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EMClarity White Paper

BASIC PATH CONSIDERATIONS FOR A

MICROWAVE LINK

Date: May 2010

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BASICPATHCONSIDERATIONSFORAMICROWAVELINK

Date:May2010

Revised:

1.0SummaryThereareanumberoffactorstobecheckedwhenconsideringapossiblepathforamicrowave

link.Ideally,amicrowavelinkwouldoperatebetweentwopointswithoutpassingthroughany

materialsubstances,includingtheatmosphereandrain,andwithoutanyobjectsbeing

anywherenearthepathofpropagation.Suchalinkiscloselyapproximatedbyatypicallink

betweenanearthstationandageostationarysatellitewheretheonlyinterveningmaterialis

about12kmofatmosphereinthe36,000kmbetweentheearthstationandthesatellite.The

closerthesatellitelinklineofpropagationistoverticalthroughtheatmosphere,thesmallerthe

atmosphericeffectsonthesignalpropagation.

Aterrestrialmicrowavelinkusuallyhasapropagationpathapproximatelyhorizontallythrough

theatmosphereandthisaddsanumberofcomplicationstotheanalysisofthepath.Itisnotthe

intentionofthisnotetogointotheseissuesingreatdetailbutrathertooutlinethebasicpoints

foragoodunderstandingofhowterrestrialmicrowavelinksworkandwhattobeawareofwhen

consideringamicrowavelinkinstallation.

2.0NoiseandInterferenceTherearetwomainsourcesofnoisethatcanlimittheperformanceofmicrowavelinks.Thefirst

isthenoiseduetonaturalsourcesandthisisasignificantdifferencebetweenaterrestriallink

andasatellitelink.Asatelliteterminalbasicallylooksintodeepspacewheretheeffectivenoise

fromallsourcesisquitesmallinthemicrowavefrequencyrange.Typically,mostofthenatural

noiseforasatelliteterminalcomesfromthenoisefromthetemperatureoftheearththatis

pickedupintheearthstationantennasidelobes.

Aterrestrialmicrowaveantennatypicallylooksalongthehorizontalandsowillpickupnearlyall

thenoiseemittedbytheearthinthemicrowaveband.Thisgivesaterrestrialmicrowave

antennaaneffectivenoisetemperatureofabout300Ksothereislittlepointinusingexpensive

lownoisereceiverswithmicrowavelinks.

Noiseoriginatingfrommanmadesourcesisusuallyreferredtoasinterference.Thisisa

significantissuewithlinksthatoperateintheclasslicensebandwherethereareonlyverybasic

constraintsontheemittedpowerlevelsofthetransmitters.Thisissuewillnotbedealtwith

furtherinthisnote.

Forlinksinthelicensedbands,theregulationsarenominallysetuptoavoiddirect,highsignal

levelinterferencefromotherlinksoperatinginthesameoradjacentbands.Furthermore,the

generalnoiselevelfromothermicrowavelinksandsourcessuchasradarandsatellite

communicationsiscontrolledsuchthattheinbandnoisefromallsuchsourcesshouldnot

materiallyaffecttheperformanceofthelink.

InterferencethatisoutofthebandwillberemovedbythesharpfrontendfilteringinEMClarity

links.Theplanningshouldensurethatsignalsinthesamechannelorintheimmediateadjacent

channelsaresufficientlysmalltobenegligible.

Itisnotonlymicrowavesignalsthatcancauseinterference.Highlevelsignalsatlower

frequenciesmayleakinatvarioussitesbutinmostcasesthiscanbepreventedbyproper

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installationtechniquessuchasfullyshieldedcables,lowimpedanceearths,useofshielded

roomswheretheinternalequipmentishousedandwellregulatedandfilteredpowersupplies.

[SEEEMCLARITYNOTEXYZformoreinformationoninterference,itscauses,identificationand

resolution]

3.0PathAnalysisThefirstbasicrequirementforamicrowavelinkisthatthereisaclearlineofsight(optical)

betweenthetwopaths.Iftheantennaatoneendcannotbeseenfromtheotherend,eitherby

eyeorwithbinoculars,thenitisveryunlikelythatthelinkwillperformancesatisfactorily.

Thenextrequirementisthatnoobjectssuchashills,buildingsandtreesarewithinacertain

radiusofthenominallineofsight.ThisradiusisreferredtoastheFresnelzoneandisdependent

onthefrequencyandthedistancebetweenthetwoantennassinceitisdeterminedwherethe

pathlengthfromeachantennatotheobjectisonehalfwavelengthlongerthanthedirectpath

lengthbetweentheantennas.

TheFresnelradiusisamaximumatthemidpointandgraduallydecreasestowardseachantenna.

