Understanding and optimizing microstrip patch antenna cross polarization radiation on element ...

18 Pages·2015·14.3 MB·English

Adv.RadioSci.,13,251–268,2015
www.adv-radio-sci.net/13/251/2015/
doi:10.5194/ars-13-251-2015
©Author(s)2015.CCAttribution3.0License.
Understanding and optimizing microstrip patch antenna cross
polarization radiation on element level for demanding phased array
antennas in weather radar applications
D.Vollbracht
FacultyofElectricalEngineeringandInformationTechnology,TechnischeUniversitätChemnitz,09126Chemnitz,Germany
Correspondenceto:D.Vollbracht([email protected])
Received:21December2014–Revised:6March2015–Accepted:16March2015–Published:3November2015
Abstract.Theantennacrosspolarizationsuppression(CPS) providenewinsightinunderstandingthegenerationofx-pol
is of significant importance for the accurate calculation of componentinmicrostrippatchantennaradiationpatterns.
polarimetricweatherradarmoments.State-of-the-artreflec-
tor antennas fulfill these requirements, but phased array an-
tennas are changing their CPS during the main beam shift,
off-broadside direction. Since the cross polarization (x-pol) 1 Introduction
of the array pattern is affected by the x-pol element factor,
thesingleantennaelementshouldbedesignedformaximum Dual polarized phased array antennas for weather radar ap-
CPS, not only at broadside, but also for the complete angu- plicationindenseradarnetworksarecurrentlyunderdiscus-
larelectronicscan(e-scan)rangeofthephasedarrayantenna sion (Vollbracht, 2014). Such antennas promise even faster
mainbeampositions. threedimensionalvolumescanningcomparedtostate-of-the-
Different methods for reducing the x-pol radiation from arthighpowerradarsystemswithreflectorantennasandtheir
microstrip patch antenna elements, available from literature twin axis mechanical drives. The change from parabolic re-
sources, are discussed and summarized. The potential x-pol flector antennas with fixed beamwidth and gain values to
sourcesfromprobefedmicrostrippatchantennasareinves- phased array antennas with inherent scan angle dependent
tigated.Duetothelackofliteraturereferences,circularand co/x-polpatternsishighlychallenging.
squareshapedX-Bandradiatorsarecomparedintheirx-pol Weather radar systems with dual polarization capability
performanceandthemicrostrippatchantennasizevariation observeechoesbackscatteredbyhydrometeorsfromillumi-
wasanalyzedforimprovedx-polpattern. nated volumes to classify the precipitation, primarily with
Furthermore,themostpromisingtechniqueforthereduc- horizontallyandverticallypolarizedelectromagneticwaves.
tion of x-pol radiation, namely “differential feeding with Foraccuratepolarimetricechomeasurementsandtheirinter-
two RF signals 180◦ out of phase”, is compared to single pretation, the antenna CPS performances for both polariza-
fed patch antennas and thoroughly investigated for phased tion planes are of significant importance. Depending on the
array applications with simulation results from CST MI- CPS, the estimation of the Differential Reflectivity (ZDR)
CROWAVE STUDIO (CST MWS). A new explanation for and the Linear Depolarization Ratio (LDR) can be biased.
theexcellentportisolationofduallinearpolarizedanddiffer- High-qualityreflectorantennasprovideCPSvaluesofupto
entialfedpatchantennasisgivengraphically.Theantennara- −30dBinS-C-X-Band,withinthehalfpowerbeamwidth.
diationpatternfromsinglefedanddifferentialfedmicrostrip A new phased array weather radar antenna should provide
patchantennasareanalyzedandtheshapesofthex-polpat- the same CPS on broadside, and even more challenging,
ternsarediscussedwiththewell-knowncavitymodel.More- the same CPS over the complete e-scan range. The e-scan
over, two new visual based electromagnetic approaches for ranges stated from (Vollbracht, 2014) are 120◦ azimuth and
theexplanationofthex-polgenerationwillbegiven:thefield 30◦elevationforoneplanarphasedarrayantennapanel.One
line approach and the surface current distribution approach 3-Dvolumeradarscannerpresentedfrom(Vollbracht,2014)
PublishedbyCopernicusPublicationsonbehalfoftheURSILandesausschussinderBundesrepublikDeutschlande.V.
