rAdiAting CABLES CABLES

CABL

ESrAdiAting CABL

ES

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.CLdefinition accordingtothestandard 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 22

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,thissymbolcorrespondstothemeanCLaveragedoverthethreeantennaorientationsasdefine inthestandard.

RC radiatingcable.

RadiatingCablesApplicationnote

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TABLEOFCONTENTS

3

INTRODUCTION

RadiatingCablesApplicationnote11/2021

Theaimofthisapplicationnoteistoprovideusefulinformationfor:

➔ theperformancesoptimisationoftheEupenradiatedmodecables;➔ reliablelinkbudget;➔ RCperformancescomparison.

Section1and2 includeabriefreminderofsome importantdefinitions Howtoperforma linkbudgetcalculationisexplainedinsection3.

The radio engineers familiar with the RC subject have certainly noticed that there is a real lack ofharmonisationconcerningthedefinitio ofthecouplingloss.Indeed,documentssuchasdatasheetsandapplicationnotespublishedbytheRCmanufacturersrevealdifferencesofinterpretationthatmayleadtosignifican errorsinlinkbudgetorwhentheRCperformanceshavetobecompared.ThesedifferencesarerathersurprisingasalltheRCmanufacturersrefertothesameIECstandard.

Thevariousmethodstomeasurethecoupling lossarepresented insection4. Itssensitivitytoantennaorientationandotherparametersisdeeplyanalysed.Suchaninformationmaybeusefulforlinkbudgetcalculations.

SomerulesallowingoptimisationofRCperformancesarealsopresentedinsection5.

1. LONGITUDINALATTENUATIONANDCOUPLINGLOSS

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

Thelongitudinal attenuationisameasureoftheattenuationofthesignalpropagatinginsidetheRC.Itisspecifie indBperunitlength(usuallyindB/100m)andisgivenbythefollowingformula:

wherePinandPoutaretheRCinputandoutputpowersrespectively.

Thelongitudinalattenuationisprimarilytheresultscopperanddielectriclossesandamountofradiatedenergy.Thelongitudinalattenuationincreaseswiththefrequencyanddecreaseswiththecablediameter.Itisalsosomewhatinfluence bytheproximityoftheRCtoothersurfaces.

Thecoupling loss (CL)characterises thecouplingbetween theenergy travelling inside theRCandareceivingantenna. It isdefine astheratioofthereceivedpowerattheantennaoutputtothepowerfl wingintheRC.Forexample,ifthepowerfl wingintheRCwas0dBmandthepowerreceivedbytheantennawas–60dBm,thentheCLwouldbe60dB.Inthedatasheet,theCLisgivenforanRCtoantennadistanceequalto2m.

Pin

➔ a=10log——Pout

4

ThelocalvalueoftheCLisgivenbythefollowingformula:

wherePcableisthepowerinsidetheRC(neartheantenna)andPantennathepowerattheantennaoutput.

Usually,CL50%andCL95%arespecifie intheRCdatasheets.TheirmeaningisillustratedinFigure1.Thecurverepresentstheprofil ofthesignal(indBm)receivedbyanantennamovedalongapathparalleltotheRC,forexampleat2m.Thehorizontallineatthetopofthediagramrepresentsthepower(indBm)insidetheRC.Thedistance(indB)betweenthehorizontallineandthecurveisequaltotheCLatthisparticularpoint.TheCL50%correspondstothe50%percentileormedianvalue.Itmeansthat50%ofthemeasuredlocalvaluesarelowerand50%arehigher.

TheCL95%correspondstothe95%percentile.Itmeansthat95%ofthemeasuredlocalvaluesarelowerthanthisfigu e.

TheCLmeasurementmethodsaccordingtotheIECstandardaredetailedandanalysedinsection4.

Figure1:CL50%andCL95%definition

Pcable

➔ CL=10log————Pantenna

CL95%

CL50%

powerinsidethecablepowerreceivedbytheantenna


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5

2. RADIATEDANDCOUPLEDMODECABLES

AllofthedifferenttypesofRCsarebasedontheeffectsofsphericalwavesexcitedbytheleakageproducedbytheaperturesintheexternalconductor.Theresultingfiel atacertaindistanceisgivenbythevectoradditionalloftheaperturescontributions.TheFigure2showsasimplecasewheretheelectromagneticwavesgeneratedbyonlythreeaperturesareconsidered.Thesewavesareidentifie bythesymbolW1,W2andW3.Let’sconsidertheresultingelectromagneticfiel atapointP.

