circular micro strip patch antennas on glass for vehicular application

8/2/2019 Circular Micro Strip Patch Antennas on Glass for Vehicular Application

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Circular m crost ri patch antenn as on glass forvehicle a plicationsL. EconomouR.J.Langley

Indexing term s: Circular patc h untennas, Autoniotiv e communication.r, icrostiip auteimas

Abstract: The performance of circular patchantennas attached to a laminated automotivewindscreen is described. Experimental work andresults computed from a spectral domain analysisinvestigate the input impedance, radiationpatterns, and the effect of surface wavepropagation within the glass. The overallefficiencies of several combinations of substratesand superstrates are presented. Variations in thedimensions of the laminate are discussed as theyhave a significant effect on patch performance.

1 IntroductionMost modern cars have little onboard communicationsequipment except a radio and in some cases a mobiletelephone. However, this is set to change very rapidlyas the car becomes part of a sophisticated electroniccontrol system. Traffic information, route guidancesystems based on GPS, emergency calls on GPS/GSMtelephones, digital audio broadcasting, satellite tele-phones, automatic toll collection and many othertelematics systems will become commonplace. Manu-facturers are planning to incorporate at least some ofthese systems into the majority of cars within a fewyears. This will result in a multiplicity of antennas onvehicles which, for aesthetic and security reasons, it willbe necessary to hide from view as far as possible. Onearea of the car that may be used is the front or rearscreen. For microwave systems, microstrip patch anten-nas are attractive since they are conformal and cheapto produce. Little has been published about the per-formance of such antennas printed on glass. Lowes etal. [l ] have reported on rectangular patch antennaswith a laminated glass superstrate, where surface wavesintroduced significant ripples into the radiation pat-terns. We have previously reported [2] on the inputimpedance of ring patch antennas with glass super-strates including the fundamental TM,, mode and thehigher order TM,, mode. Phang and Hall [3] havereported on a largely experimental study of glass-basedrectangular patches and slots using flat laminates.In this paper, we present an experimental and theo-0 EE, 1998IEE Proceedi71gs online no. 19982250Paper first received 30th October 1997 and in revised form 12th June 1998The authors are with the Electronic Engineering Laboratory, Universityof Kent, Canterbury, Kent, CT2 7NT, UK

retical study of circular patch antennas attached to alaminated car windscreen. We examine the problemscaused by surface wave excitation and due to the lossesin the glass itself which is a poor microwave dielectricmaterial. Measurements were made at frequencies of 2-6GHz covering the satellite telephone bands and up tothe 5.8GHz band favoured for tolling applications inEurope. Circular microstrip patch antennas were stud-ied, both discs and annular rings. In this paper, weconcentrate on the performance of the fundamentalTM1, mode. although several other modes have beenstudied. -

glass layer 1DJrO 0substrateDJrO 0substrate

groundplanepatcha2T

I I I glass layerplastlc layerglass

I \ atch groundplaneEM coupledfeed linebFig. 1a coaxial probe fed patch with glass superstrate

U = patch innei- radius, b = outer radiusb electromagnetically coupled patch within laminated glassGeometry o circular patch antennason wind.ween

Two approaches can be used to manufacture theantennas. The complete antenna may be made on con-ventional dielectrics and the assembly attached to thewindscreen with adhesive. In this study, these antennaswere printed on RT Duroid (relative permittivity =2.33, thickness = 0.7874") on a circular groundplane, 12cm in diameter. The patches were placed inthe centre of the screen, 5cm from the top, where therear view mirror might be attached. This method of fix-ing allows the antenna to be removed and repositionedonto a new windscreen, should a breakage occur.Fig. l a shows this geometry. The alternative (Fig. 16)is to print the entire antenna within the laminatedwindscreen, fed by an electromagnetically coupledIE E Proc.-Micr.on,Antennus Propug., Vol. 145, No 5, October 199816


