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High Gain Printed Ultra WidebandAntenna Concept

Veselin Brankovic, Adalbert Jordan, Djordje Simic, Jens Weber, Jagj it Bal

TES Electronic Solutions GmbH; Stuttgart Germany

Abstract Ultra wideband (UWB) communicationsystems are emerging on the short range communicationmarket covering variety of the application scenarios.Fact that UWB Systems are based on utilising widefrequency bands of operation makes specificrequirements for antenna solution quite demanding.Antenna dispersion behaviour is critical for the systemperformance. Printed low cost antennas with thecapability to be integrated on the same substrate withthe UWB front end are very suitable for differentapplications. In many specific cases, havingasymmetrical data traffic like access point applications,it is highly desired to have high gain printed UWBantenna solution, being able to function as sectorantenna with wide azimuth beam width and moderatebeam width in elevation. In this paper, high gain UWBprinted antenna concept covering complete frequencyrange from 3-10 GHz is presented. The antenna can beeasily integrated with UWB ICs on the same substrate,offering inherently low cost solutions with antenna againin the range of 7-10 dBi. Proposed concept has beenverified by simulations and finally confirmed bymeasurements.

Index Terms- Multi band OFDM, IEEE 802.15.4a, Ultrawideband (UWB), Printed Antenna, High Gain Antenna

I. INTRODUCTION

Ultra wideband (UWB) communication systems [1]-[2]for both High Date Rate Approach (MB-OFDM, HDR) andLow Data Rate Location and Tracking Approach (LDR-LT)have some typical application scenarios requiring high gainantenna solutions.

In the case of the HIDR applications, typical scenariosmight cover:

- Receiving multimedia streams from car dash boardto the seats having integrated displays (antenna integrated inthe seat)

Authors are with the TES Electronic Solutions GmbH, Zettachring 8,Stuttgart Germany, Tel. +49 71172877450, TES Electronic Solutions is a

leading global B2B High Tech Company with strong advanced R&D on

UW1 Technology development and products deployment. www.tesbv.com

-AReceiving multimedia and data streams to thedisplays in home and business environment (antennaintegrated in the display)

In the case of the LDR-LT typical application scenariosmight be:

- Receiving data of mobile tag in location andtracking system as antenna of the access point.

- Receiving information from wireless sensors byconcentrator, having integrated antenna radiating indedicated space sector

In the state of the art printed antenna solutions severaldifferent approaches have been reported, mainly providingsolutions based on monopole approach, for lower (3-5GHz), higher (6-10 GHz) and complete UWB band (3-10 GHz)[3]-[4]. SMD solutions have been also promoted for lowerand higher UWB bands, usually being sensitive to theimplementation of the PCB ground surface, and usuallyaddressing one of the UWB bands. The common approachis that the proposed antenna solutions provide omnidirectional radiation characteristics, with limited gaincapabilities and generally no possibility to influenceradiation direction in the simple way. Recently, printedbroadband radiation elements have been investigated anddeployed in high gain printed antenna solutions [5]-[6].

II. CHALLENGES AND CONCEPT SOLUTION

A. Challenges & Requirements

The concept solution must be able to cope withfollowing three requirements:

- Planar printed radiation elements, with 50% of theoperation bandwidth related to the center frequency in themanner that thickness of the dielectric substrates does notessentially influence the performance of the antenna. Thismay allow to deploy the concept on substrates of differentpermittivity and thickness and allow easy integration of theantenna with the front end

- Feeding network for radiation elements should beimplemented as planar printed circuit, being able to supportsignals transmission using frequency non-selective planarpower splitters.

- A metallic structure, ideally printed, should be usedas reflector to shape the radiation pattern ofthe antenna

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B. Radiation Elements

In order to provide broadband operation of the elementsbeing able to be easily fed with printed structures, printedcircular dipoles have been proposed as radiation elements.In this approach one half of the dipole is printed on one sideof the substrate and another part of the dipole on theopposite side, being fed by balanced microstrip line like it isshown in Fig. 1. Having half dipoles with circular orellipsoidal shapes as proposed in this paper, achievableantenna bandwidths have been increased for about 20%compared to approach published in literature [5] .