252 D.Vollbracht:Microstrippatchantennacrosspolarizationradiation
would be equipped with three of such planar phased array 2014) is to provide a new understanding of the x-pol radia-
antennastoachieve360◦azimuthvolumescanning. tiongenerationbyobservingthenearfielddisturbances.The
From phased array antenna theory the co- and x-pol an- explanationsofthex-polgenerationprocessinSects.4.2and
tennaradiationpatterncanbecalculatedwiththemultiplica- 4.3canbeunderstoodasanadditionalcontributiontothevi-
tion law that gives the overall antenna pattern as product of sual based near field approach from Bhardwaj and Rahmat-
the element pattern (EP) and the array factor (AF). Conse- Samii(2014),butherewithfieldlinesandsurfacecurrents.A
quentlyspecialattentionshouldbepaidtothedesignofthe comparisonofx-polperformancesofcircularandrectangu-
singleantennaelementanditsEPinordertoavoidunwanted lar/squaremicrostripradiatorsforduallinearpolarizationap-
x-polradiationrightfromtheverybeginning. plications was surprisingly not found in available literature.
AntennamanufacturerstypicallyprovideonlyEandHCut Alsonocapableliteraturereferencecouldbefoundwherethe
radiation pattern. For phased array weather radar applica- patch antenna size variation was analyzed for improved x-
tions this amount of information is inadequate. If the beam polpattern.Consequentlythecircularandrectangularshape
were to be pointed off- broadside- axis, the CPS would be types and the patch antenna size variation are investigated
muchlower.Asaconsequence,allantennasforphasedarray andcomparedinSect.3.3and3.5.
weatherapplicationsshouldbeanalyzedalsointhe45◦/135◦
Cutplanes,sincethehighestx-polradiationmaybeexpected 2.1 Feedingmethodsforlowx-polradiationin
in this region. A detailed x-pol investigation can only be microstriparraydesign
performedbythe3-Danalysisofthecompleteupperhemi-
sphereofthemicrostrippatchelementpattern. There are numerous feeding mechanisms available for
Dualpolarizedmicrostrippatchantennasaresuitablecan- microstrip patch antenna elements. Well-known feeding
didates for phased array weather radar applications. These techniquesareaperture-coupling,probe-feeding,proximity-
antennas are fabricated easily at low cost and have known coupling and insert-feeding. Aperture-coupled patch anten-
radiation characteristics. On single antenna element level, nas are often used to enhance the bandwidth with the radi-
thedifferentx-polreductionmethodsandtheirperformances atingslotsunderneaththepatchradiator.Tofurtherenhance
arenowinvestigatedanddiscussed.Finallynewvisualbased thebandwidth,asecondpatchseparatedwithfoammaterial
electromagneticapproachesfortheexplanationofthex-pol fromthefirstoneandwithslightlydifferentdimensionsand
generationareintroduced. correspondingresonancefrequencyisimplemented.Thesec-
ondpatchhasalsotheadvantageofreducingthex-polradi-
ation,sincethex-polradiationgeneratedbythelowerpatch
2 X-polsuppressiontechniquesonelementlevelknown and the slots are shadowed. The patch antenna design from
fromliterature Liangetal.(2004)showsgreatCPSperformancesforhori-
zontalandverticalportof−30to−35dB,respectively.But
Onlyafewsuppressiontechniquesonantennaelementlevel duetotheaperture-coupledradiatingslotsandthebackradi-
have been published. The authors from Liang et al. (2004), ationtothefeedingnetwork,themutualcouplingaspectbe-
Fulton and Chappell (2011) and Zhou and Chio (2011) use comes critical for array applications. Further disadvantages
differential feed excitation with two signals, 180◦ out of arethesensitivityagainstheighttolerancesofthefoamma-
phase. This method will be explained in detail by the an- terialandthegenerationofsurfacewavesduetotheradiating
tenna prototype in 2.2 and promises to be a viable solu- slotsandthecavitiesbetweenthefoamlayers.Moreover,the
tion for phased array weather radar antennas. The defective design presented in Liang et al. (2004) is working with the
groundplanesolutionpublishedinGuhaetal.(2005)isuse- reflectorgroundplanetechnique,whichisnotusefulforar-
ful for single element applications without complex feed- ray operation with multiple elements and feeding networks
ing networks and dual polarization capabilities. The radia- between.
