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Application note

➔ INTRODUCTION 3

➔ 1.LONGITUDINALATTENUATIONANDCOUPLINGLOSS 3

➔ 2.RADIATEDANDCOUPLEDMODECABLES 5

➔ 3.LINKBUDGET 73.1.RCinsertionloss 83.2.RCCouplingloss 83.3.Correctionforlongerdistance 103.4.Penetrationloss 113.5.Mobileantennalossrelativetodipole 113.6.Safetymargin 11

➔ 4.COUPLINGLOSS 124.1.Measurementprocedures 12

4.1.1.Ground–levelversusfree-spacemethod 124.1.2.Ground–levelversusor-spacemethod 154.1.3.EupenRMCRange 15

4.2Couplinglossandantennaorientations 164.2.1.CLdefinitionsaccordingtothestandard 164.2.2.Antennaorientationandlinkbudget 18

➔ 5.RCPERFORMANCESOPTIMISATION 195.1.RCpositioning 19

5.1.1.Mobileantennamountedonthevehicleroof 195.1.2.Hand-heldmobileequipmentonboardtrain 20

5.2.Multi-cablesystem 215.3.Resonantfrequencies 21

Listofabbreviations

CL designatesacouplinglossingeneral,whichevertheorientationandtheprobabilitylevel(50%,95%oranyotherpercentile).

CL50% medianvalueofthecouplingloss.

CL95% 95%percentileofthecouplingloss.

CLr,CLpandCLo Inthecontextof§3.2and4.2only,thesesymbolsdesignatetheCLsintheradial,parallelandorthogonalorientationsrespectively,whichevertheprobabilitylevel.Theseabbreviationshavenotbeenusedeverywheretosimplifythemathematicalformulas.

CLmean Inthecontextof§3.2and4.2only,thissymbolcorrespondstothemeanCLaveragedoverthethreeantennaorientationsasdefinedinthestandard.

RC radiatingcable.

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TABLEOFCONTENTS

3

INTRODUCTION

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Theaimofthisapplicationnoteistoprovideusefulinformationfor:

➔ theperformancesoptimisationoftheEupenradiatedmodecables;➔ reliablelinkbudget;➔ RCperformancescomparison.

Section1and2includeabriefreminderofsomeimportantdefinitions.Howtoperformalinkbudgetcalculationisexplainedinsection3.

The radio engineers familiar with the RC subject have certainly noticed that there is a real lack ofharmonisationconcerningthedefinitionofthecouplingloss.Indeed,documentssuchasdatasheetsandapplicationnotespublishedbytheRCmanufacturersrevealdifferencesofinterpretationthatmayleadtosignificanterrorsinlinkbudgetorwhentheRCperformanceshavetobecompared.ThesedifferencesarerathersurprisingasalltheRCmanufacturersrefertothesameIECstandard.

Thevariousmethodstomeasurethecouplinglossarepresentedinsection4.Itssensitivitytoantennaorientationandotherparametersisdeeplyanalysed.Suchaninformationmaybeusefulforlinkbudgetcalculations.

SomerulesallowingoptimisationofRCperformancesarealsopresentedinsection5.

1. LONGITUDINALATTENUATIONANDCOUPLINGLOSS

Fromtheelectricalpointofview,RCperformancesaremostlycharacterisedbythelongitudinalattenuation(indB/100m)andbythecouplingloss(indB).

Thelongitudinal attenuationisameasureoftheattenuationofthesignalpropagatinginsidetheRC.ItisspecifiedindBperunitlength(usuallyindB/100m)andisgivenbythefollowingformula:

wherePinandPoutaretheRCinputandoutputpowersrespectively.

Thelongitudinalattenuationisprimarilytheresultscopperanddielectriclossesandamountofradiatedenergy.Thelongitudinalattenuationincreaseswiththefrequencyanddecreaseswiththecablediameter.ItisalsosomewhatinfluencedbytheproximityoftheRCtoothersurfaces.

Thecoupling loss (CL)characterises thecouplingbetweentheenergy travelling inside theRCandareceivingantenna.It isdefinedastheratioofthereceivedpowerattheantennaoutputtothepowerflowingintheRC.Forexample,ifthepowerflowingintheRCwas0dBmandthepowerreceivedbytheantennawas–60dBm,thentheCLwouldbe60dB.Inthedatasheet,theCLisgivenforanRCtoantennadistanceequalto2m.

Pin

➔ a=10log——Pout

4

ThelocalvalueoftheCLisgivenbythefollowingformula:

wherePcableisthepowerinsidetheRC(neartheantenna)andPantennathepowerattheantennaoutput.

Usually,CL50%andCL95%arespecifiedintheRCdatasheets.TheirmeaningisillustratedinFigure1.Thecurverepresentstheprofileofthesignal(indBm)receivedbyanantennamovedalongapathparalleltotheRC,forexampleat2m.Thehorizontallineatthetopofthediagramrepresentsthepower(indBm)insidetheRC.Thedistance(indB)betweenthehorizontallineandthecurveisequaltotheCLatthisparticularpoint.TheCL50%correspondstothe50%percentileormedianvalue.Itmeansthat50%ofthemeasuredlocalvaluesarelowerand50%arehigher.