Theradiusinmetresisgivenby:

R=17.3(D1D2/(Fx(D1+D2))1/2(1)

WhereRisinm

D1andD2areinkmandarethedistancetotheobstaclefromeach

antenna

FisinGHz.

Closetoeitherantenna,theformulacanbeapproximatedby

R=17.3(D1/F)1/2 (2)

AndatthemiddlethemaximumvalueofRisgivenby

R=8.67(D/F)1/2 (3)

WhereD=D1+D2isthetotaldistancebetweentheantennas.

Theaboveformulaissuitableforhighgain,narrowbeamwidthantennaswithagaingreaterthan

25dBorso.

Example:Considera20kmpathwhereabuildingabout3kmfromoneendisseentobe

reasonablyclosetothelineofsightbetweentheantennas.Assumethatthelinkfrequencyis

8.2GHz.

Equation(2)givestheFresnelzoneradiusas9.6mwhichwouldbeabouttheminimumradial

spacingbetweenthebuildingandthelineofsight.

Atthe10kmpointtheFresnelradiusis13.5mwhereassay100mfromtheantenna,theradiusis

1.9m.

TheeffectoftheobstacleclosetoorwithintheFresnelzonedependsonthetypeofobjectandhow

closeitistothelineofsight.Smoothobjectssuchasgrassyhillsorabuildingroofcanhaveamuch

moresignificanteffectthanangularobstaclessuchasthecornerofabuildingordispersedobstacles

suchastrees.

Itmaybenecessarytouseprecisionopticalmeasurementsand/orGoogleEarthtodeterminehow

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stillperformquitewellwhenthisisincludedintheplanning.Ifallobstaclesarewellclearofthe

Fresnelzonethenthenextstepistoconsideratmosphericandearthcurvatureeffects.

4.0AtmosphericandEarthCurvatureEffectsMicrowavessignalsarenotsignificantlyaffectedbyfog,dustandpollutionlevels.Rainfallhas

littleeffectonfrequenciesbelowabout8GHzbuttheattenuationcausedbyrainincreasesquitequicklyandthefrequencyandrainfallrateincrease.Rainfallisthenamajorfactorforpaths

longerthan10kmandforfrequencieshigherthan10GHzorforshortpaths(5kmorso)at18GHz

andhigher.

Forpathlengthslongerthanabout10kmthecurvatureoftheearthandtheeffectofthe

atmosphereonthepropagationdirectionhavetobetakenintoaccount.Themicrowavesignal

typicallytravelshorizontallythroughtheatmosphereandthedielectricpropertiesofthe

atmospheremaychangewithheight.Thiscancausethemicrowavebeamtobendupordown

andsotochangetheeffectivecurvatureoftheearth.Thismayeffectivelymoveobjectsbelow

thepropagationpathintotheFresnelzoneorinsomecasesrightintothepathwhichcancause

largeincreasesinthepathattenuation.Thescienceofatmosphericpropagationhasbeenstudiedformanyyearsandtheengineeringof

linkstocompensateforsuchpatheffectshavebeencodifiedsothatstandardstatisticalbased

analysesareavailabletoestimatetheprobabilitythatthelinkwillfailduetotheatmospheric

andraineffects.

Forpathdistancesgreaterthanabout10kmorwherethereareanyobstaclesclosetoorwithin

theFresnelzone,EMClaritycanprovideacompletepathanalysis.

5.0DifficultPathsDifficultpathsarethosewherelargereflectionsmayoccurmainlyduetosmoothflatsurfaces

thatinterceptasignificantproportionoftheenergypropagatingbetweenthetwoantennas.

Suchpathstypicallyoccurwheretherearelongerdistancesof40kmormoreandflatsurfaces

suchaslakes,flatbaregroundortheseaalongsomeorallofthepath.Thepathisthensubject

tomultipathfadingwhichmayvarygreatlywiththetimeofdayastheatmosphericvariations

movethereflectionsinanoutofphasewiththemainsignal.

Sucheffectscanalsohappenovermuchshorterpathswhereoneorbothantennasmaybe

relativelyclosetoareflectingsurfacesuchastheroofofabuildingoronlyseveralmetresabove

thesealevel.

Thereareanumberofwaystoimprovethelinkavailabilityofsuchpathsbyproperpositioning

andalignmentoftheantennas,addingantennanullingalongthedirectionofthereflected

signal,increasingthetransmitpowerlevels,addingredundancybyusingextrafrequenciesor

extraantennasorsomecombinationoftheabove.

EMClaritycanassistwithdecidingontheoptimumconfigurationforanysuchpathsubjectto

costandavailabilityconstraints.