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microstrip feed line. This restricts the antenna design,due to the fixed thickness and electrical properties ofthe windscreen, and increases the replacement cost if abreakage occurs.This investigation concentrates on the first method,due to the difficulties in manufacturing patches withina commercial windscreen. Nevertheless, the secondmethod was simulated and some results are presented.The feed can be either a coaxial connector or an elec-tromagnetically coupled microstrip line; the overall per-formance of the antenna is similar for either feedtechnique. The electrical properties of the glass layersare E,. , = 6.75, tan 6 = 0.03 nominally 2.5"~ thick,and for the plastic laminate E , . ~= 2.9, tan 6 0.05nominally 0.7" thick. A Triplex XX X (modelF10773) Ford Escort front windscreen was used in allmeasurements. The dimensions of the screen are nomi-nal as they vary considerably in practice, and the effectof tolerances is discussed in Section 3.3. Note that, inour experimental work, with patches adhered to glass,a full size car windscreen was used,We briefly describe a C A D model developed for thestudy, based on the spectral domain approach [4-61.We also discusse the performance of circular patchesattached to the windscreen with a thin adhesive layer.In practice, these antennas are likely to be manufac-tured as active modules that may sit slightly adrift ofthe glass (about lmm) to reduce the effects of rain, iceand snow. This has no effect on the radiation patterns,but if the spacing from the glass is increased, a degra-dation of the radiation pattcrns occurs. The change inresonant frequency and input impedance are reported,together with a discussion on the bandwidth implica-tions. Losses due to the poor performance of the glassat microwave frequencies and the contribution of sur-face waves are important considerations. Measure-ments of radiation patterns illustrate the loss in gainand radiation of surface waves.2 Mo del spect r al domain ana lys isA spectral domain method of moments analysis wasused to model the antennas. The basic method is wellknown and is not repeated here. The multi-layer con-figurations were handled by extending the analysis,based on the method used by Fan and Lee [5, 61. Themethod was formulated in the Hankel domain and wasapplied to disc and annular ring patches. The Greensfunctions were derived for each layer, relating the tan-gential electric field components on each dielectric andthe electric surface currents on the patches; a matrixformulation allowed the multi-layer configuration to bemodelled. Galerkin's method was used to solve theunknown currents and the resonant frequencies.The following microstrip antenna configurations canbe handled by the model: multi-layered substrates andsuperstrates simultaneously, dielectric losses, conductorlosses, stacked and concentric patch configurations.,efficiency and the contribution of surface waves. Somecomparisons with experiments are presented below.However due to the difficulties of fabricating patches inthe glass, much of the initial design work relied heavilyon using the model. The software was tested extensivelyfor various patch/dielectric combinations, includingmulti-layer substrates and superstrates, to prove itsaccuracy and provide confidence in the CAD model.I E E Proc. -Micro~vAntcnnii, P r o p u g , Vol. 145, No . 5, Ocrohrr 1098

3 Pat ches a t t ached o nt o windscr een3. Resonant frequency and bandwidthFig. 2 plots the measured and calculated input imped-ances for a TM ,, mode circular disc patch, 40" indiameter, when attached to laminated glass. This was asample section of flat float glass, 60cm square withhigh tolerances on dimensions. The windscreen was notpresented at this point due to the known dimensionaltolerance problems (discussed below). The aim herewas to gain confidence in the modelling software. Aglue layer 0.2" thick was assumed, with E, = 2.5. Thedisc resonated at 2.79GHz without a cover, but whenattached t o the glass it falls to 2.63GHz. The calculatedvalues for the input impedance were in very goodagreement with the measurements; the resonant fre-quency was in error by about 1% in both cases. Thereal and imaginary values of the input impedance werealso accurately predicted. Many other calculations andmeasurements gave similarly good agreement, givingconfidence in the model. The feed position at 5mmfrom the centre of the ring was not optimised for theglass cover and consequently the real part of the inputimpedance was only 20Q.