tionfromthedefectivegroundareainducesamplitudetaper Proximitycoupledpatcheshavesimilardisadvantagesfor
disturbancestothearrayfeedingnetworkandwoulddegrade arrayconfigurationsasaperture-coupledantennas.Theback
the overall antenna performance. Mohanty and Das (1993) radiation close to the feeding networks for two polarization
presentedaprintedthindielectricsubstratewithstripgrating planesisundesiredinarrayconfigurationsandtheportisola-
in front of the radiating antennas for suppressing the x-pol tionwouldbecomeinsufficient.Insertfedmicrostrippatches
radiation.Butstripgratingsolutionsareonlyusefulforsin- havetheirfeedingnetworksonthesamelayerandunwanted
glepolarizationapplications.Recentlyanewinvestigationto x-pol radiation is generated. In keeping with these observa-
explain x-pol radiation from microstrip antennas was pub- tions, insert fed and proximity coupled patch antennas are
lished in Bhardwaj and Rahmat-Samii (2014). Here the x- notsuitableforphasedarrayswithhighx-polrequirements.
polfordifferentialfeeding(180◦outofphase)androtational Probe fed microstrip patch antennas are suitable candi-
feeding(mirroredantennaelementsinsideantennaarray)are dates for the phased array weather radar application with
discussedbyanalyzingthenear-fieldradiationbehavior.The high x-pol requirements. The possibility to drive the single
majorintentionoftheauthors(BhardwajandRahmat-Samii, patch antenna element with 180◦ out of phase and the low
Adv.RadioSci.,13,251–268,2015 www.adv-radio-sci.net/13/251/2015/
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10.32 0.2 0.91 1.47 4.78 edg0e. 42 1133..2200 66..0000 3300xx3300

10.55 0.4 0.91 1.31 5.13 0.42 1144..2200 55..9955 3300xx3300
mm mm mm mm mm mm dB dBi mmmm
10.66 0.5 1.2 1.42 5.19 0.42 1144..7755 55..9944 3300xx3300
10.32 0.2 0.91 1.47 4.78 0.42 1133..2200 66..0000 3300xx3300
10.80 10.55 0.7 0.4 1.5 0.91 1.419.3 1 55..1339 0.402. 50 1144..220011 55..5500 55..9955 55..990033 00xx3300 3300xx3300
10.66 10.66 0.7 0.5 1.2 1.2 1.412.4 2 55..1199 0.402. 42 1144..775511 88..3344 55..9944 55..331133 00xx3300 4400xx4400
10.80 0.7 1.5 1.49 5.39 0.50 1155..5500 55..9900 3300xx3300
2
10.66 0.7 1.2 1.42 5.19 0.42 1188..3344 55..3311 4400xx4400
D.Vollbracht:Microstrippatchantennacrosspolarizationradiation 253
2
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back radiation due to the small feed via holes through the performances. The grey layer represents in Fig. 1a and b
shiel dinggroundplanegeneratesonlysmallx-polcontribu- theRO4403prepregwithε =3.17and100µm19height.The
r
tion. RO4403 prepreg will be used to combine the two RO6002
laminates during the compression process. For suppressing
2.2 Thoroughanalysisofthe“differentialfeeding unwanted surface currents or propagation channels within
technique”forphasedarrayweatherradar the substrate right from the design beginning, this prepreg
applications shows only small εr differences compared to RO6002. The
copper has a thickness of 35µm and the diameter of the
One valuabletechnique to reducethe x-pol radiationof mi- isolation circle inside the shielding ground for the probe
crostrip antennas was presented by Liang et al. (2004) and feedtransitionis0.91mm.Theradiatingsquarepatcheswith
Fulton and Chappell (2011) and is called “differential feed- 10.55mmedgelength,pin-viadiameterof0.4and0.416mm
ing”. Differential feeding can be established with two RF pin- via to edge- distance are designed for a resonance fre-
drivesignalsexactly180◦ outofphase.Todemonstrateand quencyof9.395GHz(seeFig.2).
to analyze the advantages of differentially fed microstrip BothantennasfromFig.1aresharingthealmostsamede-
patch antennas for phased array weather radar applications signparameters.Theonlydifferencesaretheadditionalport
two probe fed antenna designs with single and differential in Fig. 1c with 180◦ out of phase excitation and the width
phasefeedingarecompared.Itwillbefurtherexplainedwith andlengthofthematchingcircuit(arrow)closetothefeed.
theseantennaprototypeshowthex-polradiationisgenerated TheRosenberger3-Dconnectormodelwascompletelyinte-
fromsurfacecurrentsandfieldlinedistributions. gratedintheCSTMWSfullwavesimulationandimpedance
The square probe fed microstrip patch antennas were de- matched with GND vias and microstrip impedance trans-
signed and simulated with CST MWS. The patch anten- former to the 50(cid:127) line impedance. The microstrip stubs
nas are consisting of two RO6002 substrates (dark green in in parallel to the probe feed are used for matching the
Fig. 1) with ε =2.94, 508µm height and excellent dissi- impedanceofthepatchantenna.InFig.1bthemanufactured
r
pation factor tanδ=0.0012 for adequate antenna efficiency patchesarevisualized.