ThevectorsE1,E2andE3inFigure3representtheelectricfiel componentatthispointPcorrespondingtoW1,W2andW3respectively.ThevectorEistheirresultant.Withoutspecialprecautions,theaperturecontributionsmaygiverisetodestructiveinterferencesatsomeplaceswithalowresultingfiel indicatedbyarelativelyshortvectorasshowninFigure3(leftside).Conversely,aperturecontributionsmaybeinphaseatotherplaces,whichgiverisetoconstructive interferences,henceastrongresultantasshowninFigure3(rightside).ItisclearthattherationaleinFigure3appliestobothelectricandmagneticfiel components.

Therealsituation isobviouslymorecomplexastheelectromagneticfiel atanypoint istheresultsofmorethan3aperturescontributions.Therationaleishoweveridenticalanditexplainsthefluctuation ofthefiel strengthalongtheRC.Typically,thesefluctuation reach20to30dBpeaktopeakandcanbemodelisedbyaRayleighdistribution.Usually,thedifferencebetweenCL95%andCL50%rangesbetween10and13dB.

Figure2:Electromagneticwaves

duetotheaperturesintheexternalconductor

W1W2W3

P


6

Theradiated modeRCsaredesignedtoproduceacoherent interferenceofthedifferentaperturescontributions incertainfrequencybandsandatallplacesaroundtheRC.Thiseffect isobtained iftheaperturespacingischoseninsuchawaythatalltheaperturecontributionsaddupinphaseintheRCradiocoverageareaasshowninFigure3(rightside).Thisisachievedifthedelaybetweenthecontributionsoftwosuccessiveaperturesisamultipleofthesignalperiod.Whenthisconditionissatisfied theresultantfiel isstrongerandthefiel strengthfluctuation alongtheRClengthareconsiderablyreduced.Itresultsthat:

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

Withradiatedmodecables,themaindifficult istomaximisethefrequencybandinwhichtheaperturescontributionsinterfereinacoherentway.

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

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

E1

^

E2

^

E

Ê

^

E2

^

E1

^

E3

Ê3

^

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. It involvesaGSM900radiocoverage inadual-boretunnelthat is900m in length. Itshallbeassumedthat:

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

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

lossisequalto1.5dB;➔ thespecificatio 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.Thelastlineindicatesthatthespecificatio issatisfied 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:Itresultsfromthespecificatio ofthisparticularexamplethatthepenetrationlossandmobileantennalossrelativetodipoleareequalto0dB.

RCinbore1

RCinbore2


8

3.1. RC insertion loss

The RC insertion loss is equal to the cable length multiplied by the longitudinal attenuation. Thislongitudinalattenuationissomewhatinfluence bythestandoffdistance(betweentheRCandthewallorceilingtowhich it ishung).Forexample, iftheRC isdirectlyagainstaconcretesurface,the impactonthelongitudinalattenuationisfrequencydependentandisobviouslynotidenticalforallRCs.IftheIECstandardconditionsareconsideredasreferencevalues,measurementscarriedwithvariousEupenRMCsindicatethatinstallingtheRCdirectlyagainstaconcretesurfaceinvolvethefollowinglongitudinalattenuationincreases:

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

The longitudinalattenuation isalso influence byhumidityanddustdepositontheRC jacket.Even inrathersevereconditions,thelongitudinalattenuationincreaseneverexceeds10%.

3.2. RC Coupling loss

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

TheCL50%andCL95%specifie intheEupendatasheetsaremeasuredwiththegroundlevelmethodaccordingthe IEC61196-4standard1.Theground levelmethodhasbeenpreferredbecause itdefine conditions which are closer to those actually met in practice. Indeed, in almost all the applications,theRCishungatshortdistancefromasurface(ceilingorwall).Adetailedanalysisofthisissueispresentedinsection4.

However,CLsmeasuredwiththefreespacemethodarealsoavailableforsomeEupenRCs.

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

1IEC61196-4standard-Coaxialcommunicationcables-Part4:Sectionalspecificatio forradiatingcables.