TheCL95%correspondstothe95%percentile.Itmeansthat95%ofthemeasuredlocalvaluesarelowerthanthisfigure.

TheCLmeasurementmethodsaccordingtotheIECstandardaredetailedandanalysedinsection4.

Figure1:CL50%andCL95%definitions

Pcable

➔ CL=10log————Pantenna

CL95%

CL50%

powerinsidethecablepowerreceivedbytheantenna

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5

2. RADIATEDANDCOUPLEDMODECABLES

AllofthedifferenttypesofRCsarebasedontheeffectsofsphericalwavesexcitedbytheleakageproducedbytheaperturesintheexternalconductor.Theresultingfieldatacertaindistanceisgivenbythevectoradditionalloftheaperturescontributions.TheFigure2showsasimplecasewheretheelectromagneticwavesgeneratedbyonlythreeaperturesareconsidered.ThesewavesareidentifiedbythesymbolW1,W2andW3.Let’sconsidertheresultingelectromagneticfieldatapointP.

ThevectorsE1,E2andE3inFigure3representtheelectricfieldcomponentatthispointPcorrespondingtoW1,W2andW3respectively.ThevectorEistheirresultant.Withoutspecialprecautions,theaperturecontributionsmaygiverisetodestructiveinterferencesatsomeplaceswithalowresultingfieldindicatedbyarelativelyshortvectorasshowninFigure3(leftside).Conversely,aperturecontributionsmaybeinphaseatotherplaces,whichgiverisetoconstructiveinterferences,henceastrongresultantasshowninFigure3(rightside).ItisclearthattherationaleinFigure3appliestobothelectricandmagneticfieldcomponents.

Therealsituation isobviouslymorecomplexastheelectromagneticfieldatanypoint istheresultsofmorethan3aperturescontributions.TherationaleishoweveridenticalanditexplainsthefluctuationsofthefieldstrengthalongtheRC.Typically,thesefluctuationsreach20to30dBpeaktopeakandcanbemodelisedbyaRayleighdistribution.Usually,thedifferencebetweenCL95%andCL50%rangesbetween10and13dB.

Figure2:Electromagneticwaves

duetotheaperturesintheexternalconductor

W1W2W3

P

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Theradiated modeRCsaredesignedtoproduceacoherent interferenceof thedifferentaperturescontributionsincertainfrequencybandsandatallplacesaroundtheRC.ThiseffectisobtainediftheaperturespacingischoseninsuchawaythatalltheaperturecontributionsaddupinphaseintheRCradiocoverageareaasshowninFigure3(rightside).Thisisachievedifthedelaybetweenthecontributionsoftwosuccessiveaperturesisamultipleofthesignalperiod.Whenthisconditionissatisfied,theresultantfieldisstrongerandthefieldstrengthfluctuationsalongtheRClengthareconsiderablyreduced.Itresultsthat:

➔ theCL50%decreases;➔ thedifferencebetweenCL95%andCL50%decreasesandtypicallyrangesfrom3to8dB.

Withradiatedmodecables,themaindifficultyistomaximisethefrequencybandinwhichtheaperturescontributionsinterfereinacoherentway.

AlltheradiatedmodeRCsworkincoupledmodebelowacertainfrequency,hereaftertermed“transition frequency”.Thistransitionislinkedtotheaperturespacing.Thisisbecauseitisimpossibletokeepthedifferentaperturescontributionsinphasewhenthewavelengthexceeds,approximately,twotimesthedistance between two successive aperture groups. However, the performances may be impaired, forvariousreasons,insomefrequencybandsabovethetransitionfrequency.Thismeansthattheradiatedmodeisnotnecessarily“superior”thancoupledmode.

TheEupenradiatedmodeRCsaredesignedtoprovidelowCLandlowfieldstrengthfluctuationsinseveralfrequencybandsallocatedtomostmobileradiosystemsstandardssuchasTETRA,TETRAPOL,GSM900,GSMR,GSM1800,PCS,DECT,UMTS,WiFi(eitherat2.4GHzorat5to6GHz),WiMax,etc.

E1

^

E2

^

E

^E

^

E2

^

E1

^

E3

^E3

^

Ě1,Ě2andĚ3notinphase Ě1,Ě2andĚ3nearlyinphaseFigure3:Aperturecontributionsoutofphase(left)andinphase(right)

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3. LINKBUDGET

The basic elements to calculate a link budget can be illustrated by considering the example showninFigure4.ItinvolvesaGSM900radiocoverageinadual-boretunnelthatis900minlength.Itshallbeassumedthat:

➔ thepowerperchannelavailableforthedownlinkis1W(+30dBm);➔ theRC ineachbore is fedviaapower splitter (“T-feed”configuration) the insertion lossofwhich

isequalto3.5dB;➔ jumper cables are used to connect the repeater, power splitter and the RCs. Their total insertion

lossisequalto1.5dB;➔ thespecification imposesa receivedsignal (measuredwithahalf-wavedipoleantenna)ofat least

-88dBmat95%ofthepointsinthevicinityofthecableendandat6mdistance.