30r-----l

-101 " " " 1 . ' I2350 2450 2550 2650 2750 2850frequency, MHz

Fig.2attuclmed to 6nimf;oat glary lunzinute~-d l ~ u l a t e d - mcd5uredInput mm edunte R + JX or ciitular patch 40mm in dianwteiPatches were then mounted onto the windscreen withthin adhesive. Fig. 3 shows the measured input imped-ances for a 16" diameter disc before, and after, itwas attached to the glass laminate; computed values

were in close agreement. The patch was fed 6 m m fromthe centre. The bandwidth for the patch alone was ini-tially 1.87'0, and this increased to 6.8% when attachedto the glass; the input impedance fell from 58Q to 47Qrespectively. This is a useful improvement in band-width, particularly a, communication systems oftenneed bandwldths exceeding 5%. This increase in band-width has been reported previously [1-3] and is due tothe thick high dielectric constant cover, rather thanlosses in the glass.417


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60 I50

C

-

6400frequency, MH z

-20

6800

-30 a bFig.3diameter ( a ) with and (h i without glass superstrate attachedMeasured input ivipedunce of circulur patch antennu 1 6 n m in

3.2 Radiat ion pat ternsAttaching the patches to an electrically thick super-strate is expected to increase surface wave propagationand radiation. The most obvious effects of this arelikely to appear as deep ripples in the radiation pat-terns. A study has been made at 3. 8 and 5.8GHz toexamine the patterns for the antennas attached to a fullwindscreen. In practice, there was little differencebetween the results at each frequency. Therefore, weconcentrate on the radiation patterns at th e higher fre-quency.

goo

2 4 0 ' 1 1 3 0 0 '270'

Fig.4H-plane~- no glass, ~ ~ ~ attached directly to glassMeasured radiation patierns of16 innz disk antenna

Radiation patterns measured in an anechoic chamberare plotted in Figs. 4-6 for three positions of the patchwith respect to the glass. In Fig. 4a the patch is directlyin contact with the glass. For clarity, the reference radi-ation patterns for the patch alone are also shown forcomparison, in the 0" plane only. Most of the ripplesappearing in Figs. 4-6 were due to the finite groundplane, 12cm in diameter. The loss measured throughthe windscreen was typically 2dB, whereas Lowes et al .[I] have measured 3.2dB. The amplitude ripple on thewindscreen patterns was of the order of 2dB over themain forward lobe, but entirely acceptable otherwisewith no significant distortions. Some broadening of thepattern is seen compared to the patch alone, and there418

is increased back radiation. N o significant differenceswere noted for the patch spaced lmm from the glass.Figs. 5 and 6 show that spacing the antenna 5 and 10mm, respectively, behind the screen produced far moreripples in the patterns due to direct reflection from thescreen. Both principal planes are plotted. The backradiation increases accordingly. There are fewer ripplesand back radiation for the H plane patterns. Reducingback radiation into the vehicle is important, both toavoid EM C problems and to allay public concern overhuman radiation absorption; hence, it is better toattach the antenna either directly to the glass, or within1 mm of the surface. Further work has been carried outon higher order modes, and these produce similar radi-ation pattern changes. Cross-polarisation patterns havenot been presented, but are typically 10-15dB belowthe copolar patterns and are acceptable for currentapplications.goo

60'1 2 y -10.

270Fi .5he2ncl g / a s

~ H-plane (0") - ~ E-plane (90")Measured radiation patterns oj 16mm disk antenna patch 5

270'Fi .6be2nd glass__ H-plane (0") - - E-plane (90")

Meusured rudiation patterns of 16mm disk antenna patch I O m m

3.3 TolerancesAutomotive glass is designed to be low cost, ratherthan manufactured to the high dimensional tolerancesrequired for electrical design considerations. Optically,changes in the thickness of the constituent layers of upIEE Proc -Microw Anlennas Propag , Vol. 145, N o. 5 , October 1998


