World Scientific Research Journal (WSRJ) ISSN: Design and Analysis of a Series-fed Microstrip Antenna Array for 24GHz

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1 World Scientific Research Journal (WSRJ) ISSN: Design and Analysis of a Series-fed Microstrip Antenna Array for 24GHz Automotive anti-collision Radar Xiaochuan Zhou a, YueYue Liu b School of Optoelectronics Engineering, Chongqing University of Posts and Telecommunications, Chongqing 465, China. a zhouxc1994@126.com, b @qq.com Abstract: In this paper, a 8 8 microstrip antenna array with series-fed network is designed. Chebyshev distribution method is adopted by the antenna sub-array and the feed network and effectively reducing the side lobe level. Simultaneously, the feed network is connected to the sub-array directly achieving the purpose of miniaturization and effectively reducing the loss of the feed network. The simulation results show that the antenna array has a gain of 21.3dBi, the side-lobe level of the pattern at the center frequency 24.1GHz is -18.9dB and -25.2dB in E and H plane respectively. The antenna array is compact, reliable and can be used in the 24GHz automotive anti-collision radar. Keywords: high gain, miniaturization, series-fed network, Chebyshev distribution, anti-collision radar. 1. INTRODUCTION With the rapid increase in the number of cars, traffic accidents such as rear-end collisions and car crash have occurred frequently. The 24GHz automotive anti-collision radar system can measure the angle, speed and distance of the target obstacles and thus can effectively reduce traffic accidents [1].Antenna is an important part of anti-collision radar system and its performance directly affects the performance of the anti-collision radar.microstrip antenna has been widely used in various radar systems Because it has many advantages such as small size, light weight, low profile, easy integration with circuits and low cost [2]. The antenna applied in automotive anti-collision should have high-gain, low-sidelobe, narrow-beam and small-size that can not only increases the radar range, but also facilitates the integration of front-end circuit. The two most common feed networks for microstrip antenna array are series-fed network and parallel-fed network [3].The series-fed network has a shorter microstrip feeder line, smaller electromagnetic loss and higher space utilization efficiency. But its impedance bandwidth is very narrow [4].The parallel-fed network can easily realize a specific power distribution ratio 67

2 and it has a wider impedance bandwidth. But its long feeder line will cause serious electromagnetic losses [5]. In order to solve the complex feed network of the array antenna, this paper designs an array antenna operating in the frequency band at 24 GHz for automotive anti-collision radar system. Compared with similar antennas, the structure of the series-fed network is simple and compact because the unequal feeder network is directly connected to the series-fed antenna sub-array. 2. ANTENNA ARRAY DESIGN 2.1 Design of the 8 1 Antenna Sub-array In the 24 GHz, the thick substrate will easily excites the more serious surface waves, and the thin substrate can avoid the influence of the substrate surface wave and reduce the volume of the entire antenna array [6]. The substrate material used in this paper is Rogers 435 ( r 3.66, tan. 4 antenna dimensions are shown as follows. ) with thickness of.5mm. The equations used to calculate the W 1 2 r 1 c 2f 2 (1) h 2 2 w r r e 1 L (.3)( Wh /.264) e.412 h ( e.258)( Wh /.8) 1 2 (2) (3) c L 2 f e 2L As seen in Fig. 1, the antenna sub-array consists of 8 patches with different length L and width W. In addition, the line width W is determined while the microstrip line has a characteristic impedance of 1-ohm, and the feed line length between antenna elements is Lr=λg/2, where λg is the wavelength in the substrate material. (4) L 1 L r L 2 L 3 L 4 L 5 L 6 L 7 L 8 W 1 W W 2 W 3 W 4 W 5 W 6 W 7 W 8 Fig. 1 Geometry of the 8 1 Antenna Sub-array 68