It shall also be assumed that the RC has the following characteristics at 960 MHz (upper limit of theGSM900frequencyband):

➔ longitudinalattenuation:3.1dB/100m;➔ CL50%andCL95%equalto58and62dBrespectively.

Table1summarisesthelinkbudget.Thelastlineindicatesthatthespecificationissatisfied,i.e.aminimumreceivedsignal(measuredwithahalf-wavedipoleantenna)higherthan-88dBmat95%ofthepointsinthevicinityofthecableendandat6mdistance.Thevariouslinesofthislinkbudgetarecommentedhereafter.

Uplinkperformances(i.e.frommobilestationtobasestation)canbecomputedinthesameway.

Table1:Linkbudgetexample

Availablepowerperchannel + 30dBm

Jumpercableloss - 1.5dB

Powersplitterinsertionloss - 3.5dB

RCinsertionloss:900mwith3.1dB/100m - 28dB

CL95%at2m=62dB - 62dB

Correctionforlongerdistance=20log(d/2)=20log(6/2)= - 9.5dB

Penetrationloss* 0dB

Mobileantennalossrelativetodipole* 0dB

Safetymargin - 10dB

Minimum received signal at 6 m from the RC (95% percentile) - 84.5 dBm

Figure4:Dual-boretunnel

withonebasestationandapowersplitter

*Note:Itresultsfromthespecificationofthisparticularexamplethatthepenetrationlossandmobileantennalossrelativetodipoleareequalto0dB.

RCinbore1

RCinbore2

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3.1. RC insertion loss

The RC insertion loss is equal to the cable length multiplied by the longitudinal attenuation. Thislongitudinalattenuationissomewhatinfluencedbythestandoffdistance(betweentheRCandthewallorceilingtowhichitishung).Forexample,iftheRCisdirectlyagainstaconcretesurface,theimpactonthelongitudinalattenuationisfrequencydependentandisobviouslynotidenticalforallRCs.IftheIECstandardconditionsareconsideredasreferencevalues,measurementscarriedwithvariousEupenRMCsindicatethatinstallingtheRCdirectlyagainstaconcretesurfaceinvolvethefollowinglongitudinalattenuationincreases:

➔ below300MHz,theimpactisnegligibleandevensometimesnegative;➔ typicallyrangesfrom5to10%around450MHz;➔ typicallyrangesfrom10to20%around900MHz;➔ typicallyrangesfrom25to60%around2000MHz.

ThelongitudinalattenuationisalsoinfluencedbyhumidityanddustdepositontheRCjacket.Eveninrathersevereconditions,thelongitudinalattenuationincreaseneverexceeds10%.

3.2. RC Coupling loss

Some RC manufacturers use the free space method or specify the CLs for the antenna orientationcorrespondingtothebestresult.ThedifferencesofinterpretationinthemeaningoftheCLparametermayleadtosignificanterrorsinlinkbudgetorwhentheperformancesofdifferentproductshavetobecompared.

TheCL50%andCL95%specifiedintheEupendatasheetsaremeasuredwiththegroundlevelmethodaccordingthe IEC61196-4standard1.Thegroundlevelmethodhasbeenpreferredbecause itdefinesconditions which are closer to those actually met in practice. Indeed, in almost all the applications,theRCishungatshortdistancefromasurface(ceilingorwall).Adetailedanalysisofthisissueispresentedinsection4.

However,CLsmeasuredwiththefreespacemethodarealsoavailableforsomeEupenRCs.

TheCL50%andCL95%specifiedintheEupendatasheetsareaveragedoverthreeantennaorientations(radial,orthogonalandparallel).Asexplainedinsection4.2.,theCL50%orCL95%thatshouldbeusedforlinkbudgetcorrespondtothesymbolsCL50%-meanorCL95%-mean.

1IEC61196-4standard-Coaxialcommunicationcables-Part4:Sectionalspecificationforradiatingcables.

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a) E.M.wavedepolarisationduetoreflectionsonobstacles

In thecaseofcommunicationswithhand-heldmobileequipmentonboard train,awavepenetratingintoacarriageexperiencesreflectionsonthecarriagewalls,ceiling,floor,seats,etc.Ateachpoint,thefieldstrengthisthevectoradditionofseveralwavesandthepolarisationofthesumcanbeconsideredaselliptical ratherthan linear.Figure5showsthesimplecasewhereadirectwaveradiatedbytheRCinterfereswithanotherwavewhichhasbeenreflectedbythecarriageceilingandwindow.Thedashedarrows(atrightanglewiththedirectionofpropagation)indicatethewavepolarisation.Ifweconsiderthedifferenceofpropagationdelay,itisclearthatthepolarisationoftheresultingE.M.fieldatthereceptionpoint R is very complex. Figure5 is a very simple casewithonlyone reflectedwave. Inpractice, thesituationsaremuchmorecomplexassuggestedinFigure6wherethedirectwavemaybeblockedbytravellersorbyanothertrain.

b) Mobileantennaorientation

Withhand-heldequipments,themobileantennaorientationisneitherperfectlyverticalnorhorizontalbutratheracombinationoftheseasshowninFigure7.Indeed,innormaluse,anhand-heldequipmentisslightlydown-tiltedandnotnecessarilyorientatedformaximumresponse.

