Experimental validation of time-domainelectromagnetic models for field coupling into theinterior of a vehicle from a nearby broadbandantenna

A.R. Ruddle, X. Ferri"eres, J.-P. Parmantier and D.D. Ward

Abstract: Numerical electromagnetic models based on a typical automotive immunity measure-ment scenario have been built from a vehicle manufacturer’s CAD data and compared withcorresponding measurements on a complete vehicle. The simulations were carried out in the time-domain using the TLM and FDTD techniques. Despite the many limitations of both the numericalmodels and the measurements, the models are found to provide a satisfactory representation of themeasured field coupled into the passenger compartment from a nearby broadband antenna.

1 Introduction

Modelling automotive immunity test scenarios is aparticularly challenging activity. Not only is the vehicleboth geometrically complex and electrically large at thefrequencies of interest (to 1GHz in current legislation,although some manufacturers already test up to 3GHz),but the measurements also employ a broadband antennaplaced in very close proximity (B1m) to the vehicle. Inaddition, the most common test environment is asemianechoic chamber. The use of idealised plane wavesand simple antennas to excite the system model is thereforeunsuitable for automotive immunity models that areintended for experimental validation purposes. To besuccessful such a model must include details of the sourceantenna as well as the vehicle geometry.

2 Experimental measurements

The measurement configuration used here was based onstandard automotive immunity test arrangements [1]. Acomplete vehicle was placed in MIRA’s large semianechoicchamber (with a working volume 22� 10� 8m) andilluminated from the front and side using a ‘biconilog’antenna. This device is essentially a log–periodic dipolearray augmented with a pair of ‘bow-tie’ elements to obtainimproved low-frequency performance. The bandwidth was20–1000MHz.

The antenna and chamber were calibrated as specified in[1], by recording the power required to generate an electricfield of 50V/m in the empty chamber at a reference pointthat is specified in relation to the vehicle geometry. Thispoint is located 0.2m behind the front axle, on the

longitudinal axis of the vehicle at 1m above the groundplane. The antenna was positioned with the feed point at aheight of 1.2m, at a distance of 2.5m to the front or side ofthe reference point (see Fig. 1). The calibration data is usedto estimate the power needed to generate the required‘‘threat’’ field during vehicle measurements.

Standard measurements allow deviations in the func-tional performance of the vehicle to be referenced to thecorresponding field at the reference point in the emptychamber. In this work, the electric field strengths measuredat selected points within the vehicle were similarly normal-ised using the field at the reference point for the emptychamber. Thus the measured relative field strengthrepresents the resulting field at the measurement pointunder a notional threat of 1V/m at the reference point. Thisapproach may also be used [2] to produce computed fieldresults that can be directly compared with results frommeasurements without the need to model the antenna

2.5 m

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Fig. 1 Antenna positions relative to vehicle and calibrationreference point

A.R. Ruddle and D.D. Ward are with MIRA Ltd, Watling Street, Nuneaton,Warks, CV10 0TU, UK

X. Ferri"eres and J.-P. Parmantier are with ONERA, BP 4025, 2 AvenueEdouard Belin, Toulouse 31055, Cedex 4, France

r IEE, 2004

IEE Proceedings online no. 20040952

doi:10.1049/ip-smt:20040952

Paper received 10th March 2004. Originally published online: 23rd November2004

430 IEE Proc.-Sci. Meas. Technol., Vol. 151, No. 6, November 2004

source characteristics in detail, and to reduce the impact ofsystematic errors.

Electric field measurements were made at selected pointsin the passenger compartment using isotropic field probes.Reproducible positioning of the probes was achieved bymounting the probes on a thin wooden board that could bereliably located between the armrests in the front and rearof the vehicle relative to readily identified fixed points. Thisallowed the field measurement points to be related to thevehicle CAD data, and also ensured that they could beeasily reproduced in the course of measurements, whichrequired the probes to be moved between the front and rearof the passenger compartment (see Fig. 2). The measure-ments were made under illumination from the front andside of the vehicle, for both horizontal and vertical antennapolarisations. Separate calibration results were used tonormalise the data for each of the four antenna illuminationconfigurations.

