Compact Lens Antenna for Automobile Anticollision Radar at ... - PIERS

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We present a 130mm length * 100mm height antenna suitable for automotive radar at 77 GHz that .... The shift of one degree is also observed in the H-plane.
Progress in Electromagnetic Research Symposium 2004, Pisa, Italy, March 28 - 31

Compact Lens Antenna for Automobile Anticollision Radar at 77 GHz C. Migliaccio1, J. Y. Dauvignac1, B.D Nguyen1, C. Pichot1, J.L. Desvilles2, L. Le-Coq3 1

LEAT, 250 rue A. Einstein.06560 Valbonne, France. e-mail : [email protected] ; [email protected] ; [email protected] 2 France Télécom R&D,Fort de la Tête de Chien, 06320 La Turbie, France. e-mail :[email protected] 3 IETR, Campus de Beaulieu Bât. 11D, 263 av. du Général Leclerc, 34042 Rennes Cédex, France. e-mail : [email protected]

Abstract We present a 130mm length * 100mm height antenna suitable for automotive radar at 77 GHz that can be included in the bumper. The antenna is simulated with a software developed by France Télécom R&D, SRSRD. The antenna is made of a primary source connected with a WR-10 waveguide feeding a dielectric lens made in Eccostock 0005 of 2.53 dielectric permittivity. The primary source is composed of a small circular horn continued by a circular waveguide and a circular to rectangular (WR-10) waveguide transition. The circular horn is optimized in order to radiate a vertical polarization. The source provides symmetrical radaition in E and H-planes with 16 dBi gain. The gain of the source is higher than in usual lens systems. It can be placed far from the lens reducing its width that implies the diminution of its dielectric losses. The complete antenna has been shielded and achieves 33.5 dBi gain, 2.7° beamwidth and side lobes level of -26 dB in E and H planes. I – Introduction The development of automotive radar at millimeter wave has become an important domain of interest over the past few years. Significant efforts have been made to design low cost systems for use in automobiles for collision warning and intelligent cruise control systems. The antenna system has to match high performances like high gain, narrow beamwidth, low side lobes, and a small size. The antenna size is crucial as the antenna has to be placed on the vehicle without modifying its aesthetics. Thus the antenna is generally inserted in the bumper. Typically, the automotive antenna system consists of a small low directivity antenna being a waveguide aperture, or a slot coupled patch on semi-conductor substrate, feeding a lens [1-2]. Printed arrays or artificial lenses have also been developed for this application [3]. Their main advantage is the compactness but their lower gain might be not sufficient regarding the detection range of the radar. On the other hand, dielectric lens antennas can be designed to produce highly shaped beams that significantly improve the system performances but have to be optimized in size and dielectric losses. In this paper, we present a high gain antenna working at 77 GHz fed by a standard WR-10 waveguide. It is made of a small horn feeding an hyperbolic dielectric lens. In section II we discuss the antenna's conception. In section III, we present the radiation pattern simulations and the final adjustments for the antenna system dimensions. In section II, we show the radiation pattern measurements. II - Antenna's conception The antenna system can be divided into three parts: (i) the rectangular-to circular waveguide transition that is not represented, (ii) the primary source made of a circular horn shown on figure 1, (iii) the dielectric lens. Dielectric lens and primary source association is represented on figure 2. The dielectric lens has an hyperbolic profile calculated in order to meet the gain requirements of 34 dBi. Considering the radiation efficiency of 80% compared to a circular plane aperture for an hyperbolic lens, we choose the diameter of the lens of 100 mm leading to a theoritical gain of 37 dBi. It remains 3 dB higher than the requirements. This safety is necessary in order to prevent the defaults due to the dielectric losses or to an eventual offset during mechanical alignment of the primary source

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Progress in Electromagnetic Research Symposium 2004, Pisa, Italy, March 28 - 31

to the lens. The depth of the lens increases as the focal distance decreases. Thus, dielectric losses, that are proportional to the amount of material increases with short focals. Therefore, a compromise has to be found between the length of the antenna system and its performances. Because of the limited room in the bumper we have limited the focal length to 100 mm before optimising it by simulation. The choice of the primary source depends on the focal length. Its directivity increases with the focal length for the same lens diameter. Considering the rather large focal length of our antenna system of 100 mm (25*λ0), the primary source has to be directive. Therefore we choose a circular horn antenna. The circular waveguide is preferred to the rectangular one because it preserves the symmetry of revolution of the dielectric lens. The small corrugations in its design, that can be seen on figure 1, are correcting the dominant TE11 mode of the circular horn. Indeed, the TE11 mode of circular horn is not radiating a perfectly vertical electric field, thus it has to be corrected by adding of the TM11 higher mode. Finally a circular-to- rectangular waveguide has to be added in order to connect the antenna to the WR-10 waveguide of the radar.

