near field focusing for nondestructive microwave testing at 24 ghz theory and experimental verification

Sectoral horn with metal plate and 2 holes The basis ofthisantennawas an E planehorn forthe K band withan aperture sizeof 20 mm×4.3 mm.. Electric field distribution of an E plane horn wit

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Christian Ziehma, Sebastian Hantschera,∗ , Johann Hinkenb, Christian Ziepb,

Maik Richterb

aMagdeburg–Stendal University of Applied Sciences, Institute of Electrical Engineering, Breitscheidstrasse 2, 39114 Magdeburg, Germany

bFI Test- und Messtechnik GmbH, Breitscheidstrasse 17, 39114 Magdeburg, Germany

a r t i c l e i n f o a b s t r a c t

Article history:

Available online 9 November 2016

Thispaper describesthe developmentofdifferentnovelantennaconceptsforimproving the spatial resolution of microwave basednon-destructive testing (NDT) at24 GHz.In

agreatnumber ofapplications the antennaofthe sensor canbe brought verycloseto the deviceunder test Inthese cases,the near field characteristics ofthe antennas are crucialfor ahighresolution However, commonsensor headsoffer eitherahigh image resolution orahigh penetrationdepth Inorder tocombine bothof the characteristics different antenna conceptshave been developed.The objectives were to obtaina high returnlosscombined with asufficienthighdynamicrangeand anear fieldfocusingof electromagneticwavesinordertoyieldahighresolution.Altogether,threeantennashave beensetup.Eachantennahasbeencalculatedanalytically,followedbyaFEMsimulation, nearfieldmeasurementsandanexperimentalverification

©2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCC

BYlicense(http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Microwavenon-destructivetestingisoftenusedfortheinspectionofcomponentsorconstructionsconsistingofdielectric materials When the object is radiated with electromagnetic waves, the reflected or transmitted signal is received and processed.Mostimagingmethodsarebasedonthesyntheticapertureprinciplegivingcross-rangeresolutionsintheorder

ofthewavelength.Toimprovetheresolution,theradiationpatternandthenearfieldfootprintcanbemeasuredandused for the image calculation [1–3] It isoften possible to bring the sensor very close to the device under test (DUT) such that the nearfield characteristicsofthe antennadirectlyinfluence theresolution aswell asthe depthin whicha defect can still be detected [4] One common approach is to use open waveguides [5] Despite the relatively low return loss, open waveguides offermoderatepenetration depthsoftheelectromagneticwavesintotheDUT.Thatcanbeimprovedby horn antennas.However, dueto the shorter distancefrom the phase centreto the middle of theaperture compared to thedistancetotheapertureedge,thehornhasbadsidelobesuppression.Otheroptionsarecoaxialprobesthatproducea smallantennafootprint atshortdistancesto thebenefitofhigherresolutions [6].Theyare mostly usedforthedetection

ofdefectsnearthesurface.ThedisadvantageisthattheradiatedfieldscannotpenetratedeepenoughintotheDUT.Thisis alsotruefortaperedwaveguideswithslitapertureforimagecontrastimprovement [7]orknifebladesasscanningprobes

at millimetre wavelengths [8].For the same purpose,dielectric rod antennas havebeen optimised for spot-focusing[9]

*Corresponding author.

E-mail address:sebastian.hantscher@hs-magdeburg.de (S Hantscher).

http://dx.doi.org/10.1016/j.csndt.2016.10.002

2214-6571/©2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license

Fig 2 Electric field distribution of two magnetic elementary antennas.

However, theseantennassuffer fromthedrawback that onlynearsurface defects (suchascorrosionpitting underpaint) can be evaluatedwithdielectric slab-loaded waveguides [10].In orderto facilitatedeeper penetration,metal plateswith definedslotsordielectriclensesinfrontofawaveguidewereused [11,12].Thistrade-off betweenpenetration depthand spatialresolutionistypicalformicrowavereal-apertureimagingradarmethodsundernearfieldconditions [13]

