TRAINING 1.0 / ALANYA 200513
MODULE B RADAR HARDWAREFigure-2.62.4. Duplexer&TR-LimiterA duplexer, sometimes called as Transmit/receive switch, is a device that isolates the receiverfrom the transmitter while permitting them to share a comm<strong>on</strong> antenna. The duplexer is thedevice that allows a single antenna to serve both the transmitter and the receiver. Ontransmissi<strong>on</strong> it must protect the receiver from burnout or damage, and <strong>on</strong> recepti<strong>on</strong> it mustchannel the echo signal to the receiver. Duplexers, especially for high-power applicati<strong>on</strong>s,sometimes employ a form of gas-discharge device,i.e. TR tube. Solid-state devices are alsoutilized.A duplexer must be designed for operati<strong>on</strong> in the frequency band used by the receiver andtransmitter, and must be capable of handling the output power of the transmitter. Furthermore, aduplexer must provide adequate rejecti<strong>on</strong> of transmitter noise occurring at the receivefrequency, and must be designed to operate at, or less than, the frequency separati<strong>on</strong> betweenthe transmitter and receiver.TURKEY RADAR TRAINING 1.0 / ALANYA 200514
MODULE B RADAR HARDWAREFigure-2.7TR Limiters (Transmit-Receive Limiters) are used in antenna between the high powertransmitter and the receiver. The TR tube, as a gas-discharge device designed to break downand i<strong>on</strong>ize quickly at the <strong>on</strong>set of high RF power, and to dei<strong>on</strong>ize quickly <strong>on</strong>ce the power isremoved, is the most comm<strong>on</strong> receiver protector technology in use today. The c<strong>on</strong>structi<strong>on</strong>of the TR tube includes <strong>on</strong>e or more res<strong>on</strong>ant filter secti<strong>on</strong>s in a piece of waveguide whichis sealed at both ends with waveguide windows. Each filter secti<strong>on</strong> is a relatively high Qparallel L-C circuit. Truncated c<strong>on</strong>es form the capacitive element and irises or post theinductive element. One comm<strong>on</strong> c<strong>on</strong>structi<strong>on</strong> of a TR c<strong>on</strong>sists of a secti<strong>on</strong> of waveguidec<strong>on</strong>taining <strong>on</strong>e or more res<strong>on</strong>ant filters two glass-to-metal windows to seal in the gas at lowpressure. A noble gas like arg<strong>on</strong> in the TR tube has a low breakdown voltage, and offersgood receiver protecti<strong>on</strong> and relatively l<strong>on</strong>g life. Pure-arg<strong>on</strong>-filled tubes, however,have relatively l<strong>on</strong>g dei<strong>on</strong>izati<strong>on</strong> times and are not suitable for short-rangeapplicati<strong>on</strong>s. The dei<strong>on</strong>izati<strong>on</strong> process can be speeded up by the additi<strong>on</strong> of watervapour or a halogen. The life of TR tubes filled with a mixture of a noble gas (arg<strong>on</strong>) andTURKEY RADAR TRAINING 1.0 / ALANYA 200515
MODULE B RADAR HARDWAREa gas with high electr<strong>on</strong> affinity (water vapour) is less than the life of tubes filled with anoble gas <strong>on</strong>ly.TR-LimiterFigure-2.82.5. Basic Characteristics of a ReceiverThe quality and the performance of receivers depend <strong>on</strong> the some basic characteristics.Those receiver characteristics are explained briefly below:2.5.1. Minimum Detectable Signal (MDS)Due to the noise generated by the receiver itself, weak signals can not be identified. Thereis thus a bottom threshold <strong>on</strong> the ability to detect echoes. This is called the minimumdetectable signal (MDS) or Sensitivity (Smin). In other words in a receiver, it is thesmallest input signal that will a produce a detectable signal at the output. In <strong>radar</strong> terms, itis the minimal amount of back scattered energy that is required to produce a target <strong>on</strong> theTURKEY RADAR TRAINING 1.0 / ALANYA 200516
MODULE B RADAR HARDWARE<strong>radar</strong> screen. In other words, MDS is a measure of the <strong>radar</strong>'s sensitivity. If this is about 15dBZ for a signal at 100 km. This means that a storm weaker than 15 dBZ at 100 km, willnot be detected by the <strong>radar</strong>.2.5.2. Dynamic RangeThe range, from the minimum, which is at a level 3 dB above the amplifier's internallygenerated floor, to a maximum input signal level that a comp<strong>on</strong>ent can accept and amplifywithout distorti<strong>on</strong>.Dynamic Range = P 1dB - P MDSWhere:P MDS = Minimum detectable signal 3 dB above the noise floor.Typical Dynamic Range Curve a ReceiverFigure-2.92.5.3. Receiver (RX) GainWe can determine the RX gain by using the linear part of the dynamic range curve.RX gain can be calculated by getting the difference between output and input value at anypoint of that linear part of the curve.At the figure-2.9 of dynamic range;TURKEY RADAR TRAINING 1.0 / ALANYA 200517
MODULE B RADAR HARDWAREOur output is -45 dBm while our input is -80 dBm.So our RX gain= -45 dBm-(-80dBm)= 35 dB2.5.4. One dB (1 dB) Compressi<strong>on</strong> PointThe 1 dB compressi<strong>on</strong> point is the point <strong>on</strong> a Pout vs. Pin graph, where an increase powerinput causes the measured gain to decrease from the linear gain by <strong>on</strong>e dB. Typically, if notexplicitly stated, the 1 dB compressi<strong>on</strong> point refers to the output power (Pout) at that point.Figure-2.10TURKEY RADAR TRAINING 1.0 / ALANYA 200518
MODULE B RADAR HARDWARE3. TRANSMITTER3.1. General OverviewTransmitter is <strong>on</strong>e of the essential comp<strong>on</strong>ents of the <strong>radar</strong> system with a functi<strong>on</strong> ofgenerating high power RF signal which will travel through the atmosphere and echoes ofthat signal will be received by the antenna after reflected by targets.Transmitter unit of <strong>radar</strong> <strong>systems</strong> c<strong>on</strong>sists of microwave tubes (or solid state devices), highvoltage power supplies and transformers, voltage regulators, filters, processors and c<strong>on</strong>trolcircuits, blowers, modulators and RF circuits. Following figure-3.1 shows the general blockdiagram of a transmitter with klystr<strong>on</strong> tube.Figure-3.1A lot of types of transmitters are available for <strong>radar</strong>s depending <strong>on</strong> type of the <strong>radar</strong>s andthe applicati<strong>on</strong>s. Within the frequency range of interest, two technologies (vacuumand solid-state electr<strong>on</strong>ics) are used to generate and amplify microwaves. Each offersadvantages for specific applicati<strong>on</strong>s within the performance domain of radio frequencyTURKEY RADAR TRAINING 1.0 / ALANYA 200519