Figure 1 - Circular printed dipole radiation element; the secondpart of the dipole is on the opposite side of the substrate.Exponentially tapered strip is introduced to allow transformationto unbalanced line and therefore assembly of the connector fortests

The robustness to manufacturing tolerances is alsoincreased, meaning that proposed shapes could be easilymanufactured by low-cost printed technologies, even formm-waves applications. Moreover, the proposed antennashapes allow easier optimisation to meat required inputimpedances, so mainly one parameter needs to be finetuned. Measured return loss of the printed antenna on theR04003 substrate of 0.51mm thickens and Er of 3.38 overthe entire frequency band from 3 to 10 GHz is better than -12 dB.

C. Feeding network implementationfor multiplieddipolesImplemented dipoles could be easily used in antenna

arrays to obtain higher directivity. Due to the nature of thebalanced microstrip line, deployment of the feeding networkon the same substrate where the radiation elements areprinted is straight forward. In order to ensure splitting of thesignals in the wideband manner, instead of using frequencyselective quarter wave-transformers, linear tapering of thestrips is proposed. This simultaneously addresses moretypical problems, like discontinuities, manufacturingtolerance vulnerability and offers matching with satisfactoryresults in entire band of interest.

t igure 2 - teeaing network outiookjor two raaiation aipoies

D. Antenna implementation with corner reflector

Symmetrical mechanical reflectors, built from bendedmetal plate, may be used for shaping radiation diagram ofthe printed antenna in azimuth. The substrate with printeddipole structures with feeding networks is placed in axialsymmetry plane of the corner reflector through the narrowcut slot, like presented in the Fig. 3a. Slight change of thereflector position in the respect to the radiation elements,which is initially around quarter wavelength on the centeroperation frequency, combined with the variation of thefeeding network lines width and radius of the circular halfdipole may be used for final optimization of the antennacharacteristics.

A~OO

Figure 3a -High gain antenna outlook andphoto with bendedmetallic reflector

UWB Antenna Array with "31D" reflector

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Figure 3b - Simulated and measured input reflection loss forhigh gain antenna with bended metallic reflector with excellentmatching between simulation and measurements; the samesubstrates and size ofthe dipoles as in Fig] are used.

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The measured gain is 9.6 dBi at 4 GHz and the reflectioncoefficient in the frequency range between 3 and 10 GHz isbelow -10 dB. The dimensions of the antenna related to Fig.3a are: 89 mm, wide, 63 mm long and the height of thereflector is 40 mm.

The concept of the corner reflector has severaldrawbacks like: increased manufacturing cost due to the useof the bended metallic plate, mechanical issues connectedwith exact placement of the substrate in the axial symmetryplane of the metallic structure, and overall mechanical size,which can be too big for some applications. Also, this typeof antenna has more nonlinear phase response since positionof its phase centre is not stable with frequency because ofreflector position. The graph on the Figure 4 shows theimpulse response of the antenna which illustrates that theantenna can be used in systems which have system responselonger of Ins, like IR-UWB with 500 MHz bandwidth,compatible with IEEE 802.5.4a standard. The deteriorationof the pulse shape for wider channels can make significantinfluence on the system performance, so applicability of theantenna in such systems is limited.

Impulse response

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Figure 4 - Time response ofthe high gain antenna with 3Dreflector

On the other hand, with the change of the corner angleand the size of the reflector radiation characteristics may beinfluenced. So the main beam width in azimuth can bevaried between 60 and 150 degrees and also maximum gaincan be changed for approximately - 2.5 to 3 dB. Of course,there are also corresponding changes in the beam width inelevation direction.