Inthispaperthreeimprovedantennaconcepts weredevelopedandtheir advantagesandlimitationscomparedto con-ventionalwaveguidesarediscussed.Especiallytheantennadescribedinthefourthsectionattaineda6 mmbetterresolution

atsimultaneouslyhigherpenetrationdepthandimproveddynamic rangeandreturnloss.Thegreatestbenefitofthis con-ceptistheindependenceofthefocusingcharacteristicsofpermittivityoftheDUT.Forexperimentalverificationofthenear fieldcharacteristicsofthedevelopedantennas,theElectromagneticInfraredmethodEMIRhasbeenused [14]

2 Sectoral horn with metal plate and 2 holes

The basis ofthisantennawas an E planehorn forthe K band withan aperture sizeof 20 mm×4.3 mm Thehorn aperturewascoveredbyametalplatewithtwoholesasradiatingelements(Fig 1)

Theholeswerepositionedsymmetricallyinrelationtothecentreofthewaveguidesuchthattheelectromagneticwaves frombothholessuperimpose constructivelywithoutanyphase shiftalong thez axis.Eachholecan bemodelledingood agreementwiththepracticebyamagneticelementaryantenna(Fitzgeralddipole).Theazimuthcomponentoftheelectric fieldofradiator1(rightholein Fig 1) E h1 ϕ locatedat(x,z = (x h1,0)isgivenby

E h1 ϕ =E0· −e−j·

2π

λ·r1

4π ·



1

β ·r1+ 1

j· β2·r2



where E0 is a reference field strength, r1=  (x P−x h1)2+y2+z2 is the distance from the hole to the computation point (x P,y P,z P),λ isthe free-space wavelength andthe θ1 is givenby the distance fromthe aperture to the compu-tationpoint[15].Theelectricfield E h2

ϕ canbe obtainedsimilarly.Theresultingfieldisgivenbyasuperimpositionofboth fields

Fig 2showsthemagnitudeoftheelectricfielddistributionfortwoholeswith12 mmdistancetoeachother,normalised

tothemaximumelectricfieldthatoccursinvicinityofthetwoholesatx= ±6 mm.Asecondmaximumoccursat(x,z =

(0,5 mm)duetotheequalphase superimpositionofbothfields.However,atadistanceofz=10 mm theratiobetween

Fig 3 Electric field distribution of an E plane horn with two holes in the metallic aperture.

Fig 4 EMIR measurement setup for near field measurement.

thefieldstrengthsatx=0 andx= ±6 mm becomesamaximumandisdenotedinthefollowingas“sidelobesuppression” similartothefarfieldantennaradiationpattern.ThisdistanceisusedlaterforimagingNDTpurposes.Atshorterdistances, the sidelobesuppressionissolow thatdefectsare imagedtwice Theanalyticalcalculation isverifiedbya finiteelement (FEM)simulationofwhichtheresultisshown Fig 3

Fora measurementbased verification,the radiatednearfieldsofthe antennawererecorded usingtheEMIRprinciple shownin Fig 4

The antennaisdirected toamicrowave absorbingfoilthat heats upslowly whenthe antennastartstransmitting.The resulting heatdistributionis recordedby aninfrared camerathat ispositioned behindthe foil.Thus,the intensityofthe recorded infrared signal is proportional to the magnitude ofthe radiated electromagnetic power ata distance from the antennatothefoil. Table 1showstheresultsatz=5 mm andz=10 mm distance.Themeasurementshowstheexpected focusing ofthe electricfield inx direction. Inthiscase, the halfpowerbeamwidth (HPBW)isjusthalf oftheHPBW of thea simpleopenwaveguide.Alittleasymmetryofthemeasurements isduetoinaccuracies inthemanufacturing ofthe metallicaperture

InordertotesttheantennaforimagingNDTpurposes,asmallcopperplateofonly1 mm×2 mm insizewasmeasured

by movingtheantennaatadistanceof10 mmabove theplate Forpurposesofcomparability,a C-scan(twodimensional scanwith30 mm×30 mm scanarea)withanopen K bandwaveguidewascarriedout.Thesetupisshownin Fig 5 Theantennaandthewaveguidewereconnectedtoavectorialnetworkanalysertoevaluatethechangesofthereflection coefficient during the movement of the antennas.Before starting the measurement, theantenna has to be calibratedto removethereflectionS11oftheantennainfreespace.Thisreflectioncoefficienthasbeensubtractedfromallmeasurements

r meas( x,y)oftheC-scan.Theresultingmagnitudeofthereflectioncoefficientthatisdisplayedintwodimensionsin Figs 6 and7isobtainedby