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254 D.Vollbracht:Microstrippatchantennacrosspolarizationradiation
Figure3.
Figure2showstheS11ofthematchedconnectorportwith x-pol(Ludwig,1973).Notethatthereferenceparameter“re-
patchantenna.TheVSWR1:2bandwidthof188MHzwas alized gain” in dBi also considers the impedance mismatch
simulated. lossesforthegaincalculations.IntheleftrowofFig.3the
Figure 3 compares the co/x-pol pattern without (Fig. 3a) single probe fed patch antenna radiation patterns are shown
andwith(Fig.3b)thedifferentialfeedingtechnique.Thera- together with the co-pol “realized gain” of 5.85dBi. The
diationpatternsareplottedwithLudwig’sthirddefinitionof rightrowofFig.3displaystheradiationpatternsofthedif-
Adv.RadioSci.,13,251–268,2015 www.adv-radio-sci.net/13/251/2015/
D.Vollbracht:Microstrippatchantennacrosspolarizationradiation 255
Figure3.
ferential probe fed patch antenna with 180◦ out of phase ECutCPSwouldshowalmostperfectresultsandtheHCut
feedingandtheco-pol“realizedgain”of5.93dBi. wouldexhibitahigh,butunrealisticx-polcontributionfrom
Thex-polpatterninFig.3a-3showstwomaximaandone the 3-D antenna radiation pattern. Only the dedicated 45◦
null in the E Cut. The radiating edge close to the feed con- Cut would show the real antenna performance in CPS. For
tributesthehighestx-polradiation.Theoppositetofeedlo- this reason, it is recommended to use 45◦ Cuts in any CPS
catedradiatingedgeaddsmuchlowerx-polradiation. discussions for patch antennas when the integrated CPS is
By further analyzing the x-pol pattern it is obvious that important.Figure4showsthecomparisonofthesingleand
the E ((cid:56)=90◦) and H Cut ((cid:56)=0◦), typically provided by differentialprobefedpatchantennaCPSperformanceusing
antenna manufacturer, would not show the maximum x-pol therecommended45◦Cuts.
radiationintensitiesfromtheupperfarfieldhemisphere.The
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256 D.Vollbracht:Microstrippatchantennacrosspolarizationradiation
Figure3.Comparisonofco/x-polpatterns(a)withoutand(b)withdifferentialfeeding.
The differences in CPS performances are clearly identi- tion,comparedtoonly6.6dBat−60◦ AZforthesinglefed
fiable by the plots in Fig. 4. The co- and x-pol plots from patch antenna. The 3dB half power beamwidth highlighted
Fig. 4 are generated by the (cid:56)=45◦ Cuts from Fig. 3a-2, with the ellipse on the center top of Fig. 4 shows another
b-2, a-3 and b-3 respectively. Typically phased array anten- advantageofthedifferentialfedantennaforphasedarrayap-
nasareprovidingazimuthe-scanrangesfrom+60to−60◦. plications:thebeamwidthisquietsymmetricandthephased
WithinthisangularscanrangetheintegratedCPSshouldbe arrayscangainlosswithrespecttotheelementpatternwould
aslowaspossible.AnintegratedCPSof14.2dBfordiffer- alsobesymmetricforpositiveandnegativescanangles.The
entialfedpatchantennascanbeachievedat±60◦ AZposi- single fed antenna shows a slight shift of the beam maxi-
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D.Vollbracht:Microstrippatchantennacrosspolarizationradiation 257
1
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77
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258 D.Vollbracht:Microstrippatchantennacrosspolarizationradiation
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mum in negative Theta direction and an asymmetric beam feedshowsabetteraxialratiowithsmallerdecayperdegree.
shape.Thiscanbeassociatedtothesinglefeedconfiguration The single feed antenna shows for (cid:50)>+25◦ better results
andthefactthatstrongerelectromagneticfieldsareradiated than the differential feed antenna, but when considering all
fromtheradiatingedgelocatedclosetothefeed.Theblack otherdisadvantagesmentionedbeforethisfactisnegligible.
circlesat(cid:50)=±120◦indicatebackradiatedenergyandhere It should also be noted here that an asymmetric axial ratio
againthedifferentialfedantennabackradiationissymmetric induces non- consistent scan angle biases for polarimetric
andwouldinducethesameamountofenergyinbothback- phasedarrayweatherradarmeasurements.