Figure5:Wavedepolarisation

duetoreflections

Figure6:Propagationintocarriage

Figure7:Mobileantenna

orientation

Figure6 Figure7

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c) Mobilestationareratherinsensitivetoantennaorientation

Asexplainedinsection4.2,thedifferencebetweentheCLsinthedifferentantennaorientationisduetothedirectivityofthehalf-wavedipolewhichisusedforCLmeasurements.

Converselymobilestationantennas(suchasGSM,PCN,UMTS,etc.)aremoresophisticatedthatthesingledipoleormonopole(whichisillustratedinFigures6and7).Theirspatialresponseisdifferentandmuchmore“isotropic”thanquarter-wavemonopoleandhalf-wavedipoleantennas.Measurementsperformedwithvariousmobilestationsdemonstratethatthereceivedpowerisnearlyindependentoftheantennaorientation.So,mobilestationantennasbehaveasnearlyisotropicantennaswhichpickupthestrongestfieldcomponentwitharatherlowgain(about–10dB).

If the symbols CLr, CLp and CLo designate the coupling losses in the radial, parallel and orthogonalorientationrespectively(whichevertheprobabilitylevel)andifthesymbolCLmeancorrespondstothemeancoupling lossasdefined in the IEC61194-4standard, it is shown insection4.2thatCLmean isgenerally about 4 dB higher than the lowest coupling loss. It results that, for practice, the followingapproximationcanbemade:

CLmean = min (CLr, CLo, CLp) + 4

wherethe“min”symboldesignatestheminimumofthevaluesinbrackets.

This results means that calculating the link budget with the lowest coupling loss (CLr, CLp or CLo) instead of the average value (CLmean) is equivalent to a 4 dB decrease of the safety margin.

3.3. Correction for longer distance

With“classical”transmittingantennas,thereceivedpowerdecreasesasafunctionofthesquareofthedistanced,i.e.:

This is a consequence of the “spherical symmetry” (the radiated energy is contained in a sphere ofradiusequaltod.WithRCs,theradiatedenergyiscontainedinacylinderofradiusequaltod,hencea“cylindricalsymmetry”.Consequently,thereceivedpowerdecreasesasafunctionofthedistanced,i.e.:

1➔ Prec÷—

d2

1➔ Prec÷—

d

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TheCL50%isspecifiedat2moftheRCaccordingtotheIEC61196-4standard.Ifitisrequiredatanotherdistance,thefollowingcorrectionshouldbeapplied:

FortheCL95%,alongerdistanceinvolvesastrongerinfluenceofscatteredradiationsandreflectionsonwallsandceiling,henceafadingincrease.Thefollowingcorrectioncanbeapplied:

3.4. Penetration loss

For communications into vehicles, the link budget must take a penetration loss into account. Thispenetrationlossisstronglyinfluencedbythefrequency,thewidowsizes,theglasstype(singleordoublelayer) and the possible presence of metal coating (for thermal insulation). For example, at 900 MHz,penetrationlossmayrangefrom2or3dBforasinglelayerglass.Itreaches30dBinthecaseofmetalcoatedglasses.

3.5. Mobile antenna loss relative to dipole

Theantennasusedinmobilephones(suchasGSM,PCN,UMTS,etc.)haveanegativegainwithrespecttothehalfwavedipolenormallyusedtomeasuretheCLs.Theirspatialresponseishowevermore“isotropic”.A10dBmobileantennalossrelativetohalfwavedipoleseemsarealisticvalue.

3.6. Safety margin

A10dBsafetymarginisrecommendedtoaccountfor:

➔ thedifferencesbetweenthestandardconditionsinwhichtheCLsaremeasuredandthoseactuallymetinarealtunnelenvironment;

➔ thevariousfactorswhichmayimpairtheRCperformances.

As explained in section 3.2., a link budget based on a CL averaged over three antenna orientationsprovidesasafetymarginwhichis4dBsuperiortoabudgetcalculatedwiththevaluemeasuredinthebestorientation.

d➔ CL50%(d)=CL50%+10log(——)

2

d➔ CL95%(d)=CL95%+20log(——)

2

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4.1. Measurement procedures

TheproceduretomeasuretheCLisdefinedbyanIEC61196-4standard.Twoconfigurationsarepermitted,i.e.:the“ground–levelmethod”andthe“free-spacemethod”.Thesetwoconfigurationsoftengiveresultsthatmaybequitedifferent.ThatisnotsurprisingasitiswellknownthattheenvironmentaffectstheRCperformances.Asexplainedisthissection,theground–levelconfigurationisclosertotheconditionsactuallyfoundintunnels.