III. HIGH GAIN PLANAR ANTENNA CONCEPT

In many application scenarios requiring high gainantenna features, three common performance andconstruction requirements are:

a) Half-space radiation pattern in azimuth (1 80° beamwidth)

b) Moderate beam width in elevation direction (not towide, not to narrow) ideally about 60 degrees.

c) Flat construction of the antenna, without externalmechanical reflector, offering easy integration of theantenna in end product, ensuring minimal production costsand time.

Achieving those requirements is possible bymodification of the corner reflector approach. In the Fig. 5novel design is presented. Instead of having bended metallicstructure as the reflector, metallic surfaces are printed on thesame substrate as the dipoles. The basic distance from thecentre of dipoles to the central part of the reflector isapproximately quarter wavelength of the central frequency.The proposed reflector is "following" the changes of theprinted dipoles and feeding network shape, forming twobanana-shaped plate reflectors. The antenna radiationpattern is influenced by the reflector geometry and may beoptimized by further change of the reflector plate size andshape. The distance of the printed metallic reflector to thefeeding line should be more than one strip width in order notto influence the characteristic impedance of the feeding lineand to avoid possible discontinuities.

Suppressing "odd" mode of propagation in the vicinity ofthe reflector may be provided in the manufacturing processby "bridging" two reflector halves by bond wires or by othermeans, ensuring the same potential of both reflector metallicsurfaces. Proposed concept has one essential feature that thereflector portion of the printed metallic structure isconcentrated near to the structure edges and is considerablysmall compared to the complete printed area. This can beessentially used for placing the RF front end components ofthe UWB communication system inside the reflectorstructure. In that case no antenna connector is required andcompact high gain UWB solution is obtained.

Figure 5reflector

!gain antenna concept with printed metallic

A. Simulation and measurement results

Verification of the proposed concept is done bysimulation and finally by producing related prototypes.

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Simulation is performed by using WIPL-D software [7], likeit has been done in the case of 3D reflector antenna.Microwave substrate R04003 with Er of 3.38 and thicknessof 0.51mm is used, with 18pm copper thickness with hardgold coating. The dimensions of the antenna are: 89 mmtimes 63 mm, which is the same as the size of 3D reflectorantenna board, but in this case, the thickness of the antennais much smaller (0.5mm, without connector), enabling easyintegration in many space-restricted applications. Of course,in some cost sensitive application low-cost substratematerials like FR-4 can be used, with some reduction of theachievable antenna gain due to the losses in the material.

In Fig 6 Input reflection loss (simulations andmeasurements) for the planar high gain antenna is presented,confirming expected performance in complete band ofoperation better than -10 dB.

Measured and simulated results

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Figure 6 - Simulated and measured reflection lossfor high gainplanar UWB antenna with printed metallic reflector

BRadiation pattern at four frequencies

Figure 7 -Measured antenna diagram in azimuth for high gainantenna concept with printed metallic reflector

Fig 7 shows measured antenna radiation pattern inazimuth for set of frequencies, showing expecting behavior.3dB beam width varies from 180 degree to 220 degree for 3and 10 GHz respectively. From 5 to 7 GHz measured 3-dBbeam width is constant and is more as 250 degrees. Figure 8shows simulated 3D radiation pattern at 4 GHz. 3dB beamwidth of 250 degrees in predicted, what corresponds very

well with the measurements. The gain of the antenna is stillsmaller as one obtained with previously described antennawith 3D reflector, but it is compensated with wider radiationangle and better phase linearity which results in more idealimpulse response ofthe antenna. (Fig. 9.).

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Figure 8 -Radiation pattern at 4 GHzfor high gain antenna withprinted metallic reflector

Measured impulse response of the proposed antennaconcept may be observed in the Fig. 9. The data is obtainedby using frequency domain measurements with two antennasas described in [8]. The measured results look promising,showing the peak of the second response maximum 7 timesless in absolute field strength, showing small antennadispersion in time, what is consequence of good antennaphase linearity. It shows that the antenna can be successfullyused for both impulse-based and MB-OFDM UWB systems.

