Reconfigurable Multi-Beam Pillbox Antenna forMillimeter Wave Automotive Radars

Mauro Ettorre and Ronan SauleauInstitut d’Electronique et de Telecommunications de Rennes (IETR)

UMR CNRS 6164, Universite de Rennes 1, 35042 Rennes Cedex, FranceEmail: mauro.ettorre, ronan.sauleau@univ-rennes1.fr

Abstract—This work proposes a multi-layer multi-beam recon-figurable pillbox antenna based on a novel transition betweentwo substrates working as a quasi-optical system. The antennaconsists of three main parts that can be designed and optimizedindependently: feeding part, quasi-optical system and radiatingpart. The novel quasi-optical system is made by several slotsetched in the common metal layer and located along the entirelength of an integrated parabolic reflector made by verticalmetallic pins. The quasi-optical system shapes and focuses theenergy traveling inside the lower layer to the radiating partmade by an array of slotted waveguides. Several integrated hornsplaced in the focal plane of the integrated parabolic reflector areused to feed the quasi-optical system. Thanks to this feedingsolution and the introduced quasi-optical system, multi-beamoperation and beam reconfiguration capability on a wide anglefield of view are achieved. The proposed structure is compatiblewith low-cost printed circuit board and substrate integratedwaveguide technology. The low profile, low cost, efficiency andpattern reconfigurability of the proposed antenna make it verypromising for the next generation of automotive radars.

I. INTRODUCTION

The recent European regulation on automotive radars in the76 GHz band has given a boost to the development of new an-tenna concepts for automotive systems. Nowadays automotiveradar systems are only used for comfort purposes but, as alsorequired by the European Commission, they should evolve toactive systems in order to improve the road safety, being ableto detect and respond to different scenarios close (short rangeoperation, SR) and far away (long range operation, LR) fromthe car. In these two operation regimes the future radar systemsshould be able to detect both moving and stationary objectsin the form of vehicles, motor vehicles, pedestrians, etc.Therefore different specifications in terms of angular accuracy,discrimination and coverage area are required. In other wordsthis means that the radiation pattern and the scanning range ofthe antenna front end should be different for the two operationregimes. Due to the impossibility for cost and space reasonsof using several antennas for both operation regimes, any kindof antenna solution should be reconfigurable, which meansthat it should be possible to switch between the two operationregimes in order to fulfil the given requirements both in termsof scanning range and pattern characteristics. Moreover theantenna should be small size to reduce the impact on thevehicle fuselage and low cost for mass production.In the present work we propose a multi-layer multi-beam

reconfigurable pillbox antenna, suitable for Printed Circuit

h1

h2

F

Input Part

Quasi-OpticalSystem

Radiating Part

εr1

εr2

Sub.1

Sub.2

ParabolicSurface

Slot

M.1

M.2

M.3

Slot

x

y

z

BFN

(a)

Sub.1

Sub.2

εr1

εr2

Input Part

M.1

M.2

M.3

Quasi-OpticalSystem

Radiating Part

Slot

x

z y

BFN

(b)

Fig. 1. Antenna view. In the figure Sub., M., BFN stand for substrate layer,metal layer and Beam Forming Network respectively. (a) 3D view. (b) 2Dview in the xz-plane.

Board (PCB) and Substrate Integrated Waveguide (SIW)technologies. The antenna proposed here is represented inFig. 1 (a). It consists of two double grounded substratesand a Beam Forming Network (BFN) layer needed to feedthe antenna. Three main building blocks corresponding todifferent functionalities can be highlighted: 1) a feeding orinput part made by several integrated horns (only one is shownin Fig. 1); 2) a quasi-optical system consisting of an integratedplanar parabolic reflector and a recently patented wide bandtransition; 3) a radiating part (in the present case it is madeby an array of slotted waveguides).The operation of the antenna can be explained from

Fig. 1 (b). The feed or input part, placed in the focal plane

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of the integrated parabolic reflector, launches the energy inthe lowest substrate in the form of a guided quasi-TransverseElectromagnetic Mode (TEM) mode presenting a cylindricalwave front. This quasi-TEM mode hits the parabolic surface ofthe quasi-optical system and, by means of optimized couplingslots, is transferred to the second substrate where it is finallyradiated by the radiating part. In particular, as it is the caseof conventional 3D parabolic systems, the energy guided inthe second substrate presents a plane wave front with a phasedistribution depending on the position of the feed in the focalplane of the parabolic reflector. In this way, by playing withthe position of the feed in the focal plane of the parabolicsurface, it is possible to scan the radiated beam in the yz-plane. Moreover, by placing several feeds in the focal planea multi-beam operation is achieved. In this case, as it will beshown in the following, several beams crossing at −3 dB canbe used to cover the complete field of view. Besides, severalbeams can be combined to form a resultant beam with differentcharacteristics, full filling specifications needed for the nextgeneration of automotive radars.The paper is organized as follows. Section II presents

the quasi-optical system and its operation for multi-beamoperation. In Section III the input part made by 13 stackedintegrated horns is introduced. The numerical results for thecomplete antenna system are given in Section IV. Even if theoperative frequency for the next radar generation is in the79 GHz band, all the results are given for a scaled versionof the proposed antenna at 24.15 GHz without affecting thegeneral concepts behind the design. Finally, conclusions aredrawn in Sectioin V.