Inaddition,thestandardallowstospecifyeitheraCLfora“singleorientation”(i.e.radial,orthogonalorparallel)orameanvaluecalculatedwithaspecificformula.Thisissueisexaminedinsection4.2.

ThefactthattheIECstandardisnotveryrestrictingmaybeconfusing,especiallywhentheperformancespublishedinthemanufacturerdatasheetshavetobecompared.Someclarificationsareprovidedhereaftertoassisttheradioengineerinmakingthemostaccuratelinkbudgets.

4.1.1. Ground–level versus free-space methodThetwoconfigurationsaredetailedintheannexBofthestandard(§B1.1and§B1.2)andareshowninFigures8and9respectively.

Intheground–levelmethod,theRCislaidat10to12cmaboveaconcreteground.Thecentreoftheantennaispositionedverticallyat2mabovetheRC.ThefieldstrengthisrecordedwhenmovingtheantennaalongapathparalleltotheRC.

Inthefree-spacemethod,theRCishungtononmetallicpostsataheightof1.5to2m.Theantennacentreisat2mfromtheRCandatthesameheight.ThefieldstrengthisrecordedwhenmovingtheantennaalongapathparalleltotheRC.

TheFigures8and9alsodefinethethreeantennaorientations,i.e.:➔ Radial: thedipoleisorientatedatrightanglewithrespecttotheRCandisinthesameplane;➔ Orthogonal: thedipoleisatrightanglewithrespecttotheplanecontainingtheRC;➔ Parallel: thedipoleisparalleltotheRC.

Figure8:RCandantennapositionswithground-levelmethod

Figure9:RCandantennapositionswithfree-spacemethod

12

4. COUPLINGLOSS

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Figure10:Reflectionmechanism

withground-levelmethod

13

Ground-level and free-space methods sometimes give rather different CL results; to explain thesedifferences,coupledmodeandradiatedmodehavetobetreatedseparately.

Coupled mode cablesThedifferencebetweenCLswithground-levelandfree-spacemethodsmayberelativelyimportantandsometimesexceed10dB.Ingeneral,theground-levelmethodgiveslowerCLs;thisisnotsurprisingasthesurfaceclosetotheRCcontributestothecoupledmodegeneration.Inthefree-spaceconfiguration,thegroundisat2mandistoofartoefficientlypromotethecoupledmode.

Itmustberemindedthattheradiatedmodecablesworkincoupledmodebelowthetransitionfrequency(whichdependsontheRCdesign).Consequently,theaboveremarksarealsoapplicabletotheseRCswhentheyareusedbelowtheirtransitionfrequency.

Radiated mode cablesFortheRCsworkinginradiatedmode,CLdifferencesof2or3dBbetweenthetwoconfigurationsareusualbutrarelyexceed6or7dB.Thisdifferencemaybeeitherpositiveornegative,dependingonRCdesignandfrequency.

TheCLdifferencesaremainlyduetotheeffectofthereflectionsonthegroundsurface.Indeed,intheground-levelconfiguration,thereflectionsproducedbytheconcretesurfacelocatedat10to12cmfromtheRChavearelativelyimportanteffect.ThereflectionmechanismisshowninFigure10whereonlyonesingleapertureAhasbeenconsideredforsimplicity.AtanypointPintheRCvicinity,thefieldstrengthisthevectoradditionofthefieldradiatedbytheapertureA(hereaftertermed“directwave”)andtheonereflectedatthepointRbytheconcreteground.

Themagnitudeoftheresultingfieldwilldependon:

➔ the magnitude of the reflected signal: thismagnitudedependson surfaceconductivity. Thereflectioncoefficientmayrangebetween0(noreflection)and1foraperfectlyconductivesurface.

➔ the phase difference between the direct and the reflected waves:thedirectandreflectedwavesdonottravelthesamedistance,henceaphasedifference.Itsvalue(indegrees)isgivenbytheexpression360°x(AR+RP–AP)/lwherelisthewavelengthintheair.

P

A

R

RC

Concreteground

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In addition there is a possible phase shift at the reflection point R. This phase shift depends on thedirectionof theelectric fieldandon theelectricalpropertiesof theconcrete surface. In thecaseofa perfectly conductive surface, there is no phase shift for the component of the electric field whichisorientatedatrightanglewiththeconcreteground.Conversely,thecomponentoftheelectricfieldparalleltothegroundexperiencesa180°phaseshift.

Figure11showshowthereflectionsimpacttheCL.InthisFigure,Ěd,Ěr,andĚ designatetheelectricfieldvectoratthepointPcorrespondingto,respectively,thedirectwave,thereflectedwaveandtheresultantfield.TheleftpartofthisFigureshowsthecasewherethevectorscorrespondingtothedirectwaveĚdandreflectedwaveĚrarenearlyinphase.ThemagnitudeoftheirresultantĚbeinghigherthanĚd asthereflectionreinforcesthedirectwave,henceaCLdecrease.