warddirections.Themutualcouplingwouldbeequaltothe The x-pol phases from Fig. 6 are generated from the
neighbor elements compared to the asymmetric back radi- (cid:56)=45◦ Cut of Fig. 3a-4 and b-4 respectively. The co-pol
ation from single fed antennas. Furthermore, the x-pol free phase plots are not visualized in 3-D. It can be recognized
area below the main beam at −20dBi is much broader for thattheco-andx-polphaseofthedifferentialfedantennais
differentialfeedswith72◦ azimuthrangeincontrasttoonly very symmetrical distributed, especially in the desired scan
49◦azimuthrangeforthesinglefeedconfiguration.Figure5 range of ±60◦. The phase is identical over diagonal plane
comparestheantennaaxialratiosoftheantennas.CSTMWS (see Fig. 3b-4). The co- and x- phases of the single fed an-
isfollowingtheIEEE-Standardforthecalculationoftheax- tennaareasymmetricandthemaximaofx-polphasesondi-
ial ratio shown in Fig. 5. The axial ratio is the ratio of the agonalplanesaremorethan70◦ outofphase.Byanalyzing
major axis to the minor axis of the polarization ellipse. E the (cid:56)=135◦ and (cid:56)=45◦ Cuts from Fig. 7, a phase dif-
1
represents the horizontal polarized field vector and E the ferenceofexactly180◦ canbeobservedforthecomplete(cid:50)
2
verticalpolarizedfieldvector.Itiscalculatedasfollows: range.Eachpointisexactly180◦ outofphasebycomparing
bothdiagonalplanes.Thispredictablediagonalphasedistri-
v
uu(cid:12)(cid:12)E2(cid:12)(cid:12)+(cid:12)(cid:12)E2(cid:12)(cid:12)+(cid:12)(cid:12)E2+E2(cid:12)(cid:12) butioncanbeusedforarrayconfigurationswheretheneigh-
AR=t 1 2 1 2 . (1) boringelementsx-polcontributionscanceleachotherinfar
(cid:12)(cid:12)E21(cid:12)(cid:12)+(cid:12)(cid:12)E22(cid:12)(cid:12)−(cid:12)(cid:12)E21+E22(cid:12)(cid:12) field.
4 Furthermore, the two designs from Fig. 3 are redesig ned
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me6tr icanddddddddddsiiiiiiiiifffffffffhfffffffffoeeeeeeeeewrrrrrrrrreeeeeeeeesnnnnnnnnnbtttttttttiiiiiiiiieaaaaaaaaattllllllllle fffffffffreeeeeeeeereeeeeeeeeedddddddddsiiiiiiiiiunnnnnnnnnlgggggggggts tttttttttfeeeeeeeeeocccccccccrhhhhhhhhhnnnnnnnnnneiiiiiiiiiqqqqqqqqqguuuuuuuuuateeeeeeeeei.........v TTTTTTTTTehhhhhhhhhaneeeeeeeee d111111111888888888p000000000o°°°°°°°°°s ippppppppptihhhhhhhhhvaaaaaaaaaesssssssssseeeeeeeeec adddddddddniiiiiiiiiffffffffffffffffffeeeeeeeeerrrrrrrrreeeeeeeeennnnnnnnnmccccccccceeeeeeeeeet iiiiiiiiirsssssssssic vvvvvvvvveeeeeeeeeprrrrrrrrroyyyyyyyyyr tuuuuuuuuuisssssssssseeeeeeeeeofffffffffluuuuuuuuualllllllllt iiiiiiiiiionnnnnnnnnn aaaaaaaaacnnnnnnnnnhttttttttteeeeeeeeeannnnnnnnnrannnnnnnnncaaaaaaaaate aaaaaaaaarrrrrrrrrirsrrrrrrrrrtaaaaaaaaaicyyyyyyyyy. cccccccccSoooooooootnnnnnnnnnafffffffftfeiiiiiiiiiggggggggg-ouuuuuuuuufrrrrrrrrr-aaaaaaaaathtttttttttiiiiiiiiieooooooooo-nnnnnnnnnasssssssssr,,,,,,,,,t weather
ang7l esofupto(cid:50)=25ff◦oo.rrF ccoaarnnncceeegllaiinntiggv eoouuattn gtthhleees xxt-hpppppeooooodlllll icccccfooooofemmmmmrepppppnoooootnnnnniaeeeeelnnnnntttttsssss iiiiinnnnn fffffaaaaarrrrr fffffiiiiieeeeelllllddddd fffffrrrrrooooommmmm nnnnneeeeeiiiiiggggghhhhhbbbbbooooorrrrriiiiinnnnnggggg eeeeellllleeeeemmmmmeeeeennnnntttttsssss.....