Figure 2: primary source + lens

Figure 1: primary source

II- Antenna simulations and realisation Simulations on the primary source and the dielectric lens are made with SRSRD software developed by France Télécom R&D. It is based on the electric field integral equations and magnetic field integral equations and was designed for antenna structures presenting a symmetry of revolution. Therefore, the circular-to-waveguide transition was simulated with Ansoft HFSS. Simulations with SRSRD were conducted under UNIX on HP station RP7410/HPPA-7700-750. The simulation time is about 15 minutes per frequency point. This short time allows fast antenna optimization. Radiation pattern of the primary source is first computed. According to the aperture of 27° at 10 dB of the circular horn, the focal length should be of 98 mm. Nevertheless, further simulations have been made with the dielectric lens in order to reduce focal length without reducing antenna's gain. Its final value is of 80 mm, leading to an antenna depth of 22mm. The antenna is also shielded in order to avoid radiations coming from parasitic directions as shown on figure 1. Simulations results are reported on figure 2. The simulated antenna gain is of 36.45 dBi for an aperture at -3 dB of 2.6° with first side lobes at -26 dB. Comparisons with measurements are shown in figures 3 and 4. Simulations of the transition on HFSS have produced a very good transmission coefficient S21 of -0.02 dB over the investigated frequency bandwidth (75-80 GHz). This value will not be verified by measurement because the transition and the circular horn will be made in one set. Final antenna's length with the transition is 130 mm. III- Antenna measurements Measurements were made at the IETR Laboratory, Rennes, France. E and H -planes are reported in figure 3 and 4 respectively. They correspond to the simulated one, in particular concerning the aperture at -3 dB of 2.6 °, the side lobes levels and their positions.

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Progress in Electromagnetic Research Symposium 2004, Pisa, Italy, March 28 - 31

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Figure 3: radiation pattern simulated and measured in E- plane at 77 GHz

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Figure 4: radiation pattern simulated and measured in H- plane at 77 GHz

The relative cross-polarization level is comprised between -30 and -40 dB around the main lobe. Nevertheless, the measured gain is of 33.5 dBi, thus 3 dB lower than predicted. It might be caused by dielectric losses and alignment default in the antenna system between the small circular horn and the lens. Indeed it is obvious on figure 5 that there is an offset in the H-plane of about 1°. Furthermore, the measurement can also generate a loss in the gain because of the difficulty of aligning the antenna to the standard horn used for measurement. This step is very critical here because of the sharpness of the pattern. Phase has also been measured and is reported in the E-and H-planes on figure 6. In simulations, the phase center has been placed 70mm inside the antenna referred to the lens plane. The phase center was defined about the same position in the measurement setup. Figure 6 shows good agreement within -3/+3°. The shift of one degree is also observed in the H-plane. This might be due to a slight shift in the definition of the phase center measurement or to an alignment default in the antenna fabrication explaining the shift in H-plane. 531

Progress in Electromagnetic Research Symposium 2004, Pisa, Italy, March 28 - 31

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Figure 5 : zoomed measured amplitude in E- and H-planes at 77 GHz

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Figure 6: zoomed phase in E- and H-planes at 77 GHz

IV - Conclusion We designed and measured a lens antenna made of dielectric Eccostock 0005 associated to a circular horn working at 77 GHz for automotive radar applications. The antenna dimensions (130mmlength*100mm height) have been optimised in order to be placed in the car's bumper. Antenna was designed and simulated with the SRSRD software showing good agreement with the measured data in terms of radiation pattern shapes. The predicted gain was 3 dB higher than the measured one due to the mechanical uncertainties occurring during the antenna's realisation, and the dielectric losses. REFERENCES [1] B. G. Porter, L. L. Rauth, J. R. Mura, S. S. Gearhart, “Dual-Polarised Slot Coupled Patch Antennas on Duroid with Teflon Lenses for 76.5 GHz Automotive Radar Systems”, IEEE Trans. Antennas Propagat., vol. 47 no 12, December 1999, pp. 1836-1842. [2] Y-J Park, A. Herschlein, W. Wiesbeck, “A photonic Bandgap (PBG) Structure for Guiding and SuppressingSurface Waves in Millimeter-Wave Antennas”, IEEE Trans. MTT., vol. 49 no 10, October 2001, pp. 1854-1859. [3] D. M. Pozar, S. D. Targonski, H. D. Syrigos, “Design of Millimeter wave Microstrip Reflectarrays”, IEEE Trans. Antennas Propagat., vol. 45 no 2, February 1997, pp. 287-296.

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