通信与电子信息工程专业英语Unit3 Expanding reading Communication Antenna.docx

通信与电子信息工程专业英语Unit3 Expanding reading Communication Antenna.docx

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通信与电子信息工程专业英语Unit3ExpandingreadingCommunicationAntenna

An antenna （or aerial）isanelectricaldevicewhichcouplesradiowavesinfreespacetoanelectricalcurrentusedbyaradioreceiverortransmitter.Inreception,theantennainterceptssomeofthepowerofan electromagneticwave inordertoproduceatinyvoltagethattheradio receiver canamplify.Alternatively,aradiotransmitterwillproducealarge radiofrequency currentthatmaybeappliedtotheterminalsofthesameantennainordertoconvertitintoanelectromagneticwave（radiowave）radiatedintofreespace.Antennasarethusessentialtotheoperationofall radio equipment,both transmitters and receivers.Theyareusedinsystemssuchas radio and television broadcasting, two-wayradio, wirelessLAN, mobiletelephony, radar,and satellitecommunications.

Typicallyanantennaconsistsofanarrangementofmetallic conductors （or"elements"）withanelectricalconnection（oftenthroughatransmissionline）tothereceiverortransmitter.Acurrentforcedthroughsuchaconductorbyaradio transmitter willcreateanalternatingmagneticfieldaccordingto Ampère'slaw.Orthealternatingmagneticfieldduetoadistantradiotransmitterwillinduceavoltageattheantennaterminals,accordingto Faraday'slaw,whichisconnectedtotheinputofa receiver.Intheso-called farfield,ataconsiderabledistanceawayfromtheantenna,theoscillatingmagneticfieldiscoupledwithasimilarlyoscillatingelectricfield;togetherthesedefinean electromagneticwave whichiscapableofpropagatinggreatdistances.

Lightisoneexampleofelectromagneticradiation,alongwithinfraredandx-rays,whileradiowavesdifferonlyintheirmuchlowerfrequency（andmuchlonger wavelength）.Electroniccircuitscanoperateattheselowerfrequencies,processingradiosignalsconductedthroughwires.Butitisonlythroughantennasthatthose radiofrequency electricalsignalsareconvertedto（andfrom）propagatingradiowaves.Dependingonthedesignoftheantenna,radiowavescanbesenttowardandreceivedfromalldirections（"omnidirectional"）,whereasadirectionalor beamantenna isdesignedtooperateinaparticulardirection.

Anantennatransmitsandreceivesradiowaveswithaparticular polarization whichcanbereorientedbytiltingtheaxisoftheantennainmany（butnotall）cases.Thephysicalsizeofanantennaisoftenapracticalissue,particularlyatlowerfrequencies（longerwavelengths）.Highlydirectionalantennasneedtobesignificantlylargerthanthewavelength.Resonantantennasusuallyusealinearconductor（or element）,orpairofsuchelements,eachofwhichisaboutaquarterofthewavelengthinlength（anoddmultipleofquarterwavelengthswillalsoberesonant）.Antennasthatarerequiredtobesmallcomparedtothewavelengthsacrificeefficiencyandcannotbeverydirectional.Fortunatelyathigherfrequencies（UHF,microwaves）tradingoffperformancetoobtainasmallerphysicalsizeisusuallynotrequired.

Whilethereare broadbanddesigns forantennas,thevastmajorityofantennasarebasedonthehalf-wave dipole whichhasaparticular resonantfrequency.Atitsresonantfrequency,the wavelength （givenbythe speedoflight dividedbytheresonantfrequency）isslightlyovertwicethelengthofthehalf-wavedipole（thusthename）.Thequarter-waveverticalantennaconsistsofonearmofahalf-wavedipole,withtheotherarmreplacedbyaconnectionto ground oranequivalent groundplane （or counterpoise）.A Yagi-Uda arrayconsistsofanumberofresonantdipoleelements,onlyoneofwhichisdirectlyconnectedtothetransmissionline.Thequarter-waveelementsofadipoleorverticalantennaimitateaseries-resonantelectricalelement,sinceiftheyaredrivenattheresonantfrequencya standingwave iscreatedwiththepeakcurrentatthefeedpointandthepeakvoltageatthefarend.

Acommonmisconceptionisthattheabilityofaresonantantennatotransmit（orreceive）failsatfrequenciesfarfromtheresonantfrequency.Thereasonadipoleantennaneedstobeusedattheresonantfrequencyhastodowiththe impedancematch betweentheantennaandthetransmitterorreceiver（anditstransmissionline）.Forinstance,adipoleusingafairlythinconductor[12] willhaveapurelyresistivefeedpointimpedanceofabout63ohmsatitsdesignfrequency.Feedingthatantennawithacurrentof1amperewillrequire63voltsofRF,andtheantennawillradiate63watts（ignoringlosses）ofradiofrequencypower.Ifthatantennaisdrivenwith1ampereatafrequency20%higher,itwillstillradiateasefficientlybutinordertodothatabout200voltswouldberequiredduetothechangeintheantenna'simpedancewhichisnowlargelyreactive（voltageoutofphasewiththecurrent）.Atypicaltransmitterwouldnotfindthatimpedanceacceptableandwoulddelivermuchlessthan63wattstoit;thetransmissionlinewouldbeoperatingatahigh（poor） standingwaveratio.Butusinganappropriatematchingnetwork,thatlargereactiveimpedancecouldbeconvertedtoaresistiveimpedancesatisfyingthetransmitterandacceptingtheavailablepowerofthetransmitter.