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to 15% are not apparent to the driver, but they mayhave a crucial effect on the performance of patchantennas at microwave frequencies. In addition, thechemical composition, and hence dielectric properties,of glass vary from batch to batch and country to coun-try due to the local silica available. We have attemptedto investigate the effect of material and dimensionalvariations in the glass superstrate on the performanceof patch antennas. A circular patch was used as the ref-erence antenna: 16" in diameter, printed on Duroid,with a permittivity of 2.33 and height 0.787mm. Theresonant frequency is a key item of interest, but band-width and input resistance are also important.Table 1 summarises the effects of changing glassdimensions and material properties on the resonant fre-quency j;., input resistance R,, and bandwidth. Toler-ances of under 10 % in material properties but 25%(0.5") in thickness have been assumed. Case 1 wastaken as the reference. Changing the permittivities ofthe glass layers (antennas 1-3) by +/- 7.5% changes theresonant frequency by lSOMHz, which is about 3% .Corresponding variations in bandwidth and ,U, are30MHz and 9 Q , respectively. For Case 4, the lpermit-tivity of the plastic laminated layer was reduced by1796, which had a negligible effect on the frequencycharacteristics. Antenna 5 reduced the outer glass layerheight from 2. 5 to 2.0". The resonant frequencyremained the same but the bandwidth increased by2SMHz and the input resistance by 7Q. In Case 6, theinner glass layer height was also reduced to 2.01nm (aswas the outer layer) and compared with values forCases 1 and 5 ; even though the resonant frequencydecreased the band by only 30MHz, R, increased sig-nificantly to 6 5 Q , thus reducing the bandwidth to335MHz. On the other hand, increasing the gla:;s layerthickness to 3.0" (Case 7) results in the same reso-nant frequency as for the 2.0" glass, but much lesschange in either the input resistance or bandwidth (7 Qand IOMHz, respectively).These results suggest that the inner glass dimensionsand permittivity have the greatest impact on theantenna frequency characteristics, which is to beexpected, and that increasing the inner glass layerthickness beyond about 2.5" has no significant effecton the patch performance. The plastic laminate layerhas little effect on performance in this context.Finally, a more random variation in the parameterswas calculated in Case 8. The resonant frequencydropped by 6SMHz and R, by 7Q compared to thereference.

Summarising the above study, the overall variation inresonant frequency is 150MHr around 5.33GHz(about 3%), with the bandwidth varying overall by1.7%. This, is significant and comparable with the nar-row bandwidth characteristics of the patch antenna. Itwould be even more significant taken with the possibledifficulty of maintaining high tolerances when screenprinting the conducting patches within the glass.4 EfficiencyTable 2 summarises the efficiencies computed for thepatches attached to a vehicle windscreen. Measure-ments in Section 3 confirm the efficiency calculatedhere of about 68%. Owing to difficulty in producingpatches within the screen for measurements, a secondset of results calculated for a patch situated within thelaminate (Fig. l b ) is shown for comparison. Note thatfor the Duroid-based antenna the glass-covering layerwas the full laminated screen, whereas for the glass-based patch the cover was the thin (2.5") upperlayer of thLe glass laminate.Table 2:Efficiencies computed for patches attache d to avehicle windscreen

S u b s t ra teEfficiency (% ) sur facen o g lass !glass cover, wa ve lossco ve r co ve r ii o loss ('0)

Du ro id 9 1 6 9 '79 12A u to mo t i ve g l a ss 54 60 '72 20

For the patches attached to the screen, the efficiencywas only 69Y0 compared with over 90 % for the patchalone. The losses in the glass reduced the efficiency bylo%, with surface waves due to Ihe glass contributinganother 12% reduction according to the model. Conse-quently, riipples on the radiation patterns would not beexpected to be very significant, confirming the rela-tively low ripples measured in the copolar radiationpatterns of Fig. 4. From the simulations, printing theglass on a glass substrate 2.5" thick reduces the effi-ciency to 54'%; this appeared to be due to trappedwaves in the glass substrate. Matters improved onincorporating the patch into the full screen laminate(Fig. 16); efficiencies of 72% were predicted, very closeto the Duroid-based patch glued to the screen. Surfacewave excilation was increased to 20%.