Adv.Ra dioSci.,13,251–268,2015 www.adv-radio-sci.net/13/22541/2015/
1 Figure 6:::: CCCCoooommmmppppaaaarrrriiiissssoooonnnn ooooffff CCCCoooo aaaannnndddd XXXX-ppppppoooooollllll PPPPPPhhhhhhaaaaaasssssseeeeee pppppplllllloooooottttttssssss ffffffrrrrrroooooommmmmm ssssssiiiiiinnnnnngggggglllllleeeeee ((((((rrrrrreeeeeedddddd)))))) aaaaaannnnnndddddd ddddddiiiiiiffffffffffffeeeeeerrrrrreeeeeennnnnnttttttiiiiiiaaaaaallllll ((((((bbbbbblllllluuuuuueeeeee)))))) ffffffeeeeeedddddd ppppppaaaaaattttttcccccchhhhhh
2 aaaaaannnnnntttttteeeeeennnnnnnnnnnnaaaaaassssss...... TTTTTThhhhhheeeeee ddddddaaaaaasssssshhhhhheeeeeedddddd ccccccuuuuuurrrrrrvvvvvveeeeeessssss aaaaaarrrrrreeeeee rrrrrreeeeeepppppprrrrrreeeeeesssssseeeeeennnnnnttttttiiiiiinnnnnngggggg tttttthhhhhheeeeee pppppphhhhhhaaaaaasssssseeeeee pppppplllllloooooottttttssssss ooooooffffff tttttthhhhhheeeeee xxxxxx-ppooll rraaddiiaattiioonn ppaatttteerrnnss..
D.Vollbracht:Microstrippatchantennacrosspolarizationradiation 259
3
4
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61 80◦ phdddddddddaiiiiiiiiisfffffffffefffffffffeeeeeeeeedrrrrrrrrrifeeeeeeeeefnnnnnnnnnerttttttttteiiiiiiiiiaaaaaaaaanlllllllllc efffffffffeeeeeeeeeieeeeeeeeesdddddddddviiiiiiiiiennnnnnnnnrgggggygggg uttttttttteeeeeeeeescccccccccefhhhhhhhhhunnnnnnnnnliiiiiiiiiiqqqqqqqqqnuuuuuuuuuaeeeeeeeeen.........t eTTTTTTTTTnhhhhhhhhhneeeeeaeeee a111111111r888888888r00000000a0y°°°°°°°°° cpppppppppohhhhhhhhhnaaaaaaaaafisssssssssgeeeeeeeeeu dddddddddraiiiiiiiiitfffffffffifffffffffoeeeeeeeeenrrrrrrrrrseeeeeeeeefnnnnnnnnnocccccccccreeeeeeeeec aiiiiiiiiisssssssssn cvvvvvvvvveeeeeeeeeelrrrrrrrrrinyyyyyyyyyg uuuuuuuuuosssssssssueeeeeeeeetfffffffffuuuuutuuuuhllllllllle iiiiiiiiinnnnnnnnnx- paaaaaaaaaonnnnnnnnnlttttttttteeeeeeeeeconnnnnnnnnmnnnnnnnnnaaaaaaaaapo aaaaaaaaanrrrrrrrrrerrrrrrrrrnaaaaaaaaatsyyyyyyyyy i cccccnccccooooooooofnnnnnnnnnarfffffffffiiiiiiiiifigggggggggeuuuuuuuuulrrrrrrrrrdaaaaaaaaaftttttttttriiiiiiiiioooooooooomnnnnnnnnnsssssssssn,,,,,,,,, e ighboring
7e lements. ffoorr ccaanncceelliinngg oouutt tthhee xx-pppppooooolllll cccccooooommmmmpppppooooonnnnneeeeennnnntttttsssss iiiiinnnnn fffffaaaaarrrrr fffffiiiiieeeeelllllddddd fffffrrrrrooooommmmm nnnnneeeeeiiiiiggggghhhhhbbbbbooooorrrrriiiiinnnnnggggg eeeeellllleeeeemmmmmeeeeennnnntttttsssss.....
