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76-81 GHz Planar Antenna Development and Utilization for Automotive Radar Applications Master’s thesis in Wireless and Photonics Engineering DAPENG WU Department of Microtechnology and Nanoscience CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2016

76-81 GHz Planar Antenna Development and Utilization for Automotive Radar Applications DAPENG WU © DAPENG WU, 2016 Department of Microtechnology and Nanoscience Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone + 46 (0)31-772 1000 Chalmers Reproservice Göteborg, Sweden 2016

ABSTRACT Abstract Automotive radars are becoming more compact and affordable thanks to the rapid development of semiconductor technology. Nowadays most vehicles are equipped with radars to enhance safety and improve driving experiences. As an essential part of any radar sensor, antenna will largely influence the size and cost of the whole system. Therefore, the development of automotive radar antenna is a critically important topic of practical interest. This thesis presents a 76.5 GHz microstrip comb-line antenna array utilized for a commercial automotive radar prototype. First a 13-element 90 degree comb-line array is realized in standing wave configuration so no additional reflection-cancelling structures are required. In order to achieve a trade-off between beamwidth and sidelobe level, a 20 dB Taylor amplitude taper is applied. Based on the conventional 90 degree array, a new array with 45 degree polarization is built to minimize the interference from cars moving in the opposite direction. All simulations are performed in Momentum Simulator of Advanced Design System. The dimensions of 90 and 45 degree comb-line arrays are 20.7×2.5 mm2 and 20.5×2.0 mm2, respectively. Both of them are implemented on Rogers RO3003 substrate. A probe-based setup is employed for the measurement of S-parameter and radiation patterns. From 76 to 78 GHz, both arrays exhibit consistent performance. At 76.5 GHz, the 45 degree array yields a maximum gain of 11.35 dBi at and a sidelobe level of -16.3 dB; the cross-polarization level is fluctuating around -10 dB. Overall, the measurement results show good agreement with simulations. Keywords: antenna, automotive radar, microstrip, comb-line, millimeter wave I

ACKNOWLEDGMENT Acknowledgment It would not have been possible to complete this thesis without the generous contributions of many great people. First and foremost, I would like to express my sincerest gratitude to my supervisor Dr. Ralf Reuter for his immense expertise, contagious enthusiasm and tremendous patience. The valuable experiences I gained under his guidance will pave the way for my future career. I am also deeply grateful to my examiner Professor Jian Yang for providing insightful advices to improve my work and refine my thesis. Furthermore, I am indebted to Dr. Ziqiang Tong for the helpful discussion and warm hospitality during my stay in Germany. Special thanks go out to Dr. Heiko Gulan and Dr. Christian Rusch for devoting enormous efforts to the antenna measurement. Finally, I want to thank my family for the unconditional love and wholehearted support throughout my study. II

TABLE OF CONTENTS Table of Contents Abstract ........................................................................................................................................... I Acknowledgment ......................................................................................................................... II List of Abbreviations .................................................................................................................. IV 1 Introduction ................................................................................................................................. 1 1.1 Overview of Automotive Radar System .......................................................................... 1 1.2 Antennas for Automotive Radars ..................................................................................... 3 1.3 Aim of the Thesis Project .................................................................................................. 4 2 Analysis of Microstrip Comb-Line Antenna Array ............................................................... 7 2.1 Characteristics of Microstrip Open-Circuit Stub as a Radiating Element ..................... 7 2.1.1 Radiation Pattern of a Microstrip Open-Circuit Stub .......................................... 7 2.1.2 Impact of Substrate Surface Waves ....................................................................... 9 2.1.3 Improved Analysis of Microstrip Open-Circuit Stub ......................................... 11 2.1.4 End Admittance of Microstrip Open-Circuit Stub .............................................13 2.2 Comb-Line Antenna Array with Microstrip Open-Circuit Stubs .................................14 2.2.1 Microstrip Open-Circuit Stub as an Array Element ...........................................15 2.2.2 Comparison of Traveling Wave Array and Standing Wave Array.....................17 3 Designs of 90 and 45 Degree Standing Wave Microstrip Comb-Line Antenna Arrays.19 3.1 90 Degree Uniform Comb-Line Antenna Array ............................................................19 3.2 90 Degree Amplitude Tapered Comb-Line Antenna Array ..........................................22 3.3 45 Degree Amplitude Tapered Comb-Line Antenna Array ..........................................26 4 Measurements of 90 and 45 Degree Standing Wave Microstrip Comb-Line Antenna Arrays ............................................................................................................................................29 4.1 Probe-Based Antenna Measurement Setup ....................................................................29 4.2 Measurement Results of 90 and 45 Degree Comb-Line Antenna Arrays ....................31 4.2.1 90 Degree Amplitude Tapered Comb-Line Aray ................................................31 4.2.2 45 Degree Amplitude Tapered Comb-Line Aray ................................................34 5 Conclusion .................................................................................................................................37 References .....................................................................................................................................39 III