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a) E.M.wavedepolarisationduetoreflection onobstacles

In thecaseofcommunicationswithhand-heldmobileequipmentonboard train,awavepenetratingintoacarriageexperiencesreflection onthecarriagewalls,ceiling,floo ,seats,etc.Ateachpoint,thefiel strengthisthevectoradditionofseveralwavesandthepolarisationofthesumcanbeconsideredaselliptical ratherthan linear.Figure5showsthesimplecasewhereadirectwave radiatedbytheRCinterfereswithanotherwavewhichhasbeenreflec edbythecarriageceilingandwindow.Thedashedarrows(atrightanglewiththedirectionofpropagation)indicatethewavepolarisation.Ifweconsiderthedifferenceofpropagationdelay,itisclearthatthepolarisationoftheresultingE.M.fiel atthereceptionpointR isverycomplex.Figure5 isavery simplecasewithonlyone reflec edwave. Inpractice, thesituationsaremuchmorecomplexassuggested inFigure6wherethedirectwavemaybeblockedbytravellersorbyanothertrain.

b) Mobileantennaorientation

Withhand-heldequipments,themobileantennaorientation isneitherperfectlyverticalnorhorizontalbutratheracombinationoftheseasshowninFigure7.Indeed,innormaluse,anhand-heldequipmentisslightlydown-tiltedandnotnecessarilyorientatedformaximumresponse.

Figure5:Wavedepolarisation

duetoreflection

Figure6:Propagationintocarriage

Figure7:Mobileantenna

orientation

Figure6 Figure7


10

c) Mobilestationareratherinsensitivetoantennaorientation

Asexplainedinsection4.2,thedifferencebetweentheCLsinthedifferentantennaorientationisduetothedirectivityofthehalf-wavedipolewhichisusedforCLmeasurements.

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

If the symbols CLr, CLp and CLo designate the coupling losses in the radial, parallel and orthogonalorientationrespectively(whichevertheprobability level)and ifthesymbolCLmeancorrespondstothemeancoupling lossasdefine in the IEC61194-4standard, it isshown insection4.2 thatCLmean 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%isspecifie at2moftheRCaccordingtotheIEC61196-4standard.Ifitisrequiredatanotherdistance,thefollowingcorrectionshouldbeapplied:

FortheCL95%,alongerdistanceinvolvesastrongerinfluenc ofscatteredradiationsandreflection onwallsandceiling,henceafadingincrease.Thefollowingcorrectioncanbeapplied:

3.4. Penetration loss

For communications into vehicles, the link budget must take a penetration loss into account. Thispenetrationlossisstronglyinfluence bythefrequency,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 orientationsprovidesasafetymarginwhich is4dBsuperiortoabudgetcalculatedwiththevaluemeasured inthebestorientation.

d➔ CL50%(d)=CL50%+10log(——)

2

d➔ CL95%(d)=CL95%+20log(——)

2


4.1. Measurement procedures

TheproceduretomeasuretheCLisdefine byanIEC61196-4standard.Twoconfigu ationsarepermitted,i.e.:the“ground–levelmethod”andthe“free-spacemethod”.Thesetwoconfigu ationsoftengiveresultsthatmaybequitedifferent.That isnotsurprisingas it iswellknownthattheenvironmentaffectstheRCperformances.Asexplainedisthissection,theground–levelconfigu ationisclosertotheconditionsactuallyfoundintunnels.

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

ThefactthattheIECstandardisnotveryrestrictingmaybeconfusing,especiallywhentheperformancespublishedinthemanufacturerdatasheetshavetobecompared.Someclarification areprovidedhereaftertoassisttheradioengineerinmakingthemostaccuratelinkbudgets.

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

Intheground–levelmethod,theRCislaidat10to12cmaboveaconcreteground.Thecentreoftheantenna ispositionedverticallyat2mabovetheRC.Thefiel strength isrecordedwhenmovingtheantennaalongapathparalleltotheRC.

Inthefree-spacemethod,theRCishungtononmetallicpostsataheightof1.5to2m.Theantennacentreisat2mfromtheRCandatthesameheight.Thefiel strengthisrecordedwhenmovingtheantennaalongapathparalleltotheRC.

TheFigures8and9alsodefin thethreeantennaorientations,i.e.:➔ Radial: thedipoleisorientatedatrightanglewithrespecttotheRCandisinthesameplane;➔ Orthogonal: thedipoleisatrightanglewithrespecttotheplanecontainingtheRC;➔ Parallel: thedipoleisparalleltotheRC.

Figure8:RCandantennapositionswithground-levelmethod

Figure9:RCandantennapositionswithfree-spacemethod

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4. COUPLINGLOSS


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Figure10:Reflectio mechanism

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-spaceconfigu ation,thegroundisat2mandistoofartoefficient ypromotethecoupledmode.