Fig 6showsanimprovementofthespatial resolutionof10 mmcomparedtotheopenwaveguidein Fig 7.Moreover, themaximumreflectioncoefficientusingthemodified sectoralhornantennaisabout3.3 dBlargercomparedtotheopen waveguide.ThispositivesideeffectofthefocusingleadstoabetterdynamicrangeofNDTmeasurements

However,ingeneral,dielectriccomponentswitharelativedielectricconstant εr>1 areanalysedfordefects.Inorderto analysetheinfluenceof ε onthefocusingcharacteristics,thesmallcopperplatewasplacedundera10 mmthickperspex

Fig 5 Open waveguide above a small metal plate as DUT (red circle).

discwith εr=2.6.Duetotheshorteningofthewavelengthbythefactor√ ε

r≈1.61,thebestsidelobesuppressionisno longerat z=10 mm distance. Fig 8showstheelectricfield distributionfordifferentrelative permittivities.Itisobvious thatthelarger εr istheworsethesidelobesuppressionisatconstantdistancez=10 mm fromtheaperture.For εr=4,the sidelobesuppressionisjust2.2 dBinsteadof6.2 dBforair.Thatmeansthedistancefortheoptimumsidelobesuppression increaseswithincreasing εr

In orderto verifythis effect,thesmall copper platewas positioned undera perspex discof6 mm thickness At this distance,thefocusedfield alongthe z axisislower thantheelectricfieldsoriginatedfromeach holeleadingto awidely spaceddistributionofreflectedenergy.Thisdefocussing leadstoghostartifacts(Fig 9)andthe plateisnolongervisible

Fig 6 Measured magnitude of reflection coefficient of an E plane horn with two holes in the metallic aperture.

Fig 7 Measured magnitude of reflection coefficient of an open waveguide.

Fig 9 Electric field distribution of an E plane horn with two holes in the metallic aperture and 6 mm thick perspex disk.

Fig 10 (a) Drawing of E plane horn with 4 holes in the metallic aperture, (b) vector decomposition of the electric field for one single hole.

3 Sectoral horn with metal plate and 4 holes

Themodifiedsectoralhorndescribedintheprevioussectionfocusedonlyinx direction.Inordertofocusin y direction

aswell,2additionalholesofthesamesizeweredrilledintothemetallicaperture(see Fig 10a)

Tocompute theoverall field distribution, the azimuthal componentof the electricfield is calculatedaccording to (1) andsplit up into y and z direction. Figs 10band10c aswell asequation (3) show thisdecompositionof the field E h1

ϕ

exemplarilyinelevationdirectionforthefirstholeonposition(x h1, y h1,0)

wherebytheangleβ isobtainedbysimpletrigonometricrelationsindependencyofthecomputationpoint(y P,x P, z P)

β = π

2 −tan−1y P−y hk

z P

(4) Afterthat,thetotalelectricfieldE totalisgivenbyasuperimpositionofthefields(denotedbytheindicesh1 to h4)ofall fourholes:

E total=





 





4

k=1

E hk y







2

+ 





4

k=1

E hk z







2

(5)

Fig 11a shows the computedelectricfields at z=10 mm distance to the aperture withfour holes As expectedthe additional two holes yield to a focusing in x and y direction. The simulatedresults are verified by an FEM simulation shownin Fig 11bandanEMIRmeasurementshownin Fig 11c.Theoryandmeasurementagreewellwithoneanother.The resolutionin y directioncouldbeimprovedby20 mmcomparedtothesectoralhornwithtwoholes

However,themaindrawbackofthisdesignisthatthemagnitudeofthereflectioncoefficientatthefeedingpointinthe

K bandwaveguideisveryclosetoone.Thus,onlyverylittlepowerisradiatedbecausethesizeoftheholesiscomparably smalltothe aperturesize.Withthe helpoftuning screwsinthewaveguide itispossibleto tunetheantennato around

50butthisdoesnotchangethatradiatedpower.Suchanarrangementresemblesaresonatorwheretheelectromagnetic energyoscillatesbetweentheapertureandthetuningscrews

Fig 11 Electricfield distribution of anE planehorn with 4 holes in the metallic aperture atz=10 mm distance to the aperture, (a) computed, (b) Simu-lated (FEM), (c) measured with EMIR.