LIST OF ABBREVIATIONS List of Abbreviations ACC ADS AiP AoC AUT CAD Adaptive Cruise Control Advanced Design System Antenna in Package Antenna on Chip Antenna Under Test Computer-Aided Design CMOS Complementary Metal-Oxide-Semiconductor CST CTA EMI eWLB FMCW GaAs GSG HFSS HPBW LRR MRR RCP SiGe SOL SRR TEM Computer Simulation Technology Cross Traffic Alert Electromagnetic Interference Embedded Wafer Level Ball Grid Array Frequency-Modulated Continuous-Wave Gallium Arsenide Ground-Signal-Ground High Frequency Structural Simulator Half-Power Beamwidth Lang Range Radar Medium Range Radar Redistributed Chip Package Silicon-Germanium Short-Open-Load Short Range Radar Transverse Electromagnetic IV

CHAPTER 1 INTRODUCTION 1 Introduction In 1904, the German inventor Christian Hülsmeyer built a device for the detection of ships in fog, which is commonly referred to as the first radar system. During World War Ⅱ, radar was put into practice and under a rapid development. Nowadays, radar is also widely used in civil areas and one of the most important applications is the automotive radar system. 1.1 Overview of Automotive Radar System As early as 1964 the use of radar system on vehicles for the prevention of collisions has been discussed [1]. In the 1970s some automotive radar prototypes were built and road tested [2]-[4]. However, at that time the high cost and large dimensions of key components were the limiting factors for commercial application. It was not until the 1990s that major automobile manufacturers and suppliers started the research on automotive radar again. Since 1992 a 24 GHz Doppler radar system developed by Eaton VORAD Technologies has been installed in 1700 Greyhound buses and it helped to reduce the accident rate by 25% [5]. In the late 90s Mercedes-Benz firstly introduced the 77GHz-radar-based DISTRONIC system [6] and other manufacturers soon followed with their own products. Today most high and middle class vehicles are equipped with radar sensors and it is safe to predict that it will be more widely available and affordable in the near future. Figure 1.1 Block diagram of a frequency Figure 1.2 Frequency-time relationships of transmitted -modulated continuous-wave (FMCW) and received signals in FMCW radar automotive radar Figure 1.1 is the general block diagram of a frequency-modulated continuous-wave (FMCW) automotive radar, it is capable of measuring both the distance and velocity of a moving object. Assuming that a linear sawtooth frequency modulation is applied to the transmitted signal, as is shown in Figure 1.2, the time delay can be calculated by (1.1) where is the frequency difference between the transmitted and received signals which 1 SignalSourcePowerDividerSignalProcessingMixerTXAntennaRXAntennaft∆t∆f Transmitted Signal Received Signal

76-81 GHZ PLANAR ANTENNA DEVELOPMENT AND UTILIZATION FOR RADAR APPLICATIONS could be measured from the mixer output and is the frequency sweep rate. The distance between the observer and target is then given by (1.2) Here is the speed of light in air and a factor of is introduced to get the one-way distance. Two different frequency bands are available for automotive radar applications: 24 GHz and 77- GHz. The 77 GHz solution offers advantages such as smaller dimension and broader bandwidth, but also faces more challenges in design and implementation. The 77 GHz band could be divided into two subbands: 76-77 GHz and 77-81 GHz (also called 79 GHz band). The former has been approved by most countries, while the latter is only available in Europe so far but has been under discussion in other countries. The functions of automotive radar sensors vary with their maximum ranges. Long range radar (LRR) has a narrow beam and it is usually mounted in the front grill to measure the distance of objects ahead (up to 250 m); short range radar (SRR) offers a broader beam and can be used to monitor the vicinity of a vehicle (within 30 m); between LRR and SRR, there is medium range radar (MRR) which can be installed on the front, the rear, or the side area for different applications. Detailed comparisons of the three sensor types are given in Table 1.1. Table 1.1 Automotive radar classifications LRR MRR SRR Maximum Range (m) Applications 150-250 60-150 30 Adaptive cruise control (ACC) Cross traffic alert (CTA), ACC Blind spot detection, parking aid The first generation of commercial automotive radar in 77 GHz was implemented in gallium arsenide (GaAs) technology [7]. Despite their excellent performance, the market share of GaAs-based products is limited by the high fabrication cost. Nowadays, most automotive radar sensors are based on silicon-germanium (SiGe) technology since it is a more cost-effective solution. As one of the main automotive semiconductor suppliers, Freescale presented its own transceiver chipset using SiGe BiCMOS technology in 2012 [8]. It consisted of a four-channel receiver and a single-channel transmitter, which covered the whole frequency range of 76-81 GHz and could be used for both LRR and SRR. Besides on-wafer measurement, the chips were also tested in redistributed chip package (RCP) and the results showed great potential for commercial applications. In 2009, Fujitsu Laboratories reported the first 77 GHz automotive radar transceiver chip in 90 nm CMOS technology [9]; one year later, researchers from National Taiwan University published a fully integrated 77 GHz FMCW radar system in 65 nm CMOS technology [10]. The advantages of CMOS technology are the lower cost and power consumption, however, the current performances of CMOS radars are not yet comparable with their SiGe counterparts. Therefore, there is still a long way to go for CMOS automotive radars to 2

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