Itmustberemindedthattheradiatedmodecablesworkincoupledmodebelowthetransitionfrequency(whichdependsontheRCdesign).Consequently,theaboveremarksarealsoapplicabletotheseRCswhentheyareusedbelowtheirtransitionfrequency.

Radiated mode cablesFortheRCsworking inradiatedmode,CLdifferencesof2or3dBbetweenthetwoconfigu ationsareusualbutrarelyexceed6or7dB.Thisdifferencemaybeeitherpositiveornegative,dependingonRCdesignandfrequency.

TheCLdifferencesaremainlyduetotheeffectofthereflection onthegroundsurface.Indeed,intheground-levelconfigu ation,thereflection producedbytheconcretesurfacelocatedat10to12cmfromtheRChavearelativelyimportanteffect.Thereflectio mechanismisshowninFigure10whereonlyonesingleapertureAhasbeenconsideredforsimplicity.AtanypointPintheRCvicinity,thefiel strengthisthevectoradditionofthefiel radiatedbytheapertureA(hereaftertermed“directwave”)andtheonereflec edatthepointRbytheconcreteground.

Themagnitudeoftheresultingfiel willdependon:

➔ the magnitude of the reflected signal: thismagnitudedependson surfaceconductivity.Thereflectio coefficien mayrangebetween0(noreflection and1foraperfectlyconductivesurface.

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

P

A

R

RC

Concreteground


In addition there is a possible phase shift at the reflectio point R. This phase shift depends on thedirectionof theelectric fiel andon theelectricalpropertiesof theconcrete surface. In thecaseofa perfectly conductive surface, there is no phase shift for the component of the electric fiel whichisorientatedatrightanglewiththeconcreteground.Conversely,thecomponentoftheelectricfiel paralleltothegroundexperiencesa180°phaseshift.

Figure11showshowthereflection impacttheCL.InthisFigure,Ěd,Ěr,andĚ designatetheelectricfiel vectoratthepointPcorrespondingto,respectively,thedirectwave,thereflec edwaveandtheresultantfield TheleftpartofthisFigureshowsthecasewherethevectorscorrespondingtothedirectwaveĚdandreflec edwaveĚrarenearlyinphase.ThemagnitudeoftheirresultantĚbeinghigherthanĚd asthereflectio reinforcesthedirectwave,henceaCLdecrease.

Conversely,therightpartofFigure11showsthecasewherethevectorscorrespondingtothedirectwaveĚdandreflec edwaveĚrarenearlyinopposition.ThemagnitudeoftheirresultantĚislowerthanĚd,henceaCLincrease.

Although it has been assumed, in Figure 11, that the reflectio coefficien was lower than 1 (the ĚrvectorisshorterthanĚd),itisobviousthattheaboveconclusionsapplywhicheverthemagnitudeofthereflec edwave.

WiththewidebandRCs,itisnoteasytokeepthedirectandreflec edwavesinphase(ornearlyinphase)inallthefrequencybandsasthisparameterdependsonl.

Inthemostfavourablecase,i.e.whenthereisatotalreflectio (reflectio coefficien =1)inphasewiththedirectwave,theresultantĚ=2Ěd,hencea6dBCLdecrease.

Conversely, theworstcaseoccurswhere there isa total reflectio inoppositionwith thedirectwavebecausetheresultingfiel dropssharply,henceasevereCLincrease.Inpracticehowever,theresultingfiel doesnotcollapsecompletelyandtheCLincreaseshouldnotexceed20dB.

14

Figure11:Vectoradditionofthedirectandreflec edwaveswhentheyarenearlyinphase(ontheleft)andnearlyinopposition(ontheright).

Ed

Êd

^

Er

^

Er

^

E

^

E

^

ĚdandĚrnearlyinphase ĚdandĚrnearlyinopposition


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Comparedtoasituationwherethereisnoreflection theground-levelconfigu ationmayeitherproduceaCLdecreaseofmaximum6dBoranincreasethatshouldnotexceed20dB.

Inthefree-spaceconfigu ation,therearealsoreflection onthegroundsurfacebuttheireffectismuchless important as illustrated in Figure 12. Of course, if the RC is at 2 m above the concrete ground,thereflection canbeseenasproducedbyanelectrical image locatedatapproximately24.5moftheantenna. As the electromagnetic fiel decreases with the inverse of the distance, the magnitude ofthereflectio isabout0.44times (i.e.2m/4.5m)themagnitudeofdirectwavewhenthereflectio coefficien isequal to 1.Itresultsthat,ifthedirectandreflec edwavesareinphase,theCLdecrease3isabout3.2dB.Iftheyareinopposition,theCLincreasedoesnotexceed5dB.

