Home > Article > Magnetron Based Radar Systems for Millimeter Wavelength Band Modern Approaches and Prospects

Magnetron Based Radar Systems for Millimeter Wavelength Band Modern Approaches and Prospects

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmenttherepetitieshould be high enough respectihich reduces unambiguous radar range or requiresdedicated techniwavelengths magnetrons demonstrated a low reliability making the result of theirtilization rather discouraging in the most cases Thus, appearance by the middle of 60thfficient power amplifiers based on both vacuum tubes and solid-state devices and grekpectations for a rapid progress in their development as well as the introduction a pulseression technique had given up the main the high performanent time, magnetrons are considered again as a rather attractive choice todevelop systems for millimeter wavelengths band namely This turn has become possibdue to: (1) a lack or low availability of other power devices operating within the indicatedfrncy range;(ii)a significant impredigital signal processinchnique;(iv)achievementsdevelopment of high voltage modulators andavelengths technique; and (v) a strong demand for millimeter wavelengths radars fromnon-military applications, which means a great interest in cost effective solutionstaken place inside it There is no aheory of magnetronsical simulatiobox characterizedoften by unpredictable and even surprising behavior Thus, generally developmentrequires profound understanding of principles of the magnetron operation, a greatnd the utilization of specific design approaches, at a system level partialHowever, often the magnetron is considered as old, well known device and respectivelare designed in aertainly

Probably, a rather uptight attitude to utilize theneurons to builddars is caused by the above reasothis paper we do not try to review comprehensively the current state of affairs or to coveras much as possible wide range ofoncerning the development of both millimeterwavelengths magnetrons and radars based on them Instead, relying on own experience weoted disadvantages preventing the magnetreutilization in the high performance radar systems are essentially weakened until nged successfully by gafrom the achievements of modernntemporary magnetron based radar systems operating withiore or less systally We hope that the bellow consideration will be helpful for radarkeep alin mind the possibilities providing by good old magnetron!ons in radars- brief overviewWe would not like to discuss here the physical principles of magnetron operation They canbe found in a variety of manuscripts (Okress, 1961: Tsimring, 2007) For us it is importantntechopen

Wavelength Band-Moden Approaches and ProspectsAntenna gain>50dBPolarization decoupling40daand noise25 km, dBc/ HzI Number of range binsVeight of Tx/ Rx units, kTable lll Parameters of Ka band meteorological radarsDoppler spectrum assuming that the contribution from other sources like a local oscillator isgligible Thus finally from formulas (1)and(2), the following expression for thegnetron pulse-to-pulse frequency instability readslues, namely, noise floor of -73 dBc/Hz,RF of 10 kHz, of 82 mm, and R of 55 km results in pulse-to-pulse frequency instabilityof abouth線邊3

Doppler spectrum from stationary target located at 55 km distance retrieved with Kaband meteorological radartype of a compact magnetron based Ka band radar is arbons related to enhancing helicopter flight safety including the detection of powerd other obstacles, monitoring meteorological conditions, and providing secureding But the achieved radar performhich can be used for other applications The radar benefits from some novel and cost-ntechopen

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentwaveguide antenna arrayystem The radar outline; a simplifiedFig 4

Outline (a), block diagram(b), and antenna pattern(c)of Ka band airborne scanningrrespondingly In the radar there is nele the radiatedpulse forprotection circuitry is used for this purp一間郵sential radar parameters are summarized in Table IvPeak transmitter power, kwull sensitivity, mAntenna section switchingcomposite wideband noise of Doppler processitTable IV Parameters of airborne Ka band scanning radarntechopen