3 Numerical models

Time-domain methods are particularly suitable for EMCapplications because of the need for broadband results. Forlarge and complex systems, methods based on structuredmeshing (using hexahedral cells) offer many advantages,including more frugal use of memory than unstructuredmeshing methods and the ability to accommodate addi-tional surfaces without increasing the memory needed. Themain disadvantage for vehicle applications is the resultingstaircase approximation of curved surfaces. The resultsreported here were generated using two such time-domain,structured meshing techniques: FDTD [3] and TLM [4].The simulations were carried out using ONERA’s ALICEFDTD code [5] and the Microstripes TLM solver [6] fromFlomerics.

Although the FDTD and TLM models aim to representthe same system, the numerical representations in fact differfor a variety of practical and operational reasons. In theFDTD model for the antenna, the dipole elements wererepresented using thin wires. For the TLMmodel, however,it was found to be more practicable to use solid bars. Theantenna used for the measurements had 20 dipoles inaddition to the bow-tie elements, all of which wererepresented in the TLM model. In the FDTD model,however, the smallest four elements (with lengths less than 8cm) were neglected to limit the size of the final systemmodel, while the bow-tie elements were modelled astriangular conducting sheets. In the real antenna and theTLM model the bow-tie elements are formed fromconducting rods. Nonetheless, both antenna models were

found to provide a good representation of the real antennaperformance.

Although the vehicle used for the measurements wascomplete and fully functional, the content of the numericalmodels had to be constrained to ensure that the resultingmodels did not exceed the available computing resources.Since the measurements were limited to the passengercompartment the engine bay and under-body componentswere not included. Comparative measurements on thecomplete vehicle [7] indicated that the window glazing andseat cushions did not impact significantly on the electricfield coupled into the vehicle interior, for frequencies up to 1GHz. This suggested that all such dielectric materials withinthe vehicle interior could reasonably be neglected. Themodels were therefore limited to the vehicle shell andsignificant metallic parts in the passenger compartment,which included the seat frames and those elements of thesteering that are located in this region.

The FDTD vehicle model (see Fig. 3) was based on ageometrical model that had been simplified and reduced tosingle surfaces, augmented with simplified models for theseat frames and steering gear. This is because the FDTDmodel was obtained by remeshing a triangular surface patchmesh that was generated by EADS CCR (France) from asimplified, single-surface model for boundary-elementsimulations. The number of surfaces is crucial for modelsize in boundary element and related techniques, but forTLM and FDTD the model size is determined primarily bythe number of cells within the computational volume, whilethe number of surfaces within this region has very littleimpact on the memory requirements. The TLM model(Fig. 4) was constructed directly from the original vehicleCAD data using specialised meshing tools, and thusincluded both the inner and outer surfaces of structuressuch as the doors, together with higher fidelity models of theseat frames and steering gear.

The TLM model for front horizontal illumination was9.1 million cells and required 860Mbytes, while that for sideillumination was almost twice as big, with 17.3 million cellsrequiring 1.6 Gbytes. The cell sizes ranged from 4mm (to

Fig. 2 Positioning of electric field probes for measurements abovethe front seats

Fig. 3 FDTD model: front vertical illumination

Fig. 4 TLM model: front horizontal illumination

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capture the antenna geometry and local field gradients) to30mm (for a model with 1GHz bandwidth). A free-spaceabsorbing boundary condition was used to truncate themodel above the ground plane.

The FDTD model for front horizontal illumination was1.85 million cells and required 85Mbytes, while that for sideillumination was 1.95 million cells and required 89 Mbytes.The cell sizes used in this model ranged from 12.5mm, tocapture the antenna geometry, up to a maximum of 40mm.A five-layer perfectly matched layer was applied at the fivefree-space boundaries.

4 Validation results

The success of the antenna models was investigated bycomparing the relative field (normalised to the field at thereference point) at selected points without the vehiclepresent. Results for the TLM and FDTD antenna models

are compared with corresponding measurements in Figs. 5and 6, for horizontal and vertical polarisation. Since thelow-frequency resonances that are present in the measure-ments are not duplicated in the simulations it is believedthat these features result from finite wall reflections in thesemianechoic chamber, which are not represented in themodels. However, the main features of these plots are dueto the antenna and its position (together with that of themeasurement points) relative to the conducting ground:these show a satisfactory correspondence between thesimulated and measured results.

The antenna calibration results were used to normalisethe field coupling results in both the real and simulatedmeasurements. Sample results, which take account of theestimated uncertainties for the measurements, are shown inFigs. 7 and 8 for a particular field measurement point under

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Fig. 5 Relative electric field at point for horizontal biconilogantenna above ground plane

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Fig. 6 Relative electric field at point for vertical biconilog antennaabove ground plane

measurement boundsMIRA TLM ONERA FDTD

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Fig. 7 Electric field at front left-hand monitoring point: verticalpolarisation, front illumination

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Fig. 8 Electric field at front left-hand monitoring point: horizontalpolarisation, side illumination

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two different illumination configurations. The low-fre-quency measurements (below 100MHz) are corrupted bychamber effects that are not represented in the models. Atthe higher frequencies, however, both the TLM and theFDTDmodels generally show very similar topologies to themeasurements, while the absolute levels are generally arewithin the estimated error bounds of the measurements.

5 Conclusions

Despite the many approximations and limitations of boththe simulations and the measurements, the TLM andFDTD models are found to provide a satisfactoryrepresentation of the measured electric field coupling. Suchmodels therefore have potential to provide objectiveinformation to support automotive EMC (electromagneticcompatibility) engineering activities, both during the designstages of vehicle development and in problem resolution.Furthermore, such models allow automotive electromag-netic measurement issues to be investigated theoreticallyusing realistic models, thus aiding the understanding anddevelopment of practical test methods.

It is also found that the quality of the two models is notmarkedly different, indicating that the heavily simplifiedgeometry used for the FDTD model can still yieldsatisfactory predictions for the field coupled into the interiorof the vehicle. This suggests that approximate vehiclegeometry, as well as more computationally efficient, lowerfidelity models, can probably be used in practical applica-tions with reasonable confidence in the quality of theirresults.

For vehicle design studies it would be more efficient touse models that are excited by idealised plane-wave sources,which would significantly reduce the size of the modelcompared with the validation test cases described here. Inthe models reported here the need to represent the details ofthe illuminating antenna represented a significant overheadfor the simulated measurements, which required both theantenna and the vehicle to be present in the same model.

In addition to EMC related applications, whole vehicleelectromagnetic models are also beginning to be used topredict installed performance characteristics for vehicleantennas, and to assess occupant field exposure due toonboard transmitters.

6 Acknowledgments

The work described was carried out as part of theGEMCAR project, a collaborative research project sup-ported by the European Commission under the Competitiveand Sustainable Growth Programme of Framework V (ECcontract G3RD-CT-1999-00024) and by the Swiss FederalOffice for Education and Science (grant 99.0377).

The project consortium included MIRA Ltd (co-ordinator), QinetiQ and Ford Motor Company Ltd ofthe UK, EADS CCR, CETIM and ONERA of France,EPFL (Switzerland), Hevrox EMC and Safety Services NV/SA (Belgium) and Volvo TDC (Sweden). The test vehicleand its associated CAD data were kindly provided byVolvo Cars (Sweden).

7 References

1 ‘‘Commission Directive 95/54/EC’’, Off. J. Eur. Communities, No. L266, 1995, 1–66

2 Ruddle, A.R., Pomeroy, S.C., and Ward, D.D.: ‘Numerical modellingof a stripline antenna in a large semianechoic chamber’. Proc. IEEEEMC Symp., Montreal, Canada, August 2001, 1, pp. 298–301

3 Taflove, A.: ‘Computational electromagnetics: the finite-difference time-domain method’, (Artech House, London, 1995)

4 Christopoulos, C.: ‘The transmission-line modelling method: TLM’,(IEEE Press, New York, 1995)

5 Grando, J., Michielsen, B., Issac, F., and Alliot, J.C.: ‘FDTDformalism including a microstrip inside an elementary cell’ IEEE Int.Symp. on Antennas and Propagation, Atlanta, CA, USA, 1998

6 Johns, D.P., Scaramuzza, R., and Wlodarczyk, A.J.: ‘Micro-Stripes–microwave design tool based on 3D-TLM’, Proc. 1st Int. Workshop onTransmission Line Matrix Modelling – Theory and Applications,Victoria, BC, Canada, August 1995, pp. 169–177

7 Ruddle, A.R.: ‘Measured impact of vehicle seats and glazing on thecoupling of electromagnetic fields into vehicles and their wiringharnesses’, Proc. 15th Int. Symp. on EMC, Zurich, Switzerland,February 2003, pp. 487–492

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