MODULE B RADAR HARDWARE(RF) <strong>systems</strong> (see Figure-3.2 ). Microwave power tubes (the principal product derivedfrom RF vacuum electr<strong>on</strong>ics) are preferred for applicati<strong>on</strong>s requiring both higherfrequency and higher power. Electr<strong>on</strong> transport in a vacuum c<strong>on</strong>veys as advantages tomicrowave power tubes such features as wide band performance, efficiency, thermalrobustness, and radiati<strong>on</strong> hardness. Alternatively, solid-state power amplifiers combinethe power from many transistors. The advantages of charge transport in a solid-statemedia yield compact devices with superior reliability, and competitive efficiency andbandwidth at lower frequency and power. Solid-state power amplifiers transmit/receive(T/R) modules, and active arrays use a variety of power combining techniques to providecompetitive total power.C<strong>on</strong>venti<strong>on</strong>al high-power-pulse <strong>radar</strong> transmitters (rated from 0.25 to 10 MW peak power)are the largest, heaviest, and most costly porti<strong>on</strong>s of a <strong>radar</strong> system. They require a lot ofprime power and need a lot of cooling. The design of a transmitter is str<strong>on</strong>gly affected bythe type of RF transmitting source selected; this in turn is governed by what frequencyneeds to be transmitted or whether the transmitter is to operate <strong>on</strong> a single spot frequency orbe completely frequency agile. That is, the frequency for each transmitted pulse deviatesfrom the mean carrier frequency.Magnetr<strong>on</strong>s, klystr<strong>on</strong>s and travelling wave tubes still c<strong>on</strong>tinue to be the main powerhouseof most <strong>radar</strong>s. They are used in <strong>radar</strong> transmitters to produce microwave energy ofrequired wavelength and intensity. Though solid-state transmitters have becomeavailable, they have not overtaken the age-old vacuum tube technology so far due to theirlimited power. Gyrotr<strong>on</strong>s are newly emerging microwave power tubes, currently withlimited applicati<strong>on</strong>s. Classificati<strong>on</strong> and characteristics of microwave tubes are shownbelow:TURKEY RADAR TRAINING 1.0 / ALANYA 200520
MODULE B RADAR HARDWAREFigure-3.2Figure 3.2 summarizes the various types of microwave power tubes in use today.TURKEY RADAR TRAINING 1.0 / ALANYA 200521
MODULE B RADAR HARDWAREFigure-3.3Figure 3.3 summarizes microwave power tube operating characteristics and applicati<strong>on</strong>s.TURKEY RADAR TRAINING 1.0 / ALANYA 200522
MODULE B RADAR HARDWARE3.2. Transmitter Types3.2.1. Magnetr<strong>on</strong> TubesThe magnetr<strong>on</strong> tube for generating microwaves was invented by John Randall andHenry Boot in November 1939 (Buderi, 1996). Randall and Boot were working <strong>on</strong> thedevelopment of <strong>radar</strong> for the British. Magnetr<strong>on</strong>s proved to be <strong>on</strong>e of the most importantdevelopments of <strong>radar</strong> for World War II. Their small size and high power outputmade them ideal for airborne use. Magnetr<strong>on</strong> transmitters are now used at a variety of<strong>radar</strong> frequencies and can generate transmitted signals in excess of 500 kW.Magnetr<strong>on</strong>s are oscillators and usually operate in a pulsed mode. They are usedextensively in commercial airliner <strong>weather</strong> and collisi<strong>on</strong> avoidance <strong>radar</strong>, in almost allmilitary and commercial navigati<strong>on</strong> and <strong>weather</strong> <strong>radar</strong> <strong>systems</strong>, in some of the lessexpensivemedical therapy equipment, and in various industrial heating applicati<strong>on</strong>s,including the ubiquitous microwave ovens. Magnetr<strong>on</strong>s can generate a lot of power in asmall volume and are very efficient. They also are relatively inexpensive.Figure-3.4TURKEY RADAR TRAINING 1.0 / ALANYA 200523
MODULE B RADAR HARDWAREMagnetr<strong>on</strong> tubes are representative of an entirely different kind of tube than the klystr<strong>on</strong>.Whereas the latter tubes use a linear electr<strong>on</strong> beam, the magnetr<strong>on</strong> directs its electr<strong>on</strong> beamin a circular pattern by means of a str<strong>on</strong>g magnetic field:In operati<strong>on</strong>, as the electr<strong>on</strong>s are released from the cathode, they are directed toward theanode. However, since the electr<strong>on</strong>s are influenced by the magnets in the res<strong>on</strong>ant cavities,they flow circularly through the res<strong>on</strong>ant chambers and set up an oscillati<strong>on</strong> based up<strong>on</strong>their speed through the chambers. These res<strong>on</strong>ating chambers comprise the anode of themagnetr<strong>on</strong>. One or more chambers c<strong>on</strong>tain collecting antennas that let the generated RFsignal flow outside the tube.3.2.2. Klystr<strong>on</strong> TubesThe klystr<strong>on</strong> tube, as a high power amplifier, was invented at Stanford University in 1937and originally used as the oscillator in <strong>radar</strong> receivers during World War II by two of theresearchers instrumental, a pair of brothers named Sigurd and Russell Varian.The klystr<strong>on</strong> generally is referred to as a “linear beam” device because it utilizes amagnetic field in line with its electric field. The energy distributi<strong>on</strong> of the beam is alteredas it travels through a res<strong>on</strong>ant cavity. The res<strong>on</strong>ant cavity is excited by an input signalsuch that the beam is “velocity modulated” (some electr<strong>on</strong>s are slowed, some accelerated).Farther down the tube, the electr<strong>on</strong>s “bunch” as the faster electr<strong>on</strong>s overtakes the slowerelectr<strong>on</strong>s. As the bunched electr<strong>on</strong>s pass through another res<strong>on</strong>ant cavity, that cavity isexcited at the same frequency as the input cavity, but at a much higher power level. Thisamplifies the signal. Klystr<strong>on</strong>s range in length from less than a foot to over 20 feet andoperate with voltages ranging from a few hundred to several thousand. Output power canrange from 10 kilowatts (c<strong>on</strong>tinuous) to 10 megawatts (pulsed)..TURKEY RADAR TRAINING 1.0 / ALANYA 200524