Fig 12 Dielectric delay lens on the aperture of a horn antenna[15]

4. E planehorn with dielectric delay lens

In order to increase the transmittedpower and the dynamic range, a dielectric delay lens can be inserted in to the

E plane horn This lens compensates the unequal phase assignment on the aperture that has its origin in the different distancefromthephasecentretodifferentpointsontheaperture. Fig 12depictssuchalensmadeofamaterialofrelative permittivity εr

Moreover,thebestfocusingalongthez axisisachievedwhentheamplitudeassignmentontheapertureishomogeneous However, considering a regular E planehorn antenna, the electricfield distribution onthe aperture isgiven bythe H10 mode

E(x,y) =E0·cosπ ·x

Forthisreason, twometallicshields coverpartsoftheaperture andthus reducethe transmittedpowerinthemiddle

oftheaperture.The shapeoftheshieldsgivenbytheir distanced(x)isinverseproportional tothemagnitudeofthe H10 modeandhasamaximumvalueequaltothewidthb oftheK bandwaveguide:

d(x) =min b, d0

cosπ·x A

(7) The distanced0 in the middleof the aperture influences the transmitted power Fig 13 showsthe sector horn with dielectriclensandmetalshield

InordertoevaluatetheresolutionofthisantennaforNDTpurposes,thesmallcopperplateshownin Fig 5wasscanned

atadistanceof10 mm.Theresultisdepictedin Fig 14.Comparedtothemeasurementwithanopenwaveguidein Fig 7

thebetterfocusingleadstoaresolutionthatisabout6 mmbetter.Moreover,duetotheconstantelectricfielddistribution (bothinmagnitudeandphase)ontheaperturethisantennaiscomparablyindependentfromthedielectricconstantofthe DUT

5 Conclusion

Thispaperdescribedthetheoreticalanalysisandtheexperimental verificationofnovelantennaconceptsfornearfield imagingmicrowaveNDTpurposeswithaspecialfocusontheresolutionat24 GHz.Currently,open K bandwaveguidesare

Fig 14 Magnitude of the reflection coefficient using the antenna inFig 13

oftenusedforthis.Toimprovethenearfieldfocusing,thewaveguidecanbeconnectedtoanE planesectoralhornantenna However,thefootprintistoolargesuchthattheimageresolutionisdegraded.Toovercomethisdisadvantage,theaperture

ofthehornantennawascoveredbyametalplatethatcontainstwoholessymmetricallytothemainradiationdirection.By constructivesuperimpositionofthefieldsradiatedbyeachhole,theresolutioncouldbeimprovedsignificantlyinx direction.

Foratwodimensionalfocusing,twoadditionalholeshavebeenaddedtotheaperture.Measurementsoftheradiatednear field aswell asrealmeasurements witha smallcopper plateforthe analysisoftheresolution confirmthe theorybased

onmagnetic elementaryantennas.Moreover, theanalysisshowedthat thefocusing distanceisdependenton therelative permittivity ofthe DUT.Based on theside lobesuppression aswell asthe relativepermittivity,an estimation aboutthe depthofapossibledefectintheDUTcanbe determined.Ataknowndepthofthedefect,therelativepermittivity canbe estimatedbasedon theobtainedimage.However, thisantennaradiatesonly littlepowerandthus degradesthe dynamic range of the imaging measurement That is why an antennawith a larger opening of the aperture for electromagnetic waveswassetup.ThesolutionwasasectoralE planehornantennawithadielectriclensandametalshield.Bothensure

ahomogeneous distributioninmagnitudeandphaseoftheelectricfieldattheapertureofthehorn.Theresolutionofthis antennais 6 mmbetter comparedto the standardopen waveguide.Moreover, thedynamic range could be improvedby

3 dB.Afurtherveryimportantadvantageisthatduetotheequalphasedistributionontheaperture,theresolutionofthis antennaisindependentofthepermittivityoftheDUT

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