2018 International Symposium on Antennas and Propagation (ISAP 2018) [WeD1-2] October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

An Element-Staggered, Wide-Angle Beam Scanning Transmitarray with Four Focuses Design

Nan-nan Wang, Bing-xu Zhao, Mu Fang, Jing-hui Qiu School of Electronics and Information Engineering Harbin Institute of Technology Harbin, 150080, China [email protected]

Li-Yi Xiao Microelectronics Center, Harbin Institute of Technology, Harbin, 150080, China

Abstract – A wide-angle beam-scanning low level transmitarray antenna which is working in Ka-band is put 2. Transmitarray Design forward in this paper. The proposed transmitarray is consist of a transmitting surface with four-layer subwavelength Fig. 1 shows the structure of the proposed transmitarray circular patch elements which are in staggered arrangement antenna. The transmitting surface is composed of 2 1 2 1 and a movable feed. Four focuses design is used for reducing four-layer square patch elements. Three dielectric the phase error and the loss of gain caused by rotating the feed. The simulation results demonstrate that the loss of gain substrates are made of Rogers RT5880 with thickness of is less than 1.2 dB, the side lobe level is lower than -17.8 dB 0.508 mm, relative of 2.2, and loss tangent of during the scanning range of ±30 degrees at typical frequency 0.0012. The element dimensions is 3.33.3 mm 2 of 35 GHz. The excellent performance shows that it can be (equivalently 0.385 0.385 ). The four-layer metal used for millimeter wave imaging system. circular patches have the same radius a. The range of a is Keywords — Beam-scanning, transmitarray, four focuses from 0.5 mm to 1.5 mm. This structure can make the range design, staggered arrangement. of element’s phase change up to 360º to meet the requirement of transmitarray design. The dimensions of the transmitarray aperture are 1. Introduction 69.3 mm69.3 mm (equivalently 8.085 8.085 ). The With the rapid development of millimeter wave distance between the feed horn and the center of technology, millimeter wave imaging technology has drawn transmitting surface is 5 . 2  . The feed is a dual-mode much attention [1]. Antenna with high gain and small conical horn with symmetrical . The track aperture is the crucial component of this imaging system. In of movable feed is a circular motion on the center of the this domain, lens antenna has been applied for this system, surface between ±30º in the yoz plane and the elements are but this antenna has great weight and large aperture. The in staggered design. microstrip transmitarray antenna [2] is a typical antenna of high gain and low profile. The combine of transmitarray and beam scanning antenna [3] will be useful for searching for target in wide angles accurately and enhancing efficiency. In this paper, four focuses design and staggered arrangement of elements are employed to design a beam- scanning transmitarray antenna for millimeter wave imaging system, this approach is more effective to compensate phase and improve performance during the scanning. This transmitarray antenna has the characteristics of stable gain, low side lobe level and wide scan angles. The entire design is simulated through CST Microwave Studio®. The paper is divided into the following sections: 1. Introduction, 2. Transmitarray design, 3. Results and Fig. 1. Structure of transmitarray and the elements design. discussions, 4. Conclusion.

35 2018 International Symposium on Antennas and Propagation (ISAP 2018) October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

The gain and side lobe level of different scan angles are shown in Fig. 4. The max gain of transmitarray is 25.9 dB. The side lobe level is lower than -17.8 dB in ±30° at 35GHz. 27 -17

26 -18 -19 25 -20 24 -21

23 -22 Gain(dB)

22 -23 Side lobe level(dB) Fig. 2. Principle of the four focuses design. -24 21 Gain Fig. 2 illustrates the schematic of the four focuses design. Side lobe level -25 Four simulated feeds replace the single feed for phase 20 compensation in this way. The phase error of active feed 0 5 10 15 20 25 30 will be reduced by this method, the stability of gain and Theta(degree) low side lobe level will be obtained. Fig. 4. Gain and side lobe level in different scan angles.

3. Results and Discussions 4. Conclusion Fig. 3 shows the simulation results of the radiation A wide-angle and low side lobe level beam scanning pattern of proposed transmitarray. Compared with the transmitarray antenna with movable feed is designed and traditional design simulation results, it can be seen that at simulated. In this design, four-layer subwavelength circular target frequency of 35 GHz, the variation of gain is 1.2 dB patch elements with staggered arrangement and four when the feed scans from 0° to 30°, the side lobe level is focuses design are applied for the phase compensation. The always lower than -17.8 dB. The loss of gain in the loss of gain is less than 1.2 dB and the side lobe level is traditional design is 3 dB. Compared with the traditional lower than -17.8 dB in the ±30° scanning angles at 35 GHz. design, the proposed transmitarray has the more stable gain It has the advantages of steadily high gain, low side lobe and lower side lobe level. level and small aperture, it can be used in millimeter wave 30 30 imaging system.

25 25 20 20 Acknowledgment 15 Traditional Four focuses 15 10 design and staggered 10 This work was supported by the National Natural Science design 5 5 Foundation of China (Grant No. 61501143 and

0 0 No.U1633202) and the Natural Science Foundation of

-5 -5 Heilongjiang Province of China (Grant No. F2015004).

-10 -10 The authors would also like to thank CST Ltd. Germany, Radiation pattