This paper presents several antenna topologies for intra-vehicle communication in the unlicensed 2.45 GHz ISM band (2.4 GHz - 2.485 GHz). The antennas are implemented using flexible foam materials compatible with the production process of dashboards, as dashboards offer a large platform for easy and unobtrusive integration. In order to obtain the optimal intra-vehicle antenna, the different topologies are compared in terms of antenna gain, efficiency, AR bandwidth and AR robustness against small fabrication errors

Boeykens, Freek

Declercq, Frederick

Doolaege, Quinten

Rogier, Hendrik

English

F. Boeykens

Q. Doolaege

F. Declercq

H. Rogier

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Optimal topology for an intra-vehicle antenna implemented on dashboard foams

Ghent University Academic Bibliography

Optimal Topology for an Intra-Vehicle AntennaImplemented on Dashboard FoamsFreek Boeykens*, Quinten Doolaege, Frederick Declercq and Hendrik RogierGhent University, Department of Information Technology (INTEC)Ghent, BelgiumEmail: freek.boeykens@intec.ugent.beAbstract—This paper presents several antenna topologies forintra-vehicle communication in the unlicensed 2.45 GHz ISMband (2.4 GHz - 2.485 GHz). The antennas are implemented usingflexible foam materials compatible with the production process ofdashboards, as dashboards offer a large platform for easy andunobtrusive integration. In order to obtain the optimal intra-vehicle antenna, the different topologies are compared in termsof antenna gain, efficiency, AR bandwidth and AR robustnessagainst small fabrication errors.I. INTRODUCTIONThe rising interest in wireless intra-vehicle communicationleads to the concept of the Cable Free Car. Within this conceptit becomes possible to connect certain wireless devices such asPDAs and GSMs to a central infotainment console of the car.These connections use protocols such as Bluetooth, ZigBeeor Wifi, which operate in the license-free 2.45 GHz ISMband (2.4 GHz - 2.485 GHz). An unobtrusively and invisiblyintegrated antenna completely covering this ISM band is thusrequired and past research taught us that this antenna isoptimally positioned in the dashboard with the main lobe ofthe radiation pattern towards the center of the car [1]–[3].The optimal antenna for this application must satisfy threemain requirements:1) easy integration in the dashboard;2) shock and stress resistance;3) quasi-circular polarisation.The first requirement leads to the use of patch antennas.These low-profile planar antennas are light in weight, com-pact and easy to fabricate in mass production. Also flexiblematerials have to be used, since extra effort would be neededfor incorporating a rigid antenna in the flexible dashboard.According to the second requirement, the use of vias shouldbe avoided because these connections are too fragile for thisenvironment. Therefore, an aperture-coupled feed structure isproposed. Such a structure has no physical connection betweenthe feed line and the patch, making the antenna shock andstress resistant. It also allows to use different materials asantenna and feed substrate, minimising back radiation [4].Moreover, an active circuit can easily be integrated onto thefeed substrate.In order to satisfy the last requirement, different antennatopologies exist in literature. In this paper, several topologiesare investigated and compared in terms of antenna gain,TABLE ISUBSTRATES FOR ALL ANTENNAS.Type Description 휖푟 tan 훿Feed substrate Two-layer complex aromatic 2.53 0.065+ aliphatic polyurethaneAntenna substrate Aromatic polyurethane 1.45 0.03with blown air bubblesTABLE IIDESIGN PARAMETERS FOR TOPOLOGY A AND ITS MODIFICATIONS.Parameter A [mm] A.1 [mm] A.2 [mm]Patch (L × W) 44.5 × 41.9 46.4 × 43.7 41 × 41Slot (L × W) 18 × 2 14.2 × 1.4 17.2 × 1.5Stub 8.27 8.57 10Feed 2.8 2.8 2.8Dogbone (l) - 5 -Slit (h) - - 6.25Slit (v) - - 8.25efficiency and axial ratio (AR) bandwidth. Also the AR-robustness is evaluated when small fabrication errors areintroduced.Section II describes the used materials and the differentantenna topologies. Section III presents the simulation results.The conclusions are drawn in Section IV.II. TOPOLOGIESAll antenna topologies in this Section use the same antennaand feed substrates. These flexible substrates were providedby Recticel R⃝ and are typically used in the production processfor dashboards. A description of the materials and theircharacteristics is shown in Table I.The rest of this Section discusses the tested topologies.These topologies were simulated with the 2.5D full-wave fieldsimulator ADS Momentum R⃝ and optimised by imposing thefollowing design criteria for the reflection coefficient (푆11) andthe AR in the 2.45 GHz ISM band:∣푆11∣ < −10 dB, (1)AR < 3 dB. (2)As a result of our design procedure, all topologies satisfy (1) inthe entire ISM band. The AR will be discussed in Section III.W (patch)L (patch)W (slot)L (slot)stubfeed45∘(a)patchground plane antenna substrate, 휖푟,1feed substrate, 휖푟,2feed line(b)Fig. 1. Topology A: top view (a) and side view (b).A. Crossed slot antenna with single feed lineThe first antenna topology, depicted in Fig. 1, consists ofa nearly-square patch, a crossed slot and a feed line placedalong the diagonal of the cross. Quasi-circular polarisation isobtained by excitation of two orthogonal modes in the crossedslot [5], [6]. The design parameters are shown in Table II.Two modifications to this topology were also simulated(their design parameters can be found in Table II as well):1) Dogbones: Here, the aperture is extended with so-called ’dogbones’, making the crossed slots H-shaped. Thesedogbones increase the coupling between the feed line andthe patch due to a more uniform equivalent magnetic fielddistribution on the slot [7]. This modification is shown inFig. 2(a).2) Patch with slits: A second modification introduces slitsin the patch in order to obtain a reduced patch size anda broader AR bandwidth [8]. This topology is depicted inFig. 2(b).B. Crossed slot antenna with single series feedThis antenna also couples the signal through a crossed slot,but unlike previous topologies, a series feed line is placedbehind the aperture. Between each arm of the aperture is aquarter-wavelength section of the feed line positioned in orderto obtain circular polarisation. This has the effect of hybridfeeding techniques and thus widens the AR bandwidth [9].The topology is shown in Fig. 3 and the design parametersare presented in Table III.W (patch)L (patch)W (slot)L (slot)stubfeed45∘l (dogbone)(a)W (patch)L (patch)W (slot)L (slot)stubfeed45∘v (slit)h (slit)(b)Fig. 2. Modifications to topology A (top view): dogbones (a) and patch withslits (b).TABLE IIIDESIGN PARAMETERS FOR TOPOLOGY B.Parameter Value [mm]Patch (L × W) 40.5 × 40Slot (L × W) 26 × 1Stub 12.5Feed 2.8휆/4 18.2C. Quadrature hybrid couplerThe last topology that is tested uses a quadrature hybridcoupler for circular polarisation. If one port of this 4-portstructure is terminated by its characteristic impedance, thenthe coupler acts as a power splitter with two output signalsequal in amplitude and with 90∘ phase difference. Now twoW (patch)L (patch)W (slot)L (slot)stubfeed휆4Fig. 3. Topology B: top view.W (patch)L (patch)W (slot)L (slot)feedstuboffset 2offset 1Fig. 4. Topology C: top view (hybrid coupler not shown).separate rectangular apertures couple the power on the twofeed lines into the patch.The use of a hybrid coupler makes the design more com-plex, but ensures circular polarisation in the complete ISMband [10]. Fig. 4 depicts this topology and Table IV showsthe design parameters.III. SIMULATION RESULTSA. Antenna gain and efficiencyTable V shows the simulated antenna gain and efficiencyin broadside direction at 2.45 GHz. From this point of view,topology A would be the best fit, as it has the highest gainand efficiency. The lowest gain and efficiency can be foundfor topology C.TABLE IVDESIGN PARAMETERS FOR TOPOLOGY C.Parameter Value [mm]Patch (L × W) 46.3 × 46.3Slot (L × W) 20 × 3Stub 10.1Feed 2.8Offset 1 4.85Offset 2 5.35TABLE VSIMULATED GAIN AND EFFICIENCY OF ALL TOPOLOGIES AT 2.45 GHZ.Topology Gain [dBi] efficiency [%]A 4.018 41.3A with dogbones 4.005 38.7A with slits 3.376 35.3B 3.153 39.6C 2.923 29.7Fig. 5. Simulated AR in the 2.45 GHz ISM band for all topologies.B. AR performanceAnother important criterion for the intra-vehicle antennais quasi-circular polarisation in the ISM band. As a rule ofthumb, circular polarisation is assumed for an AR lower than3 dB.Fig. 5 demonstrates the AR in the 2.45 GHz ISM band forall topologies. One can see that the curves for topology A andtopology A with dogbones are almost the same. Introducingslits in the patch leads to a worse AR bandwidth and thehighest values at the ISM band edges. Topology B has abroader bandwidth and lower values at the boundaries of theISM band and for topology C, there is circular polarisation inthe entire ISM band.A sensitivity analysis is also performed by introducing smallfabrication errors on the designs and evaluating the AR inthe ISM band. Only the errors on the apertures are shown,since they affect the AR performance the most. The introducederrors can be divided into two parts:1) Alignment errors: Alignment errors can be simulatedby moving the slots in all directions. As can be seen onFig. 