Wavelength Band-Moden Approaches and Prospectsical w band radar has been developed inf RadioSciences of ukraproprietary magnetron with cold secondary emission cathode (see Tablal 20123: featured bytwo separate antennas;(ii)a separato sample theer quasi-optical polarization rotators both in Tx and Rx channels The radars are summarized in table vnoticed that the radar in question has been developon the radars operating within Ka band essentiallyOperating frequency, GHzI Peak power(max),kwuency

knamical range,dBHH,VV,HV,VHessed composite wideband noise of Doppler processingTable V Parameters of prototype of w band meteorological radarTherefore a seriprovement of its parameter may be expected due to: (i)utilization of aingle antenna due to increase in availability of high power circulators and P-i-N switchesfor the)an introduction of digital receiver technique as welldigital frequency control similarly to the above Ka band radars; (iii)theplifier, which have become available during resent time; (iv) the introductionsynthesized local oscillatthe introduction ofaL, 2002) As kidebrisery serious problemequirement conceptually for such radars In theof magnetron based radarappropriate resolution may be achieved in a rather simplthout the utilization ofpulse compression, The magnetron with cold-secondary emission cathode used in thetransmitter in question provides in addition an extended life time of 10000 heleastThe parameters of the transmitter are given in TableMeasured valuentechopen

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentRF Pulse Jitterrrent consumption, max 0 28VDimensions19,5U unitTable VI Parameters of W band short pulse magnetron transmitterIn the next section solesign approaches used in the above mentioned radars are4 Magnetron based radars- design approaches41 Generoperational mode is used;(i)each R etron based radar is featured as follows: ( a pulsedconceptually in locating thed signal as respect to the radiated one in a correspondingignal parameters depending of the radar measimplest case of non-coherent pulsed radar this space is two dimensional, withdinates of amplitude and time respectively For Dopplnal in the space of signal parameterattending the magnetron utilization in the radars require introduction of specific approachesto provide a precise location of radiated signal in such space and extend its dimensions, iesimplyparameters It is the only wayphase information,key issue to implement Doppler processing

Thus each magnetron based Doppler radarshould be provided withponding circuits to sample a small portion of radiated signalre its parameters like it is depicted in Fig 5 The magnetron oscillationequency is next important parameter, whose measurement accuracy affects strongly theoverall radar performance At first, it determines how precisely a target velocity can bemeasuredFig 5 Typical block-diagram of magnetron based radarntechopen

meterWavelength Band-Moden Approaches and ProspectsIt does not require a great accuracy and may be implemented relatively easily Practically ineviation does not exceed a portion of perothese-to-pulse frequency deviation This parameter introduces both non-coherent (noisend regponents(spurs)into Dognal processing (see Fig 3)In partdetermines the ability of the radar to resolve targets with different velociin the same range bin, eg clouds in a strong rain or a moving target in presence of a muchronger reflection from a cltshould be measured with accuracy of about 10 for a period of several hundrednanoseconds typicallyprovide 70 dB spectral dynamical range for Ka band radar and the distance of 5 km Thedicated accuracy is on the edge of contemporary technical capabilities or beyond them, notthe maximal possible Doppler performance is the responsibility for the radar circuits, whichluch as possible tight control of magnetron operational parametevoltage, filament, loading etc, and, finally, its frequency stability In the nearest future due todramatically fast progress in the development of data acquisitichardware we expect that precise measurements of the parameters of the radiated pulseto this possibility will be discussed later(see Section 4

35)Below in this section we will try to analyze requirements to high performance magnetron421GeonsiderationAs mentioned above the modern requirements to the radar performance cannot be meterwise than designing the magnetron environment testability and safety of its operation Therefore, the transmitter is probably the most valuablepart of either magnetron based high performance radar Before we will proceed to discusssome design approaches used in the transmitters, let us make a simple calculation in orderItscuits shouldrk, assume that theforementioned value of pulse-to-pulse frequency stability of/f of 10- should beThe variations of the amplitude of voltage pulse across magnetron should not exceed valuegiven by the following expressionhere fosc is a magnetron oscillatpushing factor, Ra-a dynamical resistance of the maggnetron and suthat the magnetron frequency pushing factor is of 500 kHz/a-atype, and a dynamical resistance of 300 Ohms a typical value for devices with 10-100 kwpeak power Then the above expression gives an impressive value of about 2 V, or less than200 ppm typically, for the required value of pulse-to-pagnetron anode voltage! Note, that the indicated value should be ensured during thentechopen