MODULE B RADAR HARDWAREFigure-3.5Amplificati<strong>on</strong> klystr<strong>on</strong>s c<strong>on</strong>tinue to find use in high-power (Radar), high-frequency radiotransmitters and in scientific research applicati<strong>on</strong>s.Civilian agencies c<strong>on</strong>tinue to use klystr<strong>on</strong>s in many of their <strong>radar</strong> <strong>systems</strong>. For example,the Federal Aviati<strong>on</strong> Administrati<strong>on</strong>’s Terminal Doppler Weather Radar uses a klystr<strong>on</strong>amplifier because solid-state devices cannot generate the waveforms needed to detectsevere <strong>weather</strong> and wind-shear c<strong>on</strong>diti<strong>on</strong>s.Transmitter- Performance of the full-coherent Klystr<strong>on</strong> transmitter is extremely stablecompared to a Magnetr<strong>on</strong> transmitter, therefore, the system capability for "Precise WeatherMeasurement' is absolutely superior to <strong>systems</strong> using any other type of transmitters. Thetransmitter is designed solid-state based including the high-voltage modulator. Theradiati<strong>on</strong> sound is kept ultra low level so that maintenance can be performed easily andfriendly.3.2. 3. Travelling Wave Tubes (TWTs)The TWT is an important member of the linear beam family of microwave power tubes. Itis similar to the klystr<strong>on</strong> in that a pencil beam of electr<strong>on</strong>s is generated in an electr<strong>on</strong> gunand kinetic energy is c<strong>on</strong>verted into RF energy as the beam interacts with an RF circuit.Unlike the klystr<strong>on</strong>, the interacti<strong>on</strong> occurs c<strong>on</strong>tinuously as an increasing travelling waveinstead of discrete steps in cavities. The most comm<strong>on</strong> form of RF circuit used is a simplehelix. The TWT’s advantage is “bandwidth”. It can instantaneously amplify signals over arange of frequencies – several octaves. A wide bandwidth is necessary in applicati<strong>on</strong>s whereTURKEY RADAR TRAINING 1.0 / ALANYA 200525
MODULE B RADAR HARDWAREgood range-resoluti<strong>on</strong> is required or where it is desired to avoid deliberate jamming or mutualinterference with nearby <strong>radar</strong>s. Although low power TWTs are capable of octavebandwidths, bandwidths of the order of 10 to 20 percent are more typical at the power levelsrequired for l<strong>on</strong>g-range <strong>radar</strong> applicati<strong>on</strong>s. The gain, efficiency, and power levels of TWTs arelike those of the klystr<strong>on</strong>; but, in general, their values are usually slightly less than can beobtained with a klystr<strong>on</strong> of comparable design.Figure-3.63.2.4. Gyrotr<strong>on</strong>sA gyrotr<strong>on</strong>, a more recent development, looks like a linear beam device but performs like acombined linear beam and crossed-field device. It is large, heavy, expensive, and employsvery high voltages and high magnetic fields generated by superc<strong>on</strong>ducting solenoids.It can do what no other microwave power tube can do -- generate very high- power levels atmillimetre wavelengths. The gyrotr<strong>on</strong> promises important advances in fusi<strong>on</strong> and becauseof the high resoluti<strong>on</strong> possible with its millimetre wavelengths, great improvements in <strong>radar</strong>imaging.TURKEY RADAR TRAINING 1.0 / ALANYA 200526
MODULE B RADAR HARDWARE3.2.5. Solid-State Power AmplifiersSolid-state power amplifiers (SSPAs) use transistors, not vacuum electr<strong>on</strong>ic devices, toamplify microwaves. Performance depends up<strong>on</strong> the characteristics of the transistors usedand the efficiency of the combining technique. At lower frequencies and power, SSPAs areless expensive, more reliable, and inherently quieter than microwave power tubes. SSPAsfor microwave applicati<strong>on</strong>s are composed of many individual transistors c<strong>on</strong>nected inparallel, their output “summed” in a combining network to achieve power levels useful for<strong>radar</strong> or telecommunicati<strong>on</strong>s. Each type of transistor has specific limitati<strong>on</strong>s due toits material properties (defect densities, carrier mobility, thermal c<strong>on</strong>ductivity, etc.)which determine its useful operating frequency and power output. In transistors of a givenmaterial, the output power generally varies inversely with the square of the operatingfrequency.The assessments by some experts <strong>on</strong> that issue states that SSPAs so<strong>on</strong> will replacemicrowave power tubes in some applicati<strong>on</strong>s. This predicti<strong>on</strong> appears to be overstated.Microwave power tubes c<strong>on</strong>tinue to provide significantly higher power output andefficiencies than SSPAs, for mid- and higher-frequency applicati<strong>on</strong>s.Some researchers evaluated the current state of the technology and projected evoluti<strong>on</strong>aryimprovements in the power output of well known microwave SSPAs. It is c<strong>on</strong>cludedthat: (1) microwave solid-state amplifier capability would increase by a few decibels(dB) in the next decade, (2) the main solid-state progress would be in improved efficiencyand lower cost, and (3) the new solid-state devices being developed are not expected toimprove power performance in the mid- and higher- frequencies, where microwavepower tube devices are dominant. Therefore, the applicati<strong>on</strong>s being served today bymicrowave tubes will likely c<strong>on</strong>tinue without significant change, particularly for mid- andhigher-frequency applicati<strong>on</strong>s.3.6. ModulatorNo matter what kind of transmitter is used in the <strong>radar</strong>, it is usually c<strong>on</strong>trolled byanother electr<strong>on</strong>ic device called the modulator. The purpose of the modulator is toTURKEY RADAR TRAINING 1.0 / ALANYA 200527
MODULE B RADAR HARDWAREswitch the transmitter <strong>on</strong> and off and to provide the correct waveform for the transmittedpulse. That is, the modulator tells the transmitter when to transmit and for what durati<strong>on</strong>.The modulator also serves another functi<strong>on</strong>. It stores up energy between transmitterpulses so when it is time for the transmitter to fire, it will have a storehouse of energyavailable for its use.The Modulator generates high voltage pulse by switching high DC voltage from the HVREG. The switching devices are solid state. The solid state modulator represents the lateststate-of-the-art in high power solid state electr<strong>on</strong>ics. The modulator applies a high voltagepulse through the pulse transformer to the klystr<strong>on</strong> cathode. It is designed to support theinherent high stability of a coherent <strong>radar</strong>.The solid-state modulator comprises a capacitor charging power supply (CCPS), a chargingvoltage stabilizati<strong>on</strong>, a solid-state switch assembly, a capacitor bank and a pulsetransformer. The CCPS is basically a switch-mode power supply. It charges a capacitorbank. The voltage <strong>on</strong> the capacitor bank is c<strong>on</strong>trolled by means of a sensitive toppingelectr<strong>on</strong>ics resulting in a very high stability. This is mandatory for Doppler operati<strong>on</strong>.The high-power solid state switch assembly comprises the solid state switches as well astheir trigger and protecti<strong>on</strong> circuitry. As switches fast, high power semic<strong>on</strong>ductors areemployed. The selected switch modules provide sufficient margin in their criticalparameters to guarantee trouble-free operati<strong>on</strong> over the whole life time of the <strong>radar</strong> system.The solid state switch assembly is directly triggered from the <strong>radar</strong> signal processor. Thepulse width c<strong>on</strong>trol is provided by the transmitter c<strong>on</strong>trol processor. The pulse durati<strong>on</strong> isset by a pulse terminati<strong>on</strong> trigger. Two different pulse durati<strong>on</strong>s are pre-programmed.Similar to the capacitor charging SMPS the solid state switch assembly is current limited.Therefore if an arc occurs anywhere in the high voltage circuitry, an inherent protecti<strong>on</strong> isprovided for the solid state switches.TURKEY RADAR TRAINING 1.0 / ALANYA 200528