6(a), the 3 dB AR bandwidth remains almost constantfor all topologies when the slots are shifted by 1 mm to right.For topology A and its modifications, moving the slots onlyprovokes a shift of the curves. The AR curve of topology Bhas moved upwards, but the values at the edges of the ISMband are more or less the same as without alignment errors.Topology C is still circularly polarised in the entire ISM band.Similar results are obtained when the slots are moved inthe other directions. All topologies also still satisfy (1) afterintroduction of aligment errors.2) Slot dimension errors: The slot dimension errors aresimulated by enlarging and widening the apertures. The largesteffect can be observed for an asymmetrical enlargement ofthe slots, as shown in Fig. 6(b), which depicts the AR ofall topologies after an enlargement of the vertical aperturesby 1 mm. Unlike alignment errors, the slot dimension errorsintroduce a decrease in AR bandwidth (except for topology C,which remains circularly polarised in the entire ISM band).Also, the return loss of topology B only satisfies (1) forfrequencies between 2.46 GHz and 2.485 GHz.When the vertical apertures enlarge even more, the ARcurves of topologies A and B stay above 3 dB and hencethere is no frequency in the 2.45 GHz ISM band anymore forwhich these topologies are circularly polarised.The results for other slot dimension errors have less influ-ence on the AR curves, so we can conclude that asymmetricalslot dimensions lead to the worst AR performances.IV. CONCLUSIONIn the search for an optimal intra-vehicle antenna, severaltopologies were investigated and compared. Topology C isdefinitely the most robust of all topologies against fabricationerrors. However, circular polarisation is not necessary for intra-vehicle communication, since the purpose here is to minimisethe polarisation mismatch between transmitter and receiver.Therefore it is recommended to use a simpler design with ahigher gain and efficiency and hence we suggest topology Aas the optimal intra-vehicle antenna.ACKNOWLEDGMENTThe authors would like to thank Recticel R⃝ for the provisionof the dashboard materials.The work of F. Boeykens was supported by a doctoral grantfrom the Agency for Innovation by Science and Technologyin Flanders (IWT).REFERENCES[1] H.-M. Tsai, C. Saraydar, T. Talty, M. Ames, A. Macdonald, andO.K. Tonguz, ZigBee-based Intra-car Wireless Sensor Network, IEEEInt Conf on Comm, pp. 3965-3971, 2007.[2] R. de Francisco, L. Huang, G. Dolmans, H. de Groot, Coexistence ofZigBee wireless sensor networks and Bluetooth inside a vehicle, IEEEInt Symp on Personal, Indoor and Mobile Radio Comm, pp. 2700-2704,2009.(a)(b)Fig. 6. AR performance of all topologies in the 2.45 GHz ISM band: shiftof the slot to the right by 1 mm (a) and enlargement of the vertical slot with1 mm (b).[3] F. Boeykens, Q. Doolaege, F. Declercq, H. Rogier, A planar aperture-coupled patch antenna implemented on dashboard foams for intra-vehicle communication, accepted for publication in Microwave andOptical Technology Letters.[4] D.M. Pozar, Microstrip antenna aperture-coupled to a microstrip line,Electronics Letters, vol. 21, no. 2, pp. 49-50, 1985.[5] T. Vlasits, E. Korolkiewicz, A. Sambell, B. Robinson, Performance of across-aperture coupled single feed circularly polarised patch antenna,Electronics Letters, vol. 32, no. 7, pp. 612-613, 1997.[6] B. Al-Jibouri, H. Evans, E. Korolkiewicz, E.G. Lim, A. Sambell,T.Viasits, Cavity model of circularly polarised cross-aperture-coupledmicrostrip antenna, IEEE Proc-Microwave Antennas Propag, vol. 148,no. 3, pp. 147-152, 2001.[7] D. Pozar, S. Targonski, Improved coupling for aperture coupled mi-crostrip antennas, Electronic Letters, vol. 27, no. 13, pp. 1129-1131,1991.[8] K.L. Wong, J.-Y. Wu, Single-feed small circularly polarised squaremicrostrip antenna, Electronics Letters, vol. 33, no. 22, pp. 1833-1834,1997[9] H. Kim, B.M. Lee, Y.J. Yoon, A single-feeding circularly polarisedmicrostrip antenna with the effect of hybrid feeding, IEEE Ant. andWireless Propag. Letters, vol. 2, pp. 74-77, 2003.[10] D. Pozar, Microwave Engineering, Addison Wesley, 1990.

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Optimal topology for an intra-vehicle antenna implemented on dashboard foams

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Ghent University Academic Bibliography