1 24
radars are working in pulse alternating or simultaneously H-Port
transmitting dual polarization modes. For these dual polar-
2
ization modes the single fed antenna will be equipped with
two feeds; one for horizontal and one for vertica3l po lariza- L1++λ/2
tion. The differential fed patch antenna accommodates four
feeds for dual polarization mode; two probe feed4s 18 0◦ out Feed_V_2
ofphaseforhorizontalpolarizationandtwoprobefeeds180◦
5
L1
out of phase for vertical polarization. The differential fed
patchantennaindualpolarizationdesignisgraphe6d in Fig.8.
TheisolationandS11resultsfromCSTMWS3-Dfullwave
7
simulationsarevisualizedinFig.9.Itcanberecognizedthat L22+λ/2
the differential fed dual polarized patch antenna maximum Feed_H_1 Feed_H_2
8
port isolation provides on desired frequency at 9.395GHz,
meanwhile the single fed dual polarized antenna9 ver y poor
isolation characteristics established. The excellent isolation L2
10
characteristicofthedifferentialfeddualpolarizedpatchan-
Feed_V_1
tennacanbeexplainedwithFig.8.Letusconsideraninput
11
signal at the vertical port. The signal travels through the T- V-Port
Junction splitter (red point in Fig. 8) and excite1s2a t fi rst the
Feed_V_1andsubsequentlytheFeed_V_2with113 80◦Fpihguarsee 88::: GGGrrraaaFopppfhhhigiiidcccuaauarllla edddleee8psss.occcrrrGliiaippprrtttaiiiizopooennnhd ifffcoooparrra letdxccheeslaclenrnitpet tniiiiissssnoooooanllllaaaattwttfiiiioooooirnnnnth effffuuuuxdnnnnciccccfefttttliieiilooooernnnnenaaaantlllltiiiiittttisyyyyao loooolfaffff etddddieuuuuodaaaanilllln fppppguoooo.nllllaaaacrrrrtiiiiizzzzoeeenddda ppplaaiattttyccchhh aaannnttteeennnnnnaaa wwwiiittthhh
shift. Both signals exciting the patch antenna to1r4a diate, but dddiiiffffffeeerrreeennntttiiiaaalll fffeeeeeedddiiinnnggg
smallsignalparts(dashedlinesinFig.8)arerec1e5iv ed bythe
horizontal ports Feed_H_1 and Feed_H_2. Current probes
andcanceleachotheroutattheT-Junctionsplitterinfrontof
have shown from simulations that the two signals received
thehorizontalsplitterduetothelengthdifferenceofλ/2.
byFeed_H_1andFeed_H_2arein-phaseattheprobefeeds
www.adv-radio-sci.net/13/251/2015/ Adv.RadioSci.,13,251–268,2015
16
17 FFiigguurree 9:::::::: CCCCCCCCoooooooommmmmmmmppppppppaaaaaaaarrrrrrrriiiiiiiissssssssoooooooonnnnnnnn ooooooooffffffff ppppppppoooooooorrrrrrrrtttttttt iiiiiiiissssssssoooooooollllllllaaaaaaaattttttttiiiiiiiioooooooonnnnnnnn cccccccchhhhhhhhaaaaaaaarrrrrrrraaaaaaaacccccccctttttttteeeeeeeerrrrrrrriiiiiiiissssssssttttttttiiiiiiiiccccccccssssssss ffffffffrrrrrrrroooooooommmmmmmm ddddddddiiiiiiiiffffffffffffffffeeeeeeeerrrrrrrreeeeeeeennnnnnnnttttttttiiiiiiiiaaaaaaaallllllll aaaaaaaannnnnnnndddddddd ssssssssiiiiiiiinnnnnnnngggggggglllllllleeeeeeee ffffffffeeeeeeeedddddddd ppppppppaaaaaaaattttttttcccccccchhhhhhhh eeeeeeeelllllllleeeeeeeemmmmmmmmeeeeeeeennnnnnnnttttttttssssssss
25
260 D.Vollbracht:Microstrippatchantennacrosspolarizationradiation
It turns out, that the differential fed patch antennas with 3.3 CPScomparisonofcircularandsquareshaped
180◦ out of phase excitation are suitable for phased array microstrippatchantennaelements
weather radar applications and many valuable advantages
comparedtosinglefedpatchantennasareevidencedandfirst An intensive literature search did not reveal any adequate
timedocumentedwithinthispublication. reference comparing the CPS of circular and square shaped
microstrip patch radiators. Therefore, a comparison of the
twopatchantennashapeswithrespecttotheirx-polperfor-
3 Analysisofpotentialx-polsourcesinmicrostrip
mances is provided below. The comparison is based on the
patchantennadesign
sameresonancefrequency,samelayerconfiguration,slightly
differentparallelstubconfigurationduetothedifferentpatch
Open via holes, rectangular patch edges and the position of
impedancesandthesamedifferentialfeedingtechnique.
connectorandmatchingcircuitsbelowtheshieldingground
Thecircularpatchradiatorhasadiameterof12.67mm,the
areinvestigatedandverifiedaspotentialx-polsources.Addi-
probeviadiameteris0.4mmandtheprobeviatoedgedis-
tionally, circular and square patch geometries are compared
tance is 0.5mm. The diameter of the isolation circle inside
and variations of the patch sizes are analyzed in their x-pol
the shielding ground for the probe feed transition is again
performances.