MODULE B RADAR HARDWAREFigure-3.7-Fr<strong>on</strong>t Panel of a TX CabinetFigure-3.10-M<strong>on</strong>itoring points from TXFigure-3.8-Some Klystr<strong>on</strong> IndicatorsFigure-3.11-Klystr<strong>on</strong> TankFigure-3.9-High Voltage Secti<strong>on</strong> of aTXTURKEY RADAR TRAINING 1.0 / ALANYA 200529
MODULE B RADAR HARDWAREFigure-3.14-Inside of a air isolated tankFigure-3.12-A Klystr<strong>on</strong> TubeFigure-3.15- A Klystr<strong>on</strong> TubeFigure-3.13-Modulator of a TransmitterFigure-3.16-Modulator of a TransmitterTURKEY RADAR TRAINING 1.0 / ALANYA 200530
MODULE B RADAR HARDWARE4. ANTENNA4.1. General OverviewA <strong>radar</strong> antenna (the British call it aerial) may be thought of as a coupling device betweenfree space propagati<strong>on</strong> and the waveguide from the transmitter. During transmissi<strong>on</strong>,energy from the transmitter is switched to the antenna by the duplexer, and the antennacauses the energy to radiate into free space. On recepti<strong>on</strong>, a signal in free space impinges<strong>on</strong> the antenna and is coupled to the transmissi<strong>on</strong> line that is c<strong>on</strong>nected to the duplexer,where it is delivered to the receiver. By reciprocity, the coupling to free space duringtransmissi<strong>on</strong> is identical with the coupling to the transmissi<strong>on</strong> line during recepti<strong>on</strong>. Thesetwo roles were expressed by the transmitting gain and the effective receiving aperturealias receiving gain. The main functi<strong>on</strong> of the antenna is to shape the transmitted beam so thatthe radiated energy is c<strong>on</strong>centrated in the desired directi<strong>on</strong> in space. The antenna is - so tospeak - the 'eye' of the <strong>radar</strong>.The antenna collimates the microwave energy into a narrow beam while sending it out.Larger the size of the antenna, better the angular resoluti<strong>on</strong>. The antenna generally rotatesabout a vertical axis scanning the atmosphere in azimuth. It is also capable of changing itselevati<strong>on</strong> by rotating about a horiz<strong>on</strong>tal axis so that probing of the hemisphericalvolume of the atmosphere with <strong>radar</strong> as the centre is possible, according to the scanningstrategy decided by the user.The antenna performance determines the update rate of <strong>radar</strong> images. In order to supportthe most advanced scan strategies, the antenna features the highest accelerati<strong>on</strong> rates andthe best step resp<strong>on</strong>se times available <strong>on</strong> the market.A <strong>radar</strong> receiver detects and often analyzes the faint echoes produced when <strong>radar</strong> wavesbounce off of distant objects and return to the <strong>radar</strong> system. The antenna gathers the weakreturning <strong>radar</strong> signals and c<strong>on</strong>verts them into an electric current. Because a <strong>radar</strong> antennamay both transmit and receive signals, the duplexer determines whether the antenna isc<strong>on</strong>nected to the receiver or the transmitter. The receiver determines whether the signalshould be reported and often does further analysis before sending the results to the display.TURKEY RADAR TRAINING 1.0 / ALANYA 200531
MODULE B RADAR HARDWAREThe display c<strong>on</strong>veys the results to the human operator through a visual display or anaudible signal.In general, it is expected <strong>radar</strong> antennas to have the following properties:• High forward gain or directive gain• Narrow beam width• Low sidelobe level• Broad-banding - so that the antenna can transmit and receive a <strong>radar</strong> pulse withoutdistorti<strong>on</strong> and pass all operating frequencies within the allocated frequency band• The beam must be scannedFigure-4.14.2. Antenna TypesThere are three major types of antenna used in <strong>radar</strong> applicati<strong>on</strong>s. These are phased array antennas,parabolic reflector antennas and offset feed antennas. Those types of antennas will be explainedbriefly.4.2.1. Phased Array AntennaEarly <strong>radar</strong> <strong>systems</strong> used antenna arrays formed by the combinati<strong>on</strong> of individual radiators.Such antennas date back to the turn of this century. Today, after a l<strong>on</strong>g pause, theTURKEY RADAR TRAINING 1.0 / ALANYA 200532
MODULE B RADAR HARDWAREtechnology is advanced. Better comp<strong>on</strong>ents and not least the advent of computers led todesigning well matched apertures.The phased array antenna has usually a planar aperture that is assembled from a great manysimilar radiating elements, such as slots or dipoles, each element being individuallyc<strong>on</strong>trolled in phase and amplitude. Accurately predictable radiati<strong>on</strong> patterns and beampointing directi<strong>on</strong>s can be achieved. Phased arrays allow scanning the beam electr<strong>on</strong>ically toa certain degree which makes the mechanical movement of antennas unnecessary.4.2.2. Parabolic reflectorsParabolic reflectors emit all radiati<strong>on</strong> at focus emerges in a beam parallel to the axis. These type ofantennas give a narrow beam which increases performance of <strong>weather</strong> <strong>radar</strong>s significantly. Parabolicreflectors are suitable mainly at microwave frequencies because it must be large compared with thewavelength. Most of the antennas used in <strong>weather</strong> <strong>radar</strong> applicati<strong>on</strong>s are parabolic reflectors.Figure-4.2TURKEY RADAR TRAINING 1.0 / ALANYA 200533