Ifthereflectio coefficien islowerthan1,themagnitudeofthereflectio andtheimpactontheCLsisreducedaccordingly.

Itresultsthattheimpactofthereflection arelessimportantinthefree-spaceconfigu ationthanintheground-levelonewherethedirectandreflec edwavestravelnearlythesamedistance.Thisconclusionapplieswhicheverthevalueofthereflectio coefficient

4.1.2. Ground–level versus or-space methodInmostapplications,theRCishungatshortdistancefromasurface(ceilingorwall)producingreflection whichmayeither improveor impairtheCLs. It isobviousthattheground-levelmethod isclosertotheconditionsactuallymet inpractice.This is the reasonswhy theground-levelmethodseems themostsensibletoreferto.

4.1.3. Eupen RMC RangeTheEupenRMCrange isdesignedtoderivebenefi fromthereflectio phenomenon,at least inmostfrequencybandsallocatedtomobilecommunications.Thisisachievedbychoosingalaunchinganglethatminimisesthephasedifferencebetweendirectandreflec edwaves.

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

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

Figure12:Reflectiomechanismwith

free-spacemethod R

RC

Concreteground

diplode


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-spaceconfigu ationsrespectively.

MeasurementresultsindicatethattheCLdifferencebetweentheworst(highestCL)andthebest(lowestCL) orientationsmayexceed10andeven15dBinsomecases.TheexplanationofthiseffectisgiveninFigure13whereitisassumedthataverticallypolarisedelectromagneticfiel propagatesfromthelefttotherightasindicatedbythevectorv�.Thethreeconsideredantennaorientationsareandidentifie bythelettersa,bandc.

Iftheantennaarmsareorientatedhorizontallyandparalleltothedirectionofpropagation(lettera),theresponseshouldbetheoreticallynullbecausethemain lobeoftheradiationpattern ispointing intheverticalplane.

Iftheantennaarmsareorientatedhorizontallyandparalleltothedirectionofthemagneticfiel (letterb),theradiationpatternispointingtowardthesourceofthefiel buttheresponseshouldbetheoreticallynullbecausethearmsareperpendiculartotheelectricfield

Themaximumreceivedsignalisobtainedwiththeantennaarmsorientatedvertically(letterc).Indeed,theradiationpatternispointingtowardthesourceofthefiel andtheantennaarmsareparalleltotheelectricfield

The fact that the fiel produced by a RC is polarised explains the strong influenc 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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TheIECstandardalsodefine ameanCLcalculatedwiththefollowingformula:

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’sconsideranRCfeedwithan inputpowerequalto1mW(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’sconsiderthesimplecasewheretheelectromagneticfiel isperfectlypolarisedinonedirection,forexampletheparallelone.ThisinvolvesthatonlyCLphasafini evaluewhileCLrandCLo=-∞.

As10-∞=0andas-10log[1_3

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

]-10log[10-CLp/10]

Weobtainfinal y:

CLmean = 4.8 + CLp

Intheactualsituationshowever,thefiel isnearlyneverpurelypolarisedinonlyonedirection(i.e.thereisnodirectionforwhichthereceivedpowerisnull).Forinstance,ifCLr=60dB,CLo=70dBandCLp=70dB,weobtainCLmean=64dB.Othernumericalexamplesconfi mthattheCLmeanisgenerallyabout4dBhigherthanthelowestCL.Inconclusion,forpractice,thefollowingapproximationcanbemade:

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

wherethe“min”symboldesignatestheminimumofthe3valuesinbrackets.

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


4.2.2. Antenna orientation and link budgetAsstatedabove,theCLinthe“worstorientation”maybe10to15dBhigherthaninthe“bestorientation”.Thisismainlyduetothefactthatthemeasuringantennaisahalf-wavedipolewhichfeaturesdirectivity.Indeed,theradiationpatternisthetypicaleightfigu ewithanullresponseinmedianplane.Consequently,theCLdependsonthedirectionofpropagationofthewaveradiatedbytheRCandontheorientationoftheelectricalfiel asshowninFigure13.

Inpracticehowever,theantennaorientationisnearlyneverperfectlyradialorparallelororthogonalwithrespecttotheRCbutratheracombinationofthesethreepossibilities. Indeed,themobileantenna isoftendowntiltedandisrarelyinanRCplane.