Conversely,therightpartofFigure11showsthecasewherethevectorscorrespondingtothedirectwaveĚdandreflectedwaveĚrarenearlyinopposition.ThemagnitudeoftheirresultantĚislowerthanĚd,henceaCLincrease.

Although it has been assumed, in Figure 11, that the reflection coefficient was lower than 1 (the ĚrvectorisshorterthanĚd),itisobviousthattheaboveconclusionsapplywhicheverthemagnitudeofthereflectedwave.

WiththewidebandRCs,itisnoteasytokeepthedirectandreflectedwavesinphase(ornearlyinphase)inallthefrequencybandsasthisparameterdependsonl.

Inthemostfavourablecase,i.e.whenthereisatotalreflection(reflectioncoefficient=1)inphasewiththedirectwave,theresultantĚ=2Ěd,hencea6dBCLdecrease.

Conversely, theworstcaseoccurswhere there isa total reflection inoppositionwith thedirectwavebecausetheresultingfielddropssharply,henceasevereCLincrease.Inpracticehowever,theresultingfielddoesnotcollapsecompletelyandtheCLincreaseshouldnotexceed20dB.

14

Figure11:Vectoradditionofthedirectandreflectedwaveswhentheyarenearlyinphase(ontheleft)andnearlyinopposition(ontheright).

Ed

^Ed

^

Er

^

Er

^

E

^

E

^

ĚdandĚrnearlyinphase ĚdandĚrnearlyinopposition

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Comparedtoasituationwherethereisnoreflection,theground-levelconfigurationmayeitherproduceaCLdecreaseofmaximum6dBoranincreasethatshouldnotexceed20dB.

Inthefree-spaceconfiguration,therearealsoreflectionsonthegroundsurfacebuttheireffectismuchless important as illustrated in Figure 12. Of course, if the RC is at 2 m above the concrete ground,thereflectionscanbeseenasproducedbyanelectricalimagelocatedatapproximately24.5moftheantenna. As the electromagnetic field decreases with the inverse of the distance, the magnitude ofthereflection isabout0.44times(i.e.2m/4.5m)themagnitudeofdirectwavewhenthereflectioncoefficientisequal to 1.Itresultsthat,ifthedirectandreflectedwavesareinphase,theCLdecrease3isabout3.2dB.Iftheyareinopposition,theCLincreasedoesnotexceed5dB.

Ifthereflectioncoefficientislowerthan1,themagnitudeofthereflectionandtheimpactontheCLsisreducedaccordingly.

Itresultsthattheimpactofthereflectionsarelessimportantinthefree-spaceconfigurationthanintheground-levelonewherethedirectandreflectedwavestravelnearlythesamedistance.Thisconclusionapplieswhicheverthevalueofthereflectioncoefficient.

4.1.2. Ground–level versus or-space methodInmostapplications,theRCishungatshortdistancefromasurface(ceilingorwall)producingreflectionswhichmayeitherimproveorimpairtheCLs.Itisobviousthattheground-levelmethodisclosertotheconditionsactuallymet inpractice.This is the reasonswhy theground-levelmethodseems themostsensibletoreferto.

4.1.3. Eupen RMC RangeTheEupenRMCrange isdesignedtoderivebenefitfromthereflectionphenomenon,at least inmostfrequencybandsallocatedtomobilecommunications.Thisisachievedbychoosingalaunchinganglethatminimisesthephasedifferencebetweendirectandreflectedwaves.

All the Eupen RC data sheets specify the CLs (50 and 95% probability) measured in the ground-levelconfiguration. However, data sheets with the CLs measured in the free-space configuration are alsoavailableformostEupenRCs.

2AccordingtoPythagora’stheorem,(4_+2_)1/2=4.5m320log(1+0.44)=3.2dBand20log(1-0,44)=5dB

Figure12:Reflectionmechanismwith

free-spacemethod R

RC

Concreteground

diplode

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Figure13:Half-wavedipoleresponse

4.2 Coupling loss and antenna orientations

4.2.1. CL definitions according to the standardTheIECstandardallowstospecifytheCLmeasuredeitherinasingleorientation(i.e.radial,orthogonalorparallel)orameanCLcalculatedwithaparticularformulagivenhereafter.Figures8and9showthethreeorientationsfortheground-levelandfree-spaceconfigurationsrespectively.

MeasurementresultsindicatethattheCLdifferencebetweentheworst(highestCL)andthebest(lowestCL) orientationsmayexceed10andeven15dBinsomecases.TheexplanationofthiseffectisgiveninFigure13whereitisassumedthataverticallypolarisedelectromagneticfieldpropagatesfromthelefttotherightasindicatedbythevectorv�.Thethreeconsideredantennaorientationsareandidentifiedbythelettersa,bandc.