MODULE B RADAR HARDWARE4.2.3. Offset feed reflectorsAn offset-feed dish antenna has a reflector which is a secti<strong>on</strong> of a normal parabolicreflector as shown in Figure-4.3. If the secti<strong>on</strong> does not include the centre of the dish,then n<strong>on</strong>e of the radiated beam is blocked by the feed antenna and support structure.Otherwise, <strong>on</strong>ly a small bit at the edge of the beam is blocked. For small dishes, teedblockage in an axial-feed dish causes a significant loss in efficiency. Thus, we mightexpect an offset-feed dish to have higher efficiency than a c<strong>on</strong>venti<strong>on</strong>al dish of thesame aperture.Figure-4.3Figure-4.4-An Offset Feed AntennaTURKEY RADAR TRAINING 1.0 / ALANYA 200534
MODULE B RADAR HARDWARE4.3. Basic Characteristics of Antenna4.3.1. Antenna GainAn antenna which equally radiates in all directi<strong>on</strong>s is called an isotropic radiator or source,sometimes called a spherical radiator. There is no such antenna, because every antenna exhibitssome directive properties. However, the isotropic antenna is a c<strong>on</strong>venient reference point, andthus the gain or directivity of a given antenna is expressed as the increase in power radiated in agiven directi<strong>on</strong> compared to the power radiated by the fictitious isotropic antenna, assuming thesame total power in both cases accepted by the antenna. ThusG= Power gain=Maximum field strength radiated from given antennaField strength radiated from reference antennaThe gain is a ratio, i.e. a number that is usually c<strong>on</strong>verted into dB. Thus a transmitting antennawith a gain of 30 dB would radiate a signal in the directi<strong>on</strong> in which the signal is maximum 30dB greater than a signal from an isotropic or reference source which is fed with the sametransmitter.Direct measurements of the antenna gain can seldom be made and a gain figure is usuallyobtained by knowing the effective receiving aperture of the antenna or effective area A e . It maybe regarded as a measure of the effective area presented by the antenna to the incident wave.G =4πAg2λAll points <strong>on</strong> the antenna do not couple uniformly to the transmissi<strong>on</strong> line, so that the amount ofenergy reaching the receiver is reduced. The effective area of the antenna is therefore the area ofcross secti<strong>on</strong> which, when uniformly coupled to the transmissi<strong>on</strong> line, delivers the same energyas the actual antenna. The ratio of the two is called the efficiency of illuminati<strong>on</strong>, and isexpressed as K = A e / A , where A is the physical area.TURKEY RADAR TRAINING 1.0 / ALANYA 200535
MODULE B RADAR HARDWAREThe relati<strong>on</strong>ship between the gain and the beam width of an antenna depends <strong>on</strong> thedistributi<strong>on</strong> of current across the aperture. For a " typical " reflector antenna the followingexpressi<strong>on</strong> is sometimes used :G = 20000θ B Ø BWhere θ and Ø are the half-power beam widths, in degrees, measured in the two principalplanes. This is a rough rule of thumb that can be used when no other informati<strong>on</strong> isavailable, butt should not be a substitute for more exact expressi<strong>on</strong>s that account for theactual aperture illuminati<strong>on</strong>4.3.2. Antenna patternLarger diameter antenna provides more antenna gain which directly affects to the <strong>systems</strong>ensitivity. Another benefit of the bigger antenna is narrower beam width which is quiteimportant performance especially for such as the system used in airports. Radiated power isfocused into the pencil beam, so that the echoes have more precise informati<strong>on</strong> to bedetected. Since accurate <strong>weather</strong> informati<strong>on</strong> should be correctly delivered to the varioususers resp<strong>on</strong>sible for <strong>weather</strong> alert management, this "Precise Weather Measurement" is<strong>on</strong>e of the most important functi<strong>on</strong>s required to the modern Doppler Weather Radar.TURKEY RADAR TRAINING 1.0 / ALANYA 200536
MODULE B RADAR HARDWAREFigure-4.5Beam width of parabolic reflector at half power point is simply given byθ =70λDWhere θ is the beam width, λ is the wave length and D is the diameter of the antenna.In additi<strong>on</strong> to the main beam, antennas produce rays of energy called sidelobes,which surround the main beam (primary lobe) like haloes Sidelobes extend outward <strong>on</strong>lya short distance from the <strong>radar</strong> and c<strong>on</strong>tain very low power densities.Sidelobes are a direct result of diffracti<strong>on</strong> occurring near the edges of a <strong>radar</strong> antenna.However, even though they are weak, sidelobes can detect str<strong>on</strong>g n<strong>on</strong>-meteorologicaltargets near the <strong>radar</strong> and are also disturbed by nearby g-round reflecti<strong>on</strong>s. This leads toc<strong>on</strong>fusi<strong>on</strong> in interpreting close targets because sidelobe targets are displayed al<strong>on</strong>g with themain beam targets.TURKEY RADAR TRAINING 1.0 / ALANYA 200537
MODULE B RADAR HARDWAREFigure-4.64.4.Main parts of a <strong>radar</strong> antennaAntenna c<strong>on</strong>sists of microwave circuit, electric circuit and mechanical system. Themicrowave circuit is composed of a parabolic reflector and a feed horn which feeds theelectromagnetic wave to the parabolic reflector.The functi<strong>on</strong> of the microwave circuit is to radiate electromagnetic pulse wave from theTransmitter, to c<strong>on</strong>centrate the energy of radio frequency signals scattered due to rainfallparticles etc. and to transfer them to the Receiver via drive mechanisms. The Antenna alsoincludes rotary joints, which enable to transfer the radio frequency energy both waysbetween fixed waveguides and rotating Antenna.The electric circuit is comprised of drive motors, resolvers, and slip ring etc. Drive motorrotates the Antenna <strong>on</strong> both azimuth (AZ) and elevati<strong>on</strong> (EL). Resolvers detect rotati<strong>on</strong>angle <strong>on</strong> AZ and EL as an angle detector. Slip ring supplies power for AZ and EL axismotor between rotati<strong>on</strong> part and fixed part.It also transfers angle data from servo loop system. Resolver forms servo loop systemtogether with other comp<strong>on</strong>ents for accurate c<strong>on</strong>trol of rotati<strong>on</strong> speed and positi<strong>on</strong>ing of theAntenna.TURKEY RADAR TRAINING 1.0 / ALANYA 200538
MODULE B RADAR HARDWAREElectric circuit c<strong>on</strong>trols rotati<strong>on</strong> and positi<strong>on</strong>ing of the Antenna in azimuth and elevati<strong>on</strong> .Mechanical system is structured by driving power transmissi<strong>on</strong> system to rotate the Antenna.Figure-4.74.4.1. Pedestal&reflectorThe reflector generally c<strong>on</strong>sists of a partiti<strong>on</strong>ed solid surface dish with a stiffening backstructure.The pedestal carries the reflector and other equipment, which form the antenna, with acapability of c<strong>on</strong>tinuous rotati<strong>on</strong> in azimuth and elevati<strong>on</strong> scanning c<strong>on</strong>tinuously duringwhole operati<strong>on</strong> period..TURKEY RADAR TRAINING 1.0 / ALANYA 200539