Table 1 Resonant frequency for windsc reen tolerancesCase

12345678

Glasslayer 1Er 16.756.257.156.756.756.756.757 .0

Plastic Glasslayer layeir 2E r E r22.9 6.752.9 6.252.9 7.152.4 6.752.9 6.752.9 6.752.9 6.752.7 6.25

D i m e n s i on s ( m m )

-l P h22.5 0.7 2.52.5 0.7 2.52.5 0.7 2.52.5 0.7 2.52.5 0.7 2. 02.0 0.7 2.03.0 0.7 3.02.8 0.5 2.4

B a n d w i d t hM H z40 041 538 541 542 533 539 038 5

fr BinMHz Q5335 455420 505270 415355 455335 525365 655315 385270 38

-

IE E Pr.oc.-Microw Antennas Propag , Vol 145, No . 5, October.1998 419


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5 ConclusionsMicrostrip antennas attached to laminated glass auto-motive windscreens have received little attention. Inthis paper, we have shown that a model based on thespectral domain method of moments analysis givesgood agreement with experimental results for inputimpedance, and provides an insight into surface waveexcitation within the glass up to 6GHz. Circularpatches printed on RT Duroid were used, with glass-laminated superstrates which excited surface waves ofbetween 10%1and 2034 of the input power. Thisresulted in ripples of about 2dB in the radiation pat-terns, but these did not significantly reduce the effi-ciency or usefulness of the patches. An increase in backradiation into the car was also noted. Losses in theglass, which is a poor microwave dielectric, were typi-cally lo% for a structure 6mm thick. Overall efficien-cies were nearly 70 %Antennas printed within the laminate were modelled,and lower efficiencies than those attached to the glasswere found, with increased surface wave radiation andmaterial losses. The glass improved the bandwidth ofthe patches significantly, from under 2% to about 7%1,due to the thick high-permittivity glass superstrate.However, variations in the dimensions of the glass inproduction are significant, up to 15%. The effect ofsuch dimensional tolerances was to change the resonantfrequency by about 3%,, and the bandwidth also varied

by about 1.790. Screen printing patches and microwavecircuits within the glass laminate will introduce evenmore uncertainty into the resonant frequency andbandwidth. Overall, this could be a significant problemfor microstrip antennas on automotive glass wherecommunication bands demand bandwidths of 5% andabove.6 A c k n o w l e d g m e n tThis work was supported by the Onassis Foundation inGreece.

ReferencesLOWES, P. , D A Y , S.R., K O RO K IE W ICZ , E . , and S A M -BELL. A .: Performance of microstrip patch antenna with electri-cally thick laminated glass superstrate, Electron. Let t . , 1994, 30,( 2 3 ) , pp. 1903-1905ECON OMOU , L . , and LANGL EY, R.J . : Performance of patchantenas with glass superstrates, Microw. Opt . Technol. L e t t . ,1996, 11 , ( l ) , pp . 8-10PHANG. Y.H. . ai id HALL. P.S.: Vehicle oatch and slot anten-nas for 5 .8 GHa communication systems. ?CAP97 Conf., 1997,IEE Conf. Publ., Vol. 436, pp. 1.370-373LEE, J.-H. , and RA , J.-W::^ Full wave calculation of the radia-tion impedance of microstrip excited magnatic surface waves,Microw. Opt Technol. Let t . , 1993, 6 , (7), pp. 441-443F A N , Z . , and LEE, K.-F.: Input impedance of annular-ringmicrostrip antennas with a dielectric cover, IEEE Truns., 1992,AP-40, (8), pp. 992-994F A N , Z., and LEE, K.- F.: Analysis of electromagnetically cou-pled patch antennas, Microw. Opt. Technol . Let t . , 1993, 6 , ( 7 ) ,pp. 436-440

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circular micro strip patch antennas on glass for vehicular application

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