Thisprincipleisusedtoconstructverticalantennassubstantiallyshorterthanthe1/4wavelengthatwhichtheantennaisresonant.Byaddinganinductanceinserieswiththeverticalantenna（aso-called loadingcoil）thecapacitivereactanceofthisantennacanbecancelledleavingapureresistancewhichcanthenbematchedtothetransmissionline.Sometimestheresultingresonantfrequencyofsuchasystem（antennaplusmatchingnetwork）isdescribedusingtheconstructof"electricallength"andtheuseofashorterantennaatalowerfrequencythanitsresonantfrequencyistermed"electricallengthening".Forexample,at30 MHz（wavelength=10meters）atrueresonantmonopolewouldbealmost2.5meters（1/4wavelength）long,andusinganantennaonly1.5meterstallwouldrequiretheadditionofaloadingcoil.Thenitmaybesaidthatthecoilhas"lengthened"theantennatoachievean"electricallength"of2.5meters,thatis,1/4wavelengthat30 MHzwherethecombinedsystemnowresonates.However,theresultingresistiveimpedanceachievedwillbequiteabitlowerthantheimpedanceofaresonantmonopole,likelyrequiringfurtherimpedancematching.

Theantennaconductorshavethelowestfeed-pointimpedanceattheresonantfrequencywheretheyarejustunder1/4wavelengthlong;twosuchconductorsinlinefeddifferentiallythusrealizesthefamiliar"half-wavedipole".WhenfedwithanRFcurrentattheresonantfrequency,thequarterwaveelementcontainsastandingwave withthevoltageandcurrentlargely（butnotexactly）inphasequadrature,aswouldbeobtainedusingaquarterwavestuboftransmissionline.Thecurrentreachesaminimumattheendoftheelement（whereithasnowheretogo!

）andismaximumatthefeed-point.Thevoltage,ontheotherhand,isthegreatestattheendoftheconductorandreachesaminimum（butnotzero）atthefeedpoint.Makingtheconductorshorterorlongerthan1/4wavelengthmeansthatthevoltagepatternreachesitsminimumsomewherebeyondthefeed-point,sothatthefeed-pointhasahighervoltageandthusseesahigherimpedance,aswehavenoted.Sincethatvoltagepatternisalmostinphasequadraturewiththecurrent,theimpedanceseenatthefeed-pointisnotonlymuchhigherbutmainlyreactive.

Itcanbeseenthatifsuchanelementisresonantat f0 toproducesuchastandingwavepattern,thenfeedingthatelementwith 3f0 （whosewavelengthis1/3thatof f0）willleadtoastandingwavepatterninwhichthevoltageislikewiseaminimumatthefeed-point（andthecurrentatamaximumthere）.Thus,anantennaelementis also resonantwhenitslengthis3/4ofawavelength（3/2wavelengthforacompletedipole）.Thisistrueforalloddmultiplesof1/4wavelength,wherethefeed-pointimpedanceispurelyresistive,thoughlargerthantheresistiveimpedanceofthe1/4waveelement.Althoughsuchanantennaisresonantandworksperfectlywellatthehigherfrequency,theantennaradiationpatternisalsoalteredcomparedtothehalf-wavedipole.

Theuseofamonopoleordipoleatoddmultiplesofthefundamentalresonantfrequency,however,does not extendtoevenmultiples（thusa1/2wavelengthmonopoleor1wavelengthdipole）.Nowthevoltagestandingwaveisatits peak atthefeed-point,whilethatofthecurrent（whichmustbezeroattheendoftheconductor）isataminimum（butnotexactlyzero）.Theantennais anti-resonant atthisfrequency.Althoughthereactanceatthefeedpointcanbecancelledusingsuchanelementlength,thefeed-pointimpedanceisveryhigh,andishighlydependentonthediameteroftheconductor（whichmakesonlyasmalldifferenceattheactualresonantfrequency）.Suchanantennadoesnotmatchthemuchlowercharacteristicimpedanceofavailabletransmissionlines,andisgenerallynotused.Howeversomeequipmentwheretransmissionlinesarenotinvolvedwhichdesireahighdrivingpointimpedancemaytakeadvantageofthisanti-resonance.

Theuseofamonopoleordipoleatoddmultiplesofthefundamentalresonantfrequency,however,does not extendtoevenmultiples（thusa1/2wavelengthmonopoleor1wavelengthdipole）.Nowthevoltagestandingwaveisatits peak atthefeed-point,whilethatofthecurrent（whichmustbezeroattheendoftheconductor）isataminimum（butnotexactlyzero）.Theantennais anti-resonant atthisfrequency.Althoughthereactanceatthefeedpointcanbecancelledusingsuchanelementlength,thefeed-pointimpedanceisveryhigh,andishighlydependentonthediameteroftheconductor（whichmakesonlyasmalldifferenceattheactualresonantfrequency）.S