MODULE B RADAR HARDWAREFigure-4.84.4.2. Rotary jointTwo rotary joints (<strong>on</strong>e each for AZ and EL) are located in the waveguide near the Antenna.Rotary joint performs the role to relay between fixed waveguide and the Antenna whichrotates and turns both <strong>on</strong> AZ and EL. As illustrated in Fig .1-2(a), coaxial line (upper part) <strong>on</strong>the output side and the <strong>on</strong>e (lower part) <strong>on</strong> the input side are electrically joined by means of achoke coupling. Therefore, the upper part can rotate while the lower part is fixed.Figure-4.9TURKEY RADAR TRAINING 1.0 / ALANYA 200540
MODULE B RADAR HARDWARE4.4.3. Slip ringThe Slip ring sends power and signals to the EL drive mechanism when a ring part <strong>on</strong> AZrotati<strong>on</strong> side and a brush installed in a brush holder <strong>on</strong> the fixed side slide. Both the ring partand the brush are divided for signal and for electric power, and the appearances are different.The brushes are installed double in order to prevent the generati<strong>on</strong> of electric arc.Figure-4.10Figure-4.114.4.4. Waveguide&Feed hornWaveguide is a transmissi<strong>on</strong> device that c<strong>on</strong>trols the propagati<strong>on</strong> of an electromagnetic waveso that the wave is forced to follow a path defined by the physical structure of the guide.Waveguides, which are useful chiefly at microwave frequencies in such applicati<strong>on</strong>s asc<strong>on</strong>necting the output amplifier of a <strong>radar</strong> set to its antenna, typically take the form ofrectangular hollow metal tubes but have also been built into integrated circuits. A waveguideof a given dimensi<strong>on</strong> will not propagate electromagnetic waves lower than a certain frequency(the cut-off frequency). Generally speaking, the electric and magnetic fields of anelectromagnetic wave have a number of possible arrangements when the wave is travellingthrough a waveguide. Each of these arrangements is known as a mode of propagati<strong>on</strong>.Waveguides also have some use at optical frequencies. The pyramidal horn is adopted to feedthe parabolic reflector with electromagnetic wave. Since TE10 mode, which is the main modeof a rectangular waveguide, is applied to the pyramidal horn and also because of optimizati<strong>on</strong>of the aperture, symmetrical cross secti<strong>on</strong> beam width is realized. Electromagnetic waves areradiated from Feed horn. They are sprayed to the parabolic reflector, where they are formedTURKEY RADAR TRAINING 1.0 / ALANYA 200541
MODULE B RADAR HARDWAREinto pencil beam usually less than 1 degree beam width and radiated into free space. Theinterface between feed horn and waveguide is shielded with a pressure window in order toprevent leakage of pressurized air within waveguide.Figure-4.124.4.5. Dual Polarizati<strong>on</strong> units (in polarimetric <strong>radar</strong>s)Recently, some Doppler <strong>weather</strong> <strong>radar</strong>s have been developed as Polarimetric System with acapability of detecting the Differential Reflectivity ZdR. In some cases, a dual polarizati<strong>on</strong>switch unit is used to make switching between the waveguides for vertically and horiz<strong>on</strong>tallypolarized waves.TURKEY RADAR TRAINING 1.0 / ALANYA 200542
MODULE B RADAR HARDWAREFigure-4.13The dual polarizati<strong>on</strong> switch is a special kind of waveguide comp<strong>on</strong>ent. It is used togetherwith a single receiver / transmitter to c<strong>on</strong>nect an antenna with horiz<strong>on</strong>tal and verticalwaveguide feed.This comp<strong>on</strong>ent is a very fast switching magnetic type. That makes it possible to switchbetween two normal pulses with a PRF up to 1200Hz. Mechanical switches are much slowerthan this magnetic type, so that the transmitter must not generate any pulses during theswitching time of a mechanical switch.The technical principle of this magnetic switch is similar to the circulator, which separates thetransmitted and received pulses. Also the dual polarizati<strong>on</strong> switch works as a four portcirculator. This is very important for the understanding of the operati<strong>on</strong> of the whole system.Figure-4.14Figure-4.15TURKEY RADAR TRAINING 1.0 / ALANYA 200543
MODULE B RADAR HARDWAREThis kind of circulator has the opportunity to change the directi<strong>on</strong> of the circulati<strong>on</strong> byelectrical driven magnets.To change the directi<strong>on</strong> of the circulati<strong>on</strong> does not mean to switch like a normalmechanical switch !If directi<strong>on</strong> A was chosen, the electromagnetic waves travel from port 1 to port 2, fromport 2 to port 3, from port 3 to port 4 and from port 4 to port 1.If directi<strong>on</strong> B was chosen, the waves travel in the other way : from port 1 to port 4, fromport 4 to port 3, from port 3 to port 2 and from port 2 to port 1.Port 3 is terminated with a dummy load due to the reas<strong>on</strong> that normally not all ports areneeded.Figure-4.16However, the most preferred method in Polarimetric Radar Systems is to dividetransmitted power and send to antenna via two separate circulator and collecting receivedsignal by two identical receivers. Signals sent are half powered for each channel here, butradiated simultaneously in c<strong>on</strong>trast to Dual Polarizati<strong>on</strong> Switched Type. This allows userto get different Polarimetric Radar Products.TURKEY RADAR TRAINING 1.0 / ALANYA 200544
MODULE B RADAR HARDWAREFigure-4.17Figure-4.18TURKEY RADAR TRAINING 1.0 / ALANYA 200545