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

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

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11/2021

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5. RCPERFORMANCESOPTIMISATION

5.1. RC positioning

Themobileantennapositionandorientationareimportantparametersforperformancesoptimisationofradiocommunications inconfine spaces.Mobileantennamountedonthevehicleroofandhand-heldequipmentson-boardtrainarethemaincasesencounteredinpractice.Theyaredetailedhereafter.

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

Astheelectricfiel radiatedbytheRChasastrongradialcomponent,thebestcouplingisobtainedwiththeRChungfromthetunnelceilingandpreferablynearthecentrepositionoratleast1mawayfromthesidewallsasshowninFigure15.

The lowest CL and fiel strength fluctuation 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


Figure17:RecommendedRCpositionsforcommunicationwithpassengersonboardtrain

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

Theradiowavesenterscarriagesonlythroughthewindowswithapenetration losswhichdependsonglassmaterial(numberoflayers,metalcoating,...)andwindowsizes.

Inallcases,thebestcouplingisobtainedwiththeRChungalongawallasshowninFigure17(hand-heldontheRCsideandhand-heldontheoppositeside).ItisrecommendedtohangtheRCapproximatelyatthesameheightastheupperedgeofthecarriagewindowsasshowninthesefigu es.

Again,thelowestCLandfiel strengthfluctuation areobtainedwiththeapertureslocatedonmobileside.

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Figure16:Hand-heldequipmentorientation


11/2021

21

Wherethespecification imposeasecondRC,oneofthefollowingsolutionscanbeusedtomeetthereliabilityrequirementswithoutloosingthebenefi oflowfiel strengthfluctuations

➔ 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.

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 configu ation where two nearby RCs are fed with thesameRF source.This solutionwillgive rise to large fiel strength fluctuation due 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 configu ationmust absolutely be avoided.


RadiatingCables

11/2021Applicationnote

5.3.Resonant frequencies

Due to the fact that Radiating Mode Cables are based on a slot design with evenly spaced, repeating slots, it is a physical rule that this will lead to Resonances.

On Eupen’s EUCARAY® Radiating Cables, it results in clearly identified Resonant Frequencies. In most cases, the Resonant Frequencies occur in non-used Bands of the Radio Spectrum, and the even multiples are actually suppressed once the Cable has been installed. In addition, their magnitude generally decreases the higher the order.

It is good practice to verify that the Resonant Frequencies of a chosen EUCARAY® Radiating Cable do not appear in the required and used RF-Band. However, should a Resonant Frequency be found in the RF-Band to be used, it does not necessarily result in a Radiating Cable being unsuitable. On Eupen EUCARAY® Radiating Cable, Resonant Frequencies occupy only about 1 MHz of bandwidth, so that the transmission at frequencies located before and after a particular Resonant Frequency is not impacted. Further, the result at the said Resonant Frequency will typically be a slight increase in the Longitudinal Loss on the Downlink and a slightly raised VSWR in the Uplink. In both cases, the impact on the working of an RF System should be quite marginal. To compare, one should also bear in mind that a well-matched Antenna will have a typical VSWR of 1.5:1.

Resonant Frequencies versus Stop Bands

Resonant Frequencies on Eupen EUCARAY® Radiating Cables should not be compared to Stop Bands of other Manufacturers Cables. Such Stop Bands, or unauthorised Frequencies, usually occupy a much wider bandwidth, and the high level of reflected signal – with VSWR typically >> 10:1 – makes it impossible to use those Cables within the given frequency band.

The illustrations 1 & 2 below show the actual impact the Resonant Frequencies have on the VSWR and the Attenuation of one Eupen RMC-type Radiating Cable.(*)

The illustrations 3 & 4 show, at same scale, the impact of a Stop Band on a competitor cable.(*)

(*) Example with 1-5/8” size, broadband radiating cables.

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EUPEN CABLE

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Asaleadingsupplieroftransmissionlinesandaccessoriestoglobalwirelesscommunicationsmarkets,EUPENhastheexperienceandresourcestoeffectivelyservicecustomersintoday’schallengingwirelesscommunicationsmarkets.

Atatimewhenwirelesscommunicationinconfinedareas,suchasunderground,streetandservicetunnels,isimportanttothenetworkoperators,EUPENhighqualityradiatingcablesprovethatEUPENCableisyourreliablePartnerforRFCommunications!

DownloadyourowncopyofthelatestRadiatingCablecatalogueat:www.radiating-cables.com

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[email protected] [email protected]

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