0.91mm. The resonance frequency is located at 9.395GHz
with a VSWR 1:2 bandwidth of 152MHz. In Fig. 11a the
3.1 Probefedpatcheswithpluggedviaholes
“realized gain” of 6.66dBi can be recognized and the x-pol
InFig.10athecrosssectionoftheholepluggedprobefeedis pattern from Fig. 11b shows the same results featuring four
graphed.Afterdrillingtheviaholesintothemultilayersub- maximacomparabletoFig.3b-3duetothedifferentialfeed-
strate the walls of the holes are galvanized with copper to ing technique. The gain is higher compared to the square
connecttheupperandlowerRFlayer.Theholesareplugged patch but only due to the larger antenna patch area. The in-
using the following two-step process: first the via hole is tegratedCPSof14dBfordifferentialfedcircularpatchan-
filled with cured resin to generate a plane surface on both tennas was established at ±60◦ azimuth position, which is
sides and secondly an additional galvanization step takes almostequaltothe14.2dBfordifferentialfedsquarepatch
placetoclosetheviaholeandtoestablishaplanesurfacefor antennas from Fig. 4. By applying longer optimization it-
the radiation patch antenna element and the microstrip line. erations for impedance matching, the same integrated CPS
From the hole plugging process one would assume that the forsquareandcircularpatchantennageometriescanbeex-
probe feed produces lower x-pol radiation because of plane pected. It should be noted here that the value of 14dB any-
patchantennasurfaceandlowerresistanceintheareaofthe how reflects the theoretical maximum of CPS at ±60◦ for
feed,butCSTMWSsimulationshaveshownthattheCPSis microstrippatchantennas.
exactly(within±0.01dB)thesamecomparedtousualopen
probe feeds. It was verified that hole plugging does not im- 3.4 Analysisofprobefeedanditsmatchingnetwork
provetheCPSofmicrostrippatchantennasandtheexpenses positionbelowtheshieldingGNDofthepatch
fortheadditionalpluggingprocesscanbesaved. antenna
3.2 Bendedsquarepatchantennaedges Typically, for impedance matching the probe feed positions
are changed along the y axis. The maximum impedance is
By analyzing the x-pol radiation pattern in Fig. 3a-3, b-3, determined by the position of the probe feed at the edge of
two (for single fed), respectively four (for differential fed) the patch. The minimum available impedance is zero and
maximaabovethepatchedgesareobservable.Itisassumed is located at the center of the patch antenna element. For
that x-pol radiation contributions are generated by the four phased array application with high x-pol requirements the
90◦anglesfromthepatchedges.PerformedCSTMWSsim- differential feeding design verified in Sect. 2.2. should be
ulationshaveevidencedthatthepatchantennaedgebending applied.Consequently,onlyedgefeedingcanberealizedto
doesnotreducethex-polradiation.Onlytheresonancefre- inject two separated signals with 180◦ out of phase, espe-
quency will be shifted due to the change in absolute length ciallyfordualpolarizationapplications.Straightconnections
of the radiating edge. After impedance re-matching of the of the edge probe feeds are seldom possible due to the lim-
bendedpatchantennaelementalmostthesamex-polvalues itedspaceforfeedingnetworks,especiallyfordualpolariza-
withthedifferenceofonly0.04dBwasobserved.Edgebend- tion capabilities. In this analysis the direction of the probe
ingdoesnotimprovetheCPSandthefour90◦anglesarenot feeds and its matching network below the shielding ground
contributingsignificantlytothex-polradiation. ischangedwitharotationof90◦(Fig.12a)andanalyzedfor
x-pol(Fig.12b)degradation.
Forthedifferentialfedsquare-patchwith90◦ rotatedcon-
nectors an integrated CPS of only 6.2dB was observed at
±60◦ azimuth position for the 45◦ Cut of Fig. 12, which
Adv.RadioSci.,13,251–268,2015 www.adv-radio-sci.net/13/251/2015/