Impulse response

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Figure 9 - Measured impulse response ofthe planar high gainantenna

For demonstration purposes, proposed antenna conceptis successfully integrated in the automotive seat to check thetypical application scenario on unsymmetrical distributionof the video signals in car entertainment system. The datatransmission is performed from the dashboard to the seat,which had a display mounted on the back as illustrated onthe Fig. 10. The antenna was placed on the side of the seatabove arm-rest position. The same scenario is comparedusing an omni-directional antenna as a reference; showingexpected much higher performance, robustness and

-3 GFt-5 GFt-TGFt

9 GFtI-------

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resistance toward blocking with human body. The tests havebeen performed with high data rate WiMedia compliant chipsets from Wisair, being connected to the antenna via SMAconnector, using FCC radiation mask in 3-5 GHz range.

Figure 10 - Implementation of the high gain antenna fordemonstration of video streaming application in a car

entertainment system

IV. CONCLUSION

High gain UWB printed antenna concept operation in thecomplete frequency band from 3-10 GHz is presented.Coverage in azimuth of 180 degree with measured gainbetween 7-8 dBi that have been verified by simulation andmeasurements for the entire band. Proposed low cost singleprinted antenna concept is able to be used for easy

integration of the UWB ICs on the same substrate withoutcables and connectors. The design is compared to theantenna using same radiating elements and suitable 3Dreflector, showing improvements in radiation diagram andimpulse response. Proposed antenna is advantageouslysuccessfully tested and benchmarked against classic omnidirectional printed antennas on typical video streamingapplication. The practical implementation value of theconcept is therefore also confirmed. Realisation of theantennas on very low cost substrates is possible with some

reduction in achieved overall gain.

V. ACKNOWLEDGEMENT

The work presented here has been partially performed inscope of the European research project PULSERS, which ispartly being funded by the Commission of the EuropeanUnion within the 6th EU research framework.

The authors would like also to thank ENSTA- Parisantenna laboratory for kindly supporting antenna radiationpattern measurement activities on their facilities andproviding help in measurement data interpretation.

VI. REFERENCES

[1] Multiband OFDM Alliance, "MultiBand OFDM Physical LayerProposalfor IEEE 802.15 Task Group 3a ", September 2004

[2] Anuj Batra, Jaiganesh Balakrishnan, Anand Dabak, et al."TI PHY layer proposal", IEEE 802.15- 03/ 141r4, IEEE802.15.3aPHY layer Working Group, 2003

[3] Targonski, S.D.; Waterhouse, R.B.; Pozar, D.M.; "Design ofwide-band aperture-stacked patch microstrip antennas", Antennas andPropagation, IEEE Transactions on Volume 46, Issue 9, Sept. 1998Page(s): 1245 - 1251

[4] C. Y. Wu, C. L. Tang, and A. C. Chen.;"Compact Surface mountUWB Monopole Antenna for Mobile Applications", Progress InElecromagnetics Research Symposium 2006, Cambridge, USA, March 26-29.

[5] Nesic, A.; Brankovic, V.; Krupezevic, D.; Ratni, M.; Nesic, D.,"Broadband printed high gain antenna with wide angle radiation inazimuth" Antennas and Propagation Society International Symposium,2001. IEEE Volume 2, 8-13 July 2001 Page(s):468 - 471 vol.2

[6] Nesic, A.; Radnovic, I.; Brankovic, V.; Ultra widebandprintedantenna array for 60 GHz frequency range", Antennas and PropagationSociety International Symposium, 1997. IEEE., 1997 Digest Volume 2,13-18 July 1997 Page(s): 1272 - 1275 vol.2

[7] WIPL-D High Frequency 3D EM Modeling and SimulationSoftware,w

[8] W. Sorgel, F. Pivit, W. Wiesbeck ,"Comparison of FrequencyDomain and Time Domain Measurement Procedures for Ultra WidebandAntennas," IEEE EMC Magazine, September 2004, pp3 1-35