Iftheantennaarmsareorientatedhorizontallyandparalleltothedirectionofpropagation(lettera),theresponseshouldbetheoreticallynullbecausethemainlobeoftheradiationpatternispointingintheverticalplane.

Iftheantennaarmsareorientatedhorizontallyandparalleltothedirectionofthemagneticfield(letterb),theradiationpatternispointingtowardthesourceofthefieldbuttheresponseshouldbetheoreticallynullbecausethearmsareperpendiculartotheelectricfield.

Themaximumreceivedsignalisobtainedwiththeantennaarmsorientatedvertically(letterc).Indeed,theradiationpatternispointingtowardthesourceofthefieldandtheantennaarmsareparalleltotheelectricfield.

The fact that the field produced by a RC is polarised explains the strong influence of the antennaorientationontheCL.

As CL difference between orientations may exceed 10 and even 15 dB in the worst cases, correctunderstandingoftheimpactofthisparameterisrequiredforaccuratelinkbudgetcalculationsandwhentheperformancesofRCsfromdifferentmanufacturerhavetobecompared.

16

E

^

R

^

v

^

a b c

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TheIECstandardalsodefinesameanCLcalculatedwiththefollowingformula:

CLmean=-10log[1_3

(10-CLr/10+10-CLo/10+10-CLp/10)]

This particular formula is different from the usual arithmetic and geometric averages. To understanditsphysicalmeaning,let’sconsideranRCfeedwithaninputpowerequalto1mW(0dBm).Theterm10–CLr/10intheaboveformulacorrespondstothepower(inmW)receivedbyadipoleantennaorientatedin the radial direction. Likewise, the terms 10 – CLo/10 and 10 – CLp/10 correspond to the power receivedbyadipoleantennaorientatedintheorthogonalandparalleldirectionsrespectively.

Consequently,theterm(10-CLr/10+10-CLo/10+10-CLp/10)/3isthesumofthepower(inmW)receivedwiththedipoleorientatedinthethreedifferentdirectionsdividedby3,i.e.thereceivedpoweraveragedonthethreeorientations.ItappearsthattheaboveformulagivesinfacttheCLwithrespecttothemeanvalueofthepowerreceivedinradial,verticalandorthogonalorientations.

Tounderstandtheimplicationsofthisdefinition,let’sconsiderthesimplecasewheretheelectromagneticfieldisperfectlypolarisedinonedirection,forexampletheparallelone.ThisinvolvesthatonlyCLphasafinitevaluewhileCLrandCLo=-∞.

As10-∞=0andas-10log[1_3

(10-CLp/10)]=-10log[1_3

]-10log[10-CLp/10]

Weobtainfinally:

CLmean = 4.8 + CLp

Intheactualsituationshowever,thefieldisnearlyneverpurelypolarisedinonlyonedirection(i.e.thereisnodirectionforwhichthereceivedpowerisnull).Forinstance,ifCLr=60dB,CLo=70dBandCLp=70dB,weobtainCLmean=64dB.OthernumericalexamplesconfirmthattheCLmeanisgenerallyabout4dBhigherthanthelowestCL.Inconclusion,forpractice,thefollowingapproximationcanbemade:

CLmean =~ min (CLr, CLo, CLp) + 4

wherethe“min”symboldesignatestheminimumofthe3valuesinbrackets.

Although, thestandard imposes tospecify theantennaorientation, this information is lacking inmostmanufacturerdatasheets.Consequently,RCperformancescomparisonsaresometimesdifficultasthegivenCLcouldeitherbeameanvalueormeasuredinasingleunknownorientation.

RadiatingCablesApplicationnote11/2017

4.2.2. Antenna orientation and link budgetAsstatedabove,theCLinthe“worstorientation”maybe10to15dBhigherthaninthe“bestorientation”.Thisismainlyduetothefactthatthemeasuringantennaisahalf-wavedipolewhichfeaturesdirectivity.Indeed,theradiationpatternisthetypicaleightfigurewithanullresponseinmedianplane.Consequently,theCLdependsonthedirectionofpropagationofthewaveradiatedbytheRCandontheorientationoftheelectricalfieldasshowninFigure13.

Inpracticehowever,theantennaorientationisnearlyneverperfectlyradialorparallelororthogonalwithrespecttotheRCbutratheracombinationofthesethreepossibilities. Indeed,themobileantennaisoftendowntiltedandisrarelyinanRCplane.

This remark also applies with handheld equipments. Moreover, their antenna is, generally, much lessdirectivethanadipole.Itmeansthattheirradiationpatternismore“isotropic”,resultinginadecreasedCL sensitivity to theantennaorientation.Consequently, theCLmean is recommended for linkbudgetcalculationinthecaseofcommunicationwithmobilephones.Inaddition,itmustalsoberemindedthatmobilephoneantennashavegainsubstantiallylowerthanthehalf-wavedipole.