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentterval of data acculal the duration of this intervalNow, when a reference point for the magnetron transmitter design is indicated indiagram of a transmitter is depicted in Fig 6 It includes the following essential units: (i)ahigh voltage power supply;()a modulator; (iii)a filament power supply; and (iv)controller Let us leave the latter unit beyond a more detailed consideration, meding the procedures ensuring the most optimal and safeagnetron operational mode as well as provides the9 ControlerFig 6 Block-diagram of magnetron transmittertransmitter with remote control and diagnostics abilities Other above units affect directlywould like to outline their design in more deta422 High voltage pThe high voltage power supply determines essentially the short term magnetron frequencystability, ie

Doppler perfoof whole radar Thus ensuring its maximal stability isof the highest priority under the developmentdth modulatiocannot be alternatedinherent high efficiency, small dimensid light weight However the voltage stabilityprovided by such sutilization Our experience to develop the high voltage peased radars demonstrates a benefirules At first, PWmerter shouldclose to it mixRs,ode Such approach as well as the usage of a frequency compensated high voltage dividerg both rejection of the input voltage ripples and the overall stability of thetage regulation loop Next, it is mandatory to synchronize PWM converter at a frequencymultiple to the pulse repetition frequency of the radathe influenPWM operationalcy And at last, thetilization of a particular pitor is preferably In this respect, the usage of a powercoInFor information, the line of Ka band meteorological radar deDoppler performanSection O)is equipped with the highording strictly to theA flyback toPWM converter, From our opinion, such topology is the most suitable to the higntechopen

ms fometerWavelength Band-Moden Approaches and Prospectsapplications with the output power up to l kW and voltages up to 20 kV for AC poweredDC powered radars if an appropriate step-up pre-regulatis used The essential advance of such scheme is a stable operation with a capacitive loadtputas well as the ability to provide the output voltagvingwindings of the high voltage transformer much greater than aply voltage The above peculiarities meet perfectly actual operational conditions of thetron based transmitters Asbe easilFig 3 there is no regular spurious components caused by ripples of the output voltage ofhigh voltage power supply at the harmonics of both Ac power line frequency and theperational frequency of PWM converter(folded)high voltage modulators used in high performance radars In general the modulatorf rethe magnetron terminals Indevelopment Since the magnetron frequency depends strongly on the applied volta,adarsensitivity, Thus, both transients and the distortions of flat part of the pulse should benimized Especially it is important for the millimeter wavelengths magnetrons, whichharacterized by a rather short width of the output pulse, On other hand the most types ofrequires a well controllabltage rate during the leading edge of thedulation pulse to facilitate running oscillation(Kress, 1961) In this caseetter! An opposite situation appears for the trailing edge

As usual a less attentiondrawn to ensure its appropriately short duration However, not only shape of RF envelopeshould be taken into consideration there It is due to the magnetrons haNotice that at lower voltages theof back bombardment of the magnetron catholuch greater as respect to anodas indicated in Fig 7 Evidently, the shorter RFpulse duration and higher pulse repetition rate the stronger theffect affects thegnetron performance Thus the above issue should be always taken intFig 7 Waveforms of voltage pulsmetron and RF envelopentechopen

meterWavelength Band-Moden Approaches and Prospects46that the magnetron is a vcrossed field tuberadar transmitter As fairly noticed in(Skolnik, 2008), a choice of electronic device for thetransmitter end stage defines practicalletely radar structure and design approachesus outline the most important peculiarities of the magnetrons as related to theirtilization in the radars At first, the magnetrons are characterized fundamentally by a higlfor Ka band and 4izing constant frequenulse, being the simplest possible among radar signals, while keeping an appropriate radarphisticated signaln an artially that its output signalthe physicalave ratio at the output flange etc In addition it is practically impossible to manipulatethe magnetron output signal independentlycharacterizeda highly resonant design basicaloscillations frequency is essentially defined by electromagnetic properties of its internalpabilities providing by thepulse (etron Three types of modulation are used in modernparticular type of amplitude); fremodulation respectively, Practically it may be considered that the magnetron by itselfprovides no ability for a fast, highly reproducible, and well-controlled phase/frequencyelectrical frequency chirp provided by W and g band magnetrons may appeal tothegh resoldars(see Section O) Notice, since the magnetron output pulse is shapedency direct

ly, it occupies it twice wider frequency band than it is required toinite spatial resolutionAs for any other oscillator then oscillation fresubjected to fluctuationseferred as phase noise and frequency stability respectively