II. QUASI-OPTICAL SYSTEMThe role of the optical system is to efficiently transfer the

energy from the first to the second substrate. As shown inFig. 1 (b) it consists of a parabolic metallic surface made byvertical pins connecting the metal layers M. 1 and M. 3 and ofa transition located in the common metal plate M. 2 betweenthe substrates Sub. 1 and Sub. 2. This transition usually takesthe form of a long slot bordering the entire length of theparabolic profile [1] [2]. However this configuration turns outto be narrow band and efficient only for a narrow range ofillumination angles. On the contrary, the solution we proposehere (Fig. 2) is wide band and efficient for a wide range ofillumination angles.More precisely it consists of several slots bordering along

the entire length of the parabolic profile. By tuning adequatelythe size (lsi, wsi) and position (ri) of each ith slot, it ispossible to enhance the transmission of the guided energy tothe second substrate and to cancel the reflected wave comingfrom the parabolic surface and going back to the feed part.The ith slot is placed along a path described by

ri =2F

1 + cos ϕ− Δi (1)

where F is the focal distance of the parabolic profile and Δi isthe distance between the slot center and the parabolic profile.

The symmetry of the structure along xz-plane is also preservedby the slot position and size.

M.2

Parabolic ProfileSlot

x

y

z

lsi

wsi

F

r

φ

ri

Δsi

δsi

Fig. 2. Top view of the multi-slot transition. (r, ϕ) are the usual cylindricalcoordinates. ri, Δi, lsi and wsi refer to the ith slot and are respectively itsposition along r, offset from the parabolic profile, length and width. δi is thedistance between two adjacent slots. F is the focal distance of the parabolicsurface. The feed is located at the focal point.

We compare in Fig. 3 the near-field distribution of theguided field in the first and second substrates for the “longslot” and “multi-slot” transition. The fields have been com-puted using the commercial tool HFSS [4]. This figure clearlyhighlights the improvements achieved when using several slotsin terms of quality of the wave front and matching of theguided waves in both substrates due to the suppression ofspurious reflections from the parabolic surface.

(a)

(b)

Fig. 3. Near-field plots in the first (Sub.1) and second substrate (Sub.2). (a)Long slot case. (b) Multi-slot case.

III. INPUT PARTThe input part is located in the focal plane of the optical

system (first substrate) and consists of one or more elementarysources. The main task of this part is to launch and shape aTEM mode, polarized along z (Fig. 1), in the first substrateof the antenna. Ideally, the launched TEM mode shouldoriginate from the focus of the parabolic reflecting surface

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and be tapered in such a way to control the field along theparabolic profile to reduce losses and improve the patternquality. In fact the energy transferred by the quasi-opticalsystem and traveling in the second substrate will feed theradiating part with such a tapering along the yz-plane (Fig. 1),affecting directly the radiation pattern along this plane [5].Here an integrated H-plane sectoral horn has been used as anelementary source.As done in 3D multi-beam systems [6] [7], in order to have

N beams pointing at different angles in the far-field,N sourcesshould be placed in the focal plane of the optical system. Inthis paper the configuration shown in Fig. 4 is used: it consistsof two level stacked H-plane sectoral integrated horns.

laper

z

x

y

dds

h /21

h /21

Metal Plates

Fig. 4. Two level stacked horn configuration. dds is the inter-element spacingbetween two horns and laper is the horn aperture.

This solution allows getting a compact cluster of feeds. Inthis way it is possible to choose arbitrarily the radiation angleof a given beam by simply controlling the displacement of thecorresponding horn in the focal plane. This gives an enormousdegree of freedom in choosing the crossing level betweentwo contiguous beams in the far-field, which is particularlyimportant in our case where several beams will be used tocover the antenna field of view and to reconfigure the radiationpattern.

IV. NUMERICAL RESULTS

The various blocks presented previously have been used todefine the complete antenna structure. The substrates Sub. 1and Sub. 2 have the same thickness (h1=h2=0.508 mm) anddielectric constant (εr=2.2). For the radiating part an arrayof 25 slotted waveguides (along y-axis, Fig. 1) with fourslots uniformly tapered (along x-axis) have been used. Aresonant slotted waveguide array solution has been chosenin order to obtain a fixed beam along z-direction. Howeverother solutions can be envisaged, eg. leaky wave structures.The input part is made by 13 integrated horns as shown inFig. 5 with laper=1.5 λd (λd is the wavelength in the di-electric at the central frequency) and an inter-element spacingdds=0.5 laper in order to obtain adjacent beams crossing at−3 dB. The optical system has a focal distance F=10 λd anda diameter D=14.8 λd (F/D=0.67). A −10 dB tapering along

the parabolic surface is provided by the input part. The fullstructure has been analyzed using HFSS [4].

Sub.1

Sub.2 Input Part

M.1

M.2

M.3

y

zx

12 4 6

5 78

9

Feed legendlaper

dds

h /21

310

1112

13

h2

Fig. 5. 2D view of the stacked integrated horn configuration. The input partis placed in the focal plane of the quasi-optical system.

The E-field radiation patterns in both principle planes at thecentral frequency are shown in Fig. 6 for the feed labeled 1 inFig. 5. Good performance in terms of side lobe level (SLL<−20 dB) and beamwidth (6◦) are achieved in E-plane (yz-plane) due to the tapering along the parabolic profile. The H-plane (xz-plane) pattern beamwidth (20◦) and side lobe level(−13.5 dB) can be improved by respectively increasing thesize of the array along the x-axis and using a defined taperingscheme in feeding the slots.

-25

-20

-15

-10

-5

0

-60 -40 -20 0 20 40 60

Gain[dB] E-plane

H-plane

[deg]θ

Fig. 6. E-Field radiation pattern in E-plane and H-plane for the central feed(Feed 1 in Fig. 5). θ is the usual elevation angle of the spherical coordinatesystem.

The complete radiation patterns in E-plane for the 13 feedsare given in Fig. 7. Note that the small overlapping level(−3 dB) among the various beams could not be achievedwithout overlapped stacked integrated horns. Good radiationcharacteristics have been achieved both in terms of scanningloss (< 2.5 dB) and side lobe level for all the beams in theangular range of ±40◦. It is worth mentioning that for anoptical system the scan losses and deterioration of the patternsfor squinted beams are principally dictated by the F/D of thesystem, and increasing this ratio would improve directly theperformance in terms of scanning capabilities.Finally the basic idea to combine several beams in E-plane

to produce a broader beam as needed for the short range mode

89

-25

-20

-15

-10

-5

0

-60 -40 -20 0 20 40 60

Gain[dB]

[deg]θ

Fig. 7. E-field radiation pattern in the yz-plane for the 13 integrated horns.

-25

-20

-15

-10

-5

0

-60 -40 -20 20 40 60

8-1-2

9-8-1-2-3

10-9-8-1-2-3-4

11-10-9

Active horns

Gain[dB]

[deg]θ0

Fig. 8. Reconfiguration of the main beam by using several sources at thesame time in the E-plane. The legend for the horn is reported in Fig. 5

is shown in Fig. 8. In particular, the central beam has beenenlarged using several feeds instead of just one. Moreover asshown by the lateral beam in the same figure, it is also possibleto scan a new redefined beam.

V. CONCLUSION

In this work, a novel multi-feed multi-layer pillbox antennahas been presented. A particular attention has been made indesigning the optical system. The latter is based on an arrayof coupling slots with optimized dimensions and locations.This allows to efficiently transfer energy between both layersof a pillbox antenna configuration. In order to extend theoperation of the antenna to multi-beam system, an inputpart made by interleaved stacked integrated horns has beenintroduced to feed the optimized optical system. The presentednumerical results are really promising in terms of efficiency,pattern quality and scanning loss. Thanks to the simplicity andsuitability of the structure for PCB and SIW technology, theproposed antenna is a very good candidate for all applicationswhere low-cost, small size scanning antennas are required, asit is the case for the next generation automotive radars.

ACKNOWLEDGMENTPart of the project was carried out in the frame of the

”Radar ACC” project from the French competitiveness cluster”ID4CAR” and in the frame of the FP-7 coordination actionARTIC.

REFERENCES[1] W. Rotman, “Wide-Angle Scanning with Microwave Double-Layer Pill-

boxes”, IRE Transactions on Antennas and Propagation, Vol. 6, no.1,pp. 96-105, Jan. 1958.

[2] T. Teshirogi, Y. Kawahara, A. Yamamoto, Y. Sekine, N. Baba, M.Kobayashi, “Dielectric Slab Based Leaky-Wave Antennas for Millimeter-Wave Applications”, IEEE Antennas and Propagation Society Interna-tional Symposium, 2001, Vol. 1, pp. 346-349, Jul. 2001.

[3] M. Ettorre, R. Sauleau, “Antenne multicouche a plans paralleles, de typepillbox, et systemes d’antennes correspondants”, PatentFR0952158, Apr.2009.

[4] Ansoft HFSS version 11.0, 1984-2007 Ansoft Corporation.[5] M. Ettorre, A. Neto, G. Gerini, S. Maci, “Leaky-Wave Slot Array Antenna

Fed by a Dual Reflector System”, IEEE Transactions on Antennas andPropagation, Vol. 56, no.10, pp. 3143-3149, Oct. 2008.

[6] N. Llombart, A. Neto, G. Gerini, M. D. Bonnedal, P. De Maagt, “LeakyWave Enhanced Feed Arrays for the Improvement of the Edge ofCoverage Gain in Multibeam Reflector Antennas”, IEEE Transactionson Antennas and Propagation, Vol. 56, no.5, pp. 1280-1291, May 2008.

[7] Y. Rahmat-Samii, “Reflector Antennas”, in Antenna Handbook, Y. T. Loand S. W. Lee, Eds. New York: Van Nostrand Reinhold, 1988.

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