3 Table 1. Calculated value of the 8 1 Antenna Sub-array I-th patch Current Width (mm) ratio Length (mm) The Chebyshev synthesis method is used in designing the sub-array to reduce the side lobe. The current amplitude ratio of each antenna element can be calculated by MATLAB programming. The width of the antenna elements are changed as the current amplitude ratio change. The width of the antenna elements is tapered and the length of the antenna elements do not change significantly. Table I shows the initial calculated value of the 8 1 antenna sub-array. 1 E-plane H-plane -1 S 11 (db) Gain(dB) Frequency(Ghz) Angle(deg) (a) (b) Fig. 2 Simulated results of the designed 8 1 sub-array (a) Return loss (b) radiation pattern at center frequency Fig. 2 shows the simulated results for the 8 1 antenna sub-array. It shows that the 8 1 antenna sub-array resonates at GHz and the return loss is less than -1dB over the range of 23.99GHz~24.27GHz. As can be seen from Figures 2(b), the side lobe level is-21.2db and the half power beam-width is 13.2 in E-plane. 2.2 Design of the 8 8 antenna array The 8 1 antenna sub-array is expanded to form an 8 8 antenna array. As shown in Fig. 3 The antenna array is directly connected to the unequal power feeding network. The unequal power feed network is designed so that the H plane obtains a lower side lobe level. The quarter-wavelength impedance matching section can control the current amplitude of each sub-array. In order to ensure that the antenna sub-arrays are fed in the same phase, the distance between the H planes is taken as a dielectric wavelength [7]. 69

4 feeder port quarter-wavelength impedance matching section Fig. 3 The 8 8 transmitting antenna array -5-1 E-plane H-plane S11(dB) Gain(dB) Frequency(GHz) Angle(deg) (a) (b) Fig. 4 Simulated results of the designed 8 8 array (a) Return loss (b) radiation pattern at center frequency As can be seen from Figure 4(a) and 4(b), the 8 8 antenna array achieves a good impedance match at 23.79GHz-24.28GHz. The gain reaches 21.3 dbi at the center frequency 24.1GHz.The side lobe level of the antenna array is reduced by optimizing the width of the antenna element and the line width of the quarter-wavelength impedance matching section. 7

5 The first side lobe levels of the E plane and the H plane are db and db and the half power beam-width are 13.4 and 14.4 respectively. 3. CONCLUSION This paper designs a 8 8 series-fed microstrip antenna array for 24GHz automotive anti-collision radar. Simulation results show that the antenna array obtain 21.3dB gain and has low side lobe level with Chebyshev distribution feeding network.this compact and high-performance antenna array can effectively meet the requirements of the 24GHz automotive anti-collision radar and has a broad application prospect in this field. REFERENCES [1] Lee M S, Kim Y H, Design and Performance of a 24GHz Switch Antenna Array FMCW Radar System for Automotive Applications, IEEE Transactions on Vehicular Technology, 21, Vol. 59 (5), p [2] Song Huixuan, Design of 24GHz Radar Radio Front-end for Radar, Xi'an University of Electronic Science and Technology, 214 [3] S.H. Jeong, H.Y Yu, J.E. Lee et al, A Multi-Beam and Multi-Range Radar with FMCW and Digital Beam Forming For Automotive Applications, Progress in Electromagnetics Research, 212, Vol. 124 (1), p [4] Wincza K, Gruszczynski S and Borgosz J, Microstrip Antenna Array with Series-Fed 'Through-Element' Coupled Patches, Electronics Letters, 27, Vol. 43 (9), p [5] Wang Shao-long, Wang Wei, Design of a 24GHz Miniaturized Low Sidelobe Microstrip Antenna Array, Journal of Microwaves, 214,Vol. (s1), p32-34 [6] Y. I. Chong, D. Wenbin, Microstrip series fed antenna array for millimeter wave automotive radar applications, IEEE MTT-S International Microwave Workshop Series on Millimeter Wave Wireless Technology and Applications, Nanjing, 212: 1-3. [7] Xue Zhenghui, Li Weiming, Ren Wu. Array Antenna Analysis and Synthesis [M].Beijing: Beijing University of Aeronautics and Astronautics Press,