Concerning to radarce the first defines quality of Doppler processing wportant generally except a number of rather special cases Certainly, the total frequencyfrequencies should be ensured ever Usually the relatedand this fact is a byword to utilize such devices in the radars On otherhand the maximum possible variation of magnetron operational frequency includingo nufacturing tolerances is less than +1% over all millimeter wavelength bands inAs for the phaseeferred as a pulse-to-pulse frequency instability in the magnetron based systemng peculiarities of Doppler processor implementation, the followinned Prima facie itto be difficult actually expecting a high pulse-to-pulsefrequency stability for theQ-factorvely low even for coaxial devices especially within millimeter wavelengths band asGenerally, the behavior of electron cloud haserable noise component andntechopen

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentin a value of the pulse-to-pulse frequencybserved experimentally when even a single pulse altered theI auneetron cathodeharsperational conditions featured by a very high peak power dissipated on as well as a higholtage applied to the elements the magnetron comprises ofOn other hand, in general it is not easily to predict even very roughly what ultimate value ofuencymagnetron canpected for each definiteNumerical simulation is hardly useful as well as an independent directnt ofagnetron frequency stability meets also significant difficulties if achievement of a higlnd reproducible shape of the high voltage pulse across the magnetron For this reason, ashave found, despite the above seemingly evident factors, the magnetrons demonstrate aapproaches are utilized (see Section 0)Doppler radar systems, certainly if appropriate designall above can be summarized as follows: (i) magnetron based radars operate always in alow duty cycle operational condition; (iii)since each RF pulse ischaracterized by an arbitrary phase, a special procedure should be introduced in order toprovide Doppler processing capability for the radar; and (iv) especial attention should bece,As related to the latter two pointsshould be noticed that there is a possibility to lock the phase and frequencyoscillations with highly stable external oscillator Unfortunately, an extremelyprovided, ie relation between the output peak magnetron power and the required power ofing signal, especially for millimeter wavelengths frequency region prevent the abovtraditionally tousability in high performance radar systems It reflects essentially a state of affair existing inthe past, when the magnetron demonstrated actually rather lotd essentiallyby a limited cathode lifetime

It should be noticed that the magnetron cathode operatesvery high current densitiesmetron cathode is exposed strongly to electron back bombardment inherentfield devices(Okress, 1961)as far as it is sited insidhand, sucheffect results in increase in the emission capabilities of cathode greatly dueemission induced On other hand it cathode overheatingthode surface It is considered that the cathode dissipates about 10 %oeans that a peakhigh as several kiIt leadsgnetron filament power dependingalue of the anode current The problemuch induced overheating is not well controlled and depends on many parametersused in the magnetrons Dueally for millimeterfine internal layout inherent to them Our experience exposed that lifetime ofd with oxide cathodeshort as several hours only! Nentechopen

meterWavelength Band-Moden Approaches and Prospectsthe usage so called impregnated cathodes(Okress, 1961)in the magnetrons It has allowedeasIngHowever despite magnetrons equipped with such cathode keptbility to start oscillating stably within the abovequency drift even for coaxial magnetrons caused by evaporation of the substance, fromwhich the cathode was made of, with further absorption on the surface of magnetron cavityRelative failures to manufacturely reliabld by other reasons On other hand the millimeterices for military application exclusivand foapplications the achieved life time seems to beor less suitable all above have resultedthe development of magnetron and partially investigations in cathode manufacturinghave been curtailed worldwide excepting probably the former USSR The investigationcarried over there have given a new lease of life into the development of millimetelengths magnetron and allowedrovement of their characteristicsthe end of 80th