MODULE B RADAR HARDWARE4.4.6. Antenna C<strong>on</strong>trol Unit, Servo Motors and Angle DetectorAntenna C<strong>on</strong>trol UnitAntenna c<strong>on</strong>trol unit (ACU) performs all functi<strong>on</strong>s regarding positi<strong>on</strong>ing, scanning,checking and safety c<strong>on</strong>trol of antenna. It also generates BITE informati<strong>on</strong> and send tothe <strong>radar</strong> c<strong>on</strong>trol processor. ACU reports the actual angle of the antenna dynamically toexternal devices. Resolver system, as an angle detector, is being used for AZ and EL axis,independently. ACU has another functi<strong>on</strong> for safety c<strong>on</strong>trol that, for example, the servo powercommanded from external Servo Processor is cut when any interlock system is activated. Thissafety c<strong>on</strong>trol is made with the status signals of ACU itself and those reported from ANT.Servo MotorsIt is generally required for an antenna used in a Weather Radar System to be capable ofscanning its narrow pencil beam in any directi<strong>on</strong>, and reporting accurate positi<strong>on</strong> towhere the Antenna is facing. So, Servo Motor- Driver system is preferred in bothAzimuth and Elevati<strong>on</strong> scanning c<strong>on</strong>trol.Servomechanism is an automatic device for the c<strong>on</strong>trol of a large power output by meansof small power input or for maintaining correct operating c<strong>on</strong>diti<strong>on</strong>s in a mechanism. Itis a type of feedback c<strong>on</strong>trol system. The c<strong>on</strong>stant speed c<strong>on</strong>trol system of a Servo Motoris a mechanism that m<strong>on</strong>itors any variati<strong>on</strong>s in the motor's speed so that it can quicklyand automatically return the speed to its correct value .Angle DetectorAngle detector, resolver system, detects rotati<strong>on</strong> angle <strong>on</strong> AZ and EL. Angle detectorforms servo loop system together with other comp<strong>on</strong>ents for accurate c<strong>on</strong>trol of rotati<strong>on</strong>speed and positi<strong>on</strong>ing of the Antenna.Angle detector forms servo loop system together with other comp<strong>on</strong>ents for accuratec<strong>on</strong>trol of rotati<strong>on</strong> speed and positi<strong>on</strong>ing of the Antenna.TURKEY RADAR TRAINING 1.0 / ALANYA 200546
MODULE B RADAR HARDWAREFigure-4.20-ServoMotorFigure-4.21-Angle DetectorFigure-4.19-C<strong>on</strong>trollerFigure-4.22-Servo AMPFigure-4.23-Antenna C<strong>on</strong>trol PCBsFigure-4.24.Antenna C<strong>on</strong>trol Unit andServo Amp.TURKEY RADAR TRAINING 1.0 / ALANYA 200547
MODULE B RADAR HARDWAREFigure-4.27-7m Parabolic AntennaFigure-4.25.WaveguideFigure-4.28-Cable and Waveguide TrayFigure-4.26.Positi<strong>on</strong> Resolver of AntennaFigure-4.29-TX Cabinet&DehydratorTURKEY RADAR TRAINING 1.0 / ALANYA 200548
MODULE B RADAR HARDWAREFigure-4.32-Antenna PedestalFigure-4.30-Another DehydratorFigure-4.31-Obstructi<strong>on</strong> LightsTURKEY RADAR TRAINING 1.0 / ALANYA 200549
MODULE B RADAR HARDWARE5. RADOME5.1. General OverviewAntennas of ground based <strong>radar</strong>s are often subject to severe <strong>weather</strong>. So some enclosure isneeded for antennas to survive and to perform under adverse <strong>weather</strong> c<strong>on</strong>diti<strong>on</strong>s. Theseenclosures are called as RADOM.A radome (<strong>radar</strong> dome) is a <strong>weather</strong>proof enclosure used to protect an antenna. It is usedmainly to prevent ice (especially freezing rain) from accumulating directly <strong>on</strong>to the metalsurface of the antenna. In the case of a spinning <strong>radar</strong> dish antenna, the radome also protectsfrom debris and rotati<strong>on</strong>al irregularities due to wind.For stati<strong>on</strong>ary antennas, excessive amounts of ice can de-tune the antenna to the pointwhere its impedance at the input frequency rises drastically, causing VSWR (VoltageStanding Wave Ratio) to rise as well. This reflected power goes back to the transmitter,where it can cause overheating. A foldback circuit activates to prevent this; however, itcauses the stati<strong>on</strong>'s output power to drop dramatically, reducing its range.A radome prevents this by covering the antenna's exposed parts with a sturdy, <strong>weather</strong>proofmaterial, typically fibreglass, which keeps the ice far enough away from the antenna toprevent any serious issues. A radome does however add to the wind load and the ice load, inadditi<strong>on</strong> to its own weight, and so must be planned for when c<strong>on</strong>sidering overall structuralload.For this reas<strong>on</strong>, and the fact that radomes may be unsightly if near the ground, heaters areoften used instead. Usually running <strong>on</strong> DC, the heaters do not interfere physically orelectrically with the AC of the radio transmissi<strong>on</strong>.Radomes must be mechanically str<strong>on</strong>g if they are to provide the necessary protecti<strong>on</strong> andthey must not more interfere with the normal operati<strong>on</strong> of the antenna as absolutelynecessary.A properly designed radome should distort the antenna pattern as little as possible. Thepresence of a radome can affect the gain, the beam width, sidelobe level, the polarisati<strong>on</strong>,and the directi<strong>on</strong> of the boresight, as well as change the VSWR (voltage-standing-waveTURKEY RADAR TRAINING 1.0 / ALANYA 200550
MODULE B RADAR HARDWAREratio) and the antenna noise temperature. In order to keep the affects as low as possible, thefollowing radome characteristics are specified:• One way transmissi<strong>on</strong> loss for the dry and wet state• Increase of sidelobe level• Reflected power• Cross polarisati<strong>on</strong> degradati<strong>on</strong>• Boresight error• Beam width increase.These parameters have a close relati<strong>on</strong> to the antenna that is covered, thus the radomeperformance can <strong>on</strong>ly be estimated <strong>on</strong> the basis of the antenna characteristics.There are several radome types available as far as the supporting structure and the wallmaterial is c<strong>on</strong>cerned. There are foam radomes and space-frame radomes. The latter <strong>on</strong>esuse different types of sandwich panels. Their properties may be adjusted to the antenna andlocal envir<strong>on</strong>ment.5.2. Radome TypesDue to its size the radome has to be subdivided into panels for producti<strong>on</strong> and transportati<strong>on</strong>reas<strong>on</strong>s. Out of an infinite number of possible soluti<strong>on</strong>s most of them can be assigned to <strong>on</strong>ethe following types :Igloo :The radome is subdivided into small regular pieces with either vertical or horiz<strong>on</strong>taljoints.Orange Peel: The radome is subdivided into relatively large vertical pieces with mostlyvertical joints.Quasi-Random: The radome is subdivided into a number of irregular pieces withoutpreferred joint directi<strong>on</strong>.Quasi Random Orange Peel IglooFigure-5.1TURKEY RADAR TRAINING 1.0 / ALANYA 200551