Forallthesereasons,theEupenRMCdatasheetsspecifytheCL50%-meanandCL95%-mean.DetailedmeasurementreportswiththeCLsforthethreeorientationsareavailableonrequest.

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RadiatingCablesApplicationnote

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19

5. RCPERFORMANCESOPTIMISATION

5.1. RC positioning

Themobileantennapositionandorientationareimportantparametersforperformancesoptimisationofradiocommunicationsinconfinedspaces.Mobileantennamountedonthevehicleroofandhand-heldequipmentson-boardtrainarethemaincasesencounteredinpractice.Theyaredetailedhereafter.

5.1.1. Mobile antenna mounted on the vehicle roofFigure14showstwotypicalexampleswiththemobileantennainstalledonthevehicleroof(train,car,...).Itisgenerallyaquarterwavemonopoleorawhipverticallyorientedordowntilted.

AstheelectricfieldradiatedbytheRChasastrongradialcomponent,thebestcouplingisobtainedwiththeRChungfromthetunnelceilingandpreferablynearthecentrepositionoratleast1mawayfromthesidewallsasshowninFigure15.

The lowest CL and field strength fluctuations are obtained with the apertures located on the mobileside. Theaperture side ismarkedon theRC jacket. Figure12 shows theRCandaperturepositioningrecommendationsifthemobileantennaisinstalledonthevehicleroof.

Figure14:Mobileantennainstalled

onthevehicleroof

Figure15:RCpositionsifthemobileantennais

installedonthevehicleroof

min.1m aa

a

a=RecommendedzoneforRCposition Aperturesorientedtowardsvehicles

RadiatingCablesApplicationnote11/2017

Figure17:RecommendedRCpositionsforcommunicationwithpassengersonboardtrain

5.1.2. Hand-held mobile equipment on board trainItisobviousthattheorientationofahand-heldequipmentantennaisnearlyneververticalinnormaluseasshowninFigure16.

Theradiowavesenterscarriagesonlythroughthewindowswithapenetrationlosswhichdependsonglassmaterial(numberoflayers,metalcoating,...)andwindowsizes.

Inallcases,thebestcouplingisobtainedwiththeRChungalongawallasshowninFigure17(hand-heldontheRCsideandhand-heldontheoppositeside).ItisrecommendedtohangtheRCapproximatelyatthesameheightastheupperedgeofthecarriagewindowsasshowninthesefigures.

Again,thelowestCLandfieldstrengthfluctuationsareobtainedwiththeapertureslocatedonmobileside.

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Figure16:Hand-heldequipmentorientation

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21

WherethespecificationsimposeasecondRC,oneofthefollowingsolutionscanbeusedtomeetthereliabilityrequirementswithoutloosingthebenefitoflowfieldstrengthfluctuations:

➔ Feeding the 2 RCs with different carrier frequencies

The2RCsarefedwithdifferentcarrierfrequencysetsasshownintheseconddiagramofFigure18.ThusRC

1radiatesf

1onlyandRC

2radiatesf

3only.Thesameprincipleappliesfortheup-linkwithf

2andf

4.

➔ Use of only 1 RC at a time

OnlyRC1(mainlink)isactiveinnormaloperationconditionsasshowninthethirddiagramofFigure18.

RC2(sparelink)isactivatedincaseofRC

1linkfailure.

5.3.Resonant frequencies

Due to theirdesign, EupenRadiatedModesRCcannotbeusedat some resonant frequencies. Theseforbiddenfrequenciesarespecifiedinthedatasheet.

Figure18:Multi-cablesystem

RC1

RC2f1

RC1

RC1

GOOD

GOOD

RC2

RC2

f2

f3

f4

f1f3

f2f4

5.2. Multi-cable system

Insomecases,twoparallelRCsareusedinthesametubetoimprovethesystemreliability.Thediagramin the upper left corner of Figure 18 shows a configuration where two nearby RCs are fed with thesameRF source.This solutionwillgive rise to largefield strengthfluctuationsdue to theconstructiveand destructive interferences between the signals radiated by the two RCs. Of course, the signal atfrequency f1 and f3 are simultaneously radiated by both RC1 and RC2. Hence, such a configurationmust absolutely be avoided.

RadiatingCablesApplicationnote11/2017

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EUPEN CABLE

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Asaleadingsupplieroftransmissionlinesandaccessoriestoglobalwirelesscommunicationsmarkets,EUPENhastheexperienceandresourcestoeffectivelyservicecustomersintoday’schallengingwirelesscommunicationsmarkets.

Atatimewhenwirelesscommunicationinconfinedareas,suchasunderground,streetandservicetunnels,isimportanttothenetworkoperators,EUPENhighqualityradiatingcablesprovethatEUPENCableisyourreliablePartnerforRFCommunications!

DownloadyourowncopyofthelatestRadiatingCablecatalogueat:www.radiating-cables.com

Tel. +32 87 597343 Fax. +32 87 597060

www.eupen.comrf_sales@eupen.com rf_products@eupen.com

RadiatingCablesApplicationnote

11/2017

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