It has been achieved due to: (i) utilization of metallic alloy cathodes;successes in the development of the magnetrons with cold secondary ernd (iii)utilization of spatial harmonics different from i type The latter wins eletron interaction space and the cathode diameter, which resultsdraber as well as the maximal pulse duration In addition, thebalt magnet system has allowed reducing the magnetron dimensionsnd weight as well as developing rather miniature devices, The parameters of severalvelengths magnetrons developed with utilizationsummarized in TabFrequency band Katputduty0100065ode voltage,VHarmonicType of cathode Metallic alloy Cold with spikeold with auxiliary Metallic alloy (?autoemmitersthermionic cathodeNo information20200>1000Reached duringLiquiFull productionTable I Parameters of millimeter wavelengths magnetrontechopen

464Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentThe indicated values for lifetime of Ka band magnetrons have been obtained during thetested inf a particular procedure Achieved value of theally as outstanding! It shoulde considered reaching 4000 hours only in thebest case for Ka band magnetrons There weselection for Ka band magnetrons- eithertechnohould Be mentioned, whichkeeping a highhole utilization term in addition we dare to claim thatagnetron modulator helps significantly the magnetrons exposing its actualpotential by automating providing as mSince w band magnetrons in question are manufactured in one of branch of Institute ofRadieof National Academy of Sciences of Ukraine we would like to describeand production in somdevices had been developed in the middle of 60th (Usikov, 1972) Ab initio they utilizeperation at spatial harmonics different from i type Suchlent performance eg peak output power achieved was 80 kw and 10 kw formm wavelengths devices correspondingly However due to L type oxide cathodeused in the magnetrons, their life timlimited bof several tens hours only Indary emission cathode has been introduced inthe magnetron design

An auxiliary thermionic cathode placed aside an interaction spaceefficiency, they are much more promising to extend their lifetime As a result wband i kw dbeen developed and industrialized by the middle of 80th(Naumenkot al, 1999) It characterized by a guaranteed life time of 2000 hours By the end of 90thtial efforts has been coated to develop w band magnetrons with expected lifecle of 01 for meteorological radars and, a bit later, 1 kw devices with target life time of0000airport debris radar These efforts have resulted in a stableband magnetrons characterized by peak power within the range from 1000 to 4000 W andpected lifetime of 10000 hours (Gritsaenko et al, 2005of thisSuch fact can bederedclaimed to extend the magnetronlife timelesign of the auxiliary thermionic cathode: (ii) correct choiceoperational spatial harmonics; and (ii] equipping the magnetron with a magnetic-dischargelarge duty cycle, or a high pulse repetition rate(see Section 0) A photo of 4 kw W banagnetronof radio astrois depicted in Fig 1 During resenttime a low voltage, compact Ka band magnetrons withntechopen

Wavelength Band-Moden Approaches and Prospects1 4 kW w bandsecondary emission cathode and extended lifeevelopment They are intended to be used in low-cred the peculiarities of the magnetron utilization in the radars aswell as demonstrated that the life time is not an issue preventinghigh performance radars Now we would like to discuss further benefits and disadvantagesof such approach in a comparative manner

Table Il provides a brief comparison betweenhigh power millimeter wavelengths devices available at the time being in respect totheir possiblmillimeterbe found in( Barker et al, 2005)As usualssibility to introducesophisticated signal modulationioned as the essential benefit provided by using anmplifier in the radar transmitter Actually the utilization of a pulse compressionlows attaining the highest possible resolution for the radars, which is of centimenow Certainly, the magnetron based radars cannot provide a similar perfoHowever for the most applications an extreme resolution is not requiredompression is used only tosuitable radar sensitivitytubki(W)Ka band 20(100band4501501501002002)mplifier/n achievableow(single chip)pressionghsimilarTable Il Parameters of power microwave devicesntechopen

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentppression utilization, namely, appearancetrong reflectivity due to side-lobes of the autocorrelation function, should be always takenty to monitor simultaneously meteoroclouds, precipitation) with a reflectivity range of about 100 dB restrictssentially the application of pulse compression technique for weather radars Certainly thehardware relatihardware, suitablein mainstream commercial radars, has become to be allowableNext issue, mentioned usually to accent benefits of truly coherent systems based on utilizingmuch higher quality of Doppler processingwithin at least Ka band where coaxialavailable In addition furtherin the magnetronutilization of a sophisticated digital signal processing allexpect over and aboveA o provements in this area, especially for w band radarsg in the magnetron based radarsus to suggest retrievingpplications requiring the utilizature

Interestingly, the approach, basedthe radiated signal to provide ainherent to the mamprove proche influence of the distortionstroduced by Tx/Rx chains(Zhu, 2008)uly and pseudo coherent radarsProbably the only area the magnetron based radars cannot compete beyond any doubts withtruly coherent systems is radars requiring fast frequency agilityallows conceptually developing the op ovanced radarto utilization ofCertainly, within millimeter wavelength band such approach is on the technology edgerrently and requires enormous efforts to be implementedAll above mentioned allow us to declare that the achievallevel of theapplications, Theirrely low cost and complexity make theelengths, as for meteorological applications (1/i2 law for the reflectivity of meteorsApplications where dimensions and weight is among major requirements can be consideredof preferable utiliaf the magnetron based radars especially operatinghin w and g bane3 Magnetron based radar systems-development experienceDevelopment of the magnetron based radar systems had started in the Institute of RadioAstronomy of National Academy of Sciences of Ukraine since the middle of 90th of the lastentury By the time indicated we experienced withduring many years in thentechopen

meterWavelength Band-Moden Approaches and Prospects467nt and manufacturing of millimetre wavelengths magnetrons with coldclear thating circumstance for the magnetron usage did not meet modernts and didconsiderable efforts were made in order to develop advanced modulators to drive the abovemagnetrons, which require furthermore a tighter contnmodulation pulse shape ascompared to traditional types, Such situation has coincided with a growing interest inperating withthe course of above tendency the first magnetron based radaor has been developedther simple Ka and w band double frequencyairborne side looking system intended to detect oil spills on water surface Since this tasklengths shortening, the utilization of magnetrons hasevertheless it demonstratedpectedly good perfand was capable to detectes condition inherent for internObtained experience has allowed us to proceed with radar development and, naturally, thenext step was the introduction of Doppler processing capabilities in the radar(Schunemanet al, 2000) After the first usage of a traditional analogue coherence-on-receiyeteorological radar demonstrated a good Doppler performance, which was appropriatefor the most atmospheric researches as well as to monitor atmospheric conditions Infirst full functional Doppler polarimetricmitters in order toreliable unattended continuous operation for at least several months interval Theexperience of first year utilization of this radar has disclosed a surprisingly high stabilitythe magnetron operation

It has allowedh vertically pointed and scanningted in Fig 2 Until nown radars has been produced and deliveredwith metek gmbHhorn, Germany) Some of them are included into European weather radar networkntechopen

Microwave and Millimeter Wave Technologies: Modern UwB antennas and equipmentCoaxial magnetronsreceiver;a digital automatic frequency control; a digital receiver technique implementation;protection circuitry; introdof the circuits ensuringagnetron operation safety etc The essential parameters of most resummarized in lThe quality of Doppler processing provided bDopplerocated at the distancdepicted Signal processingrs were as follows: pulse repetition frequency -1Hz: fast fotransform length- 512; spectrum avlows from this figure, Doppler dynamical range exceeds 60 dBc, which corresponding tohe value of wideband noise floor of73 dBc/Hz from this data it is possible to estimfrequency instability

Actually, the total power of thebedered as a sum of coherentSkolnik, 2008), taking into account that the above data are product of a discrete Fouriertransform(DFT), and assuming that the magnetron introdtributed evenly instationary target, producing definitely monochromatic response, can we at wie ttered by aency domain, the ratio between the above components for the signal backscatNFL PRFconwhere nfl is theflcthe radar Doppler pfrequency of the radar On other hand, the phase lag of the signal reflected from a stationarytarget located at a fixed distance Rhere Aa is the deviation of theelengths for ith pulse from a constant value of do Assuming that Ai do, thecorresponding discrete time complex signal s at the input of DFT may be written as followswhere Po=4rR/, The second term in the above equation reflects the entity of incoherentponents in the received signal due to the magnetron pulse-to-pulse frequency instabilityOperating frequency, GHz355+015Peak transmitter power, kwAverage power (max),wLosses, dB, Tx pathdar instantaneous dynamical range includingntechopen