MODULE B RADAR HARDWARE6. RADAR CONTROL AND SIGNAL PROCESSORS6.1. General OverviewIn modern <strong>radar</strong>s, c<strong>on</strong>trol of the functi<strong>on</strong> of each sub-unit and signal processing are d<strong>on</strong>e bydedicated computers incorporated within the system. They are generally known as c<strong>on</strong>troland signal processors.6.2. C<strong>on</strong>trol ProcessorsThe <strong>radar</strong> c<strong>on</strong>trol processor is resp<strong>on</strong>sible for the c<strong>on</strong>trol and supervisi<strong>on</strong> of the <strong>radar</strong>system. In particular the Built-in Test Equipment (BITE) is c<strong>on</strong>trolled by this processor.The states of a large number of subsystem parameters are m<strong>on</strong>itored and if a fault isdetected the c<strong>on</strong>trol processor acts according to the severity. Additi<strong>on</strong>ally the c<strong>on</strong>trolprocessor interfaces the <strong>radar</strong> system to the data processing system.The signal processor computes the reflectivity (with and without correcti<strong>on</strong>s for clutter,sec<strong>on</strong>d trip echo, attenuati<strong>on</strong> and partial beam filling), mean velocity and spectrum width.Many commercially available off-the-shelf digital signal processors (DSPs) may belinked together to meet the computati<strong>on</strong>al requirement. The signal processor unit performsanalogue to digital c<strong>on</strong>versi<strong>on</strong>, quality assurance and applying various correcti<strong>on</strong>s to thedataset in additi<strong>on</strong> to performing complex statistical signal-processing jobs. Radar displaysare of different types. Plan positi<strong>on</strong> indicator (PPI), range height indicator (RHI)amplitude range scope (AR scope), c<strong>on</strong>stant altitude PPI (CAPPI), described later in the inthe other module of the <str<strong>on</strong>g>course</str<strong>on</strong>g>, Module D, are a few to menti<strong>on</strong>. Earlier <strong>radar</strong>s useddedicated cathode ray tubes to display the echoes. Modern <strong>radar</strong>s use computer displaysfor displaying their products.6.3. Signal ProcessorThe signal processor is a critical unit of <strong>radar</strong> system with important functi<strong>on</strong>s. Itperforms three main tasks: (1)the triggering of the <strong>radar</strong> system, especially of thetransmitter, (2) the phase and frequency modulati<strong>on</strong> of the carrier signal and (3) thedigitizing, polar coordinate tagging and pre-processing of the received signals.TURKEY RADAR TRAINING 1.0 / ALANYA 200552
MODULE B RADAR HARDWARESignal processor accepts analogue linear channel I and Q as well as a LOG receiver input.The LOG receiver input is <strong>on</strong>ly used for an intensity estimati<strong>on</strong> which is used to drive anattenuator which prevents a saturati<strong>on</strong> of the linear channels.Since the signal to be processed by signal processor is a complex linear signal, intensityestimati<strong>on</strong> c<strong>on</strong>sists simply of integrating the power in the linear channel (power = I 2 + Q 2 )over range and azimuth, as specified by the user. The resulting power estimate is correctedfor system noise (by subtracting the average system noise level), for target range (using anR-squared relati<strong>on</strong>ship), for atmospheric attenuati<strong>on</strong> (using an attenuati<strong>on</strong> factor suppliedby the user), and opti<strong>on</strong>ally for transmitter power.Figure-6.1TURKEY RADAR TRAINING 1.0 / ALANYA 200553
MODULE B RADAR HARDWAREFigure-6.2-Signal Processor CardFigure-6.3-Digitizer(before Signal Processing)Figure-6.4-A Radar C<strong>on</strong>trol ProcessorTURKEY RADAR TRAINING 1.0 / ALANYA 200554
MODULE B RADAR HARDWARESome View of Radar SystemsFigure-6.8-Scenic View of Ankara Radar-TurkeyFigure-6.6-Balıkesir Radar Site-TurkeyFigure-6.7-Hardware of Z<strong>on</strong>guldakRadar/TurkeyFigure-6.9-General view of Radar HardwareTURKEY RADAR TRAINING 1.0 / ALANYA 200555
MODULE B RADAR HARDWAREREFERENCES:1. Radar for Meteorologist, R<strong>on</strong>ald E. Rinehart August 19972. Radar Handbook, Merill I. Skolnik3. Doppler Radar and Weather Observati<strong>on</strong>s, Doviak R.J. & Zrnic D.S.4. Introducti<strong>on</strong> to Radar System, Merrill I. Skolnik5. Field and Wave Electromagnetics, David K. Cheng,19836. Weather Radar Calibrati<strong>on</strong>, R. Jeffrey Keeler January, 20017. Doppler Weather Radar System- Meteor 1000CUser Manuel and Documentati<strong>on</strong>-Gematr<strong>on</strong>ik GmbH12.July.20018. RC-57A Weather Radar <strong>Training</strong> Document and User Manuel- Mitsubishi ElectricCorp. 20029. Radome Influence <strong>on</strong> Weather Radar Systems, Principle and Calibrati<strong>on</strong> IssuesGematr<strong>on</strong>ik GmbH Alexander Manz10. Principles of Radar- Wolfgang Manz 12.March .199911. Radar Meteorology- Jürg Joss July.200412. Technical Descripti<strong>on</strong> TDR Series-C Band Doppler Radar, Radtec Engineering13. Radar Range Folding and The Doppler Dilemma, Jeff Haby14. Doppler Radar, A detecting tool and measuring instrument in meteorologyCurrent Science, Vol. 85, No. 3, 10 August 2003A.K. Bhatnagar, P. Rajesh Rao, S.Kalyanasundorom, S.B. Thampi, R. Suresh and J.P.Gupta15. Doppler Weather Radar System, Enterprise Electric Corp.16. Industrial Assessment of the Microwave Power Tube Industry, Department ofDefense, U.S.A. April 199717. Weather Watch Radar, BoM, Australia18. Radar Meteorology Doppler, Heikki Pohjoa, FMI19. Data Quality Improvements <strong>on</strong> AP Mitigati<strong>on</strong>, Range Velocity Mitigati<strong>on</strong>, Nati<strong>on</strong>alWeather Service, U.S.A20. Radar <strong>Training</strong> Informati<strong>on</strong>, NOAA21. Detecti<strong>on</strong> of ZDR abnormalities <strong>on</strong> operati<strong>on</strong>al polarimetric <strong>radar</strong> in Turkish<strong>weather</strong> <strong>radar</strong> network, WMO- TECO 2005, TSMS, Oguzhan Sireci, 4th.May.200522. Modernizati<strong>on</strong> of Observati<strong>on</strong> Network in Turkey, TECO 2005, TSMS, ErcanBuyukbas, 4th.May.200523. Radar Basics, Renato CrociTURKEY RADAR TRAINING 1.0 / ALANYA 200556
MODULE B RADAR HARDWARE24. Feasibility Report for Turkey Radar Network, BoM, Australia,200025. Weather Radar Principles, Firat Bestepe, TSMS, 200526. Principles of Meteorological Doppler Radar, Distance Learning Operati<strong>on</strong>s Course,Instructi<strong>on</strong>al Comp<strong>on</strong>ent 5.3. Ver: 030727. Notes <strong>on</strong> Radar Basics, Serkan Eminoglu, TSMS,200428. Radar Basics, Radar <strong>Training</strong> Informati<strong>on</strong>,NOAA29. Turkish Radar Network, Hardware Maintenance of Weather Radars, <strong>Training</strong>Notes, Ercan Büyükbas, Oguzhan Sireci, Aytac Hazer, Ismail Temir, Cihan Gozubuyuk,Abdurrahman Macit, M.Kemal Aydin, Mustafa Kocaman, 200230. Weather Radar Maintenance Procedures and Measurements, TSMS, Aytac Hazer,Cihan Gozubuyuk, 200531. Operati<strong>on</strong>al Use of Radar for Precipitati<strong>on</strong> Measurements in SwitzerlandJürg Joss(1)Bruno Schädler(2) Gianmario Galli(1) Remo Cavalli(1) Marco Boscacci(1)Edi Held(1) Guido Della runa(1) Giovanni Kappenberger(1) Vladislav Nespor(3)Roman Spiess(3) Locarno, 23.Sep.199732. Radar Lecture Notes and Articles available in internet33. Booklets, reports and guidelines published by WMO34. Technical Brochures of Radar ManufacturersTURKEY RADAR TRAINING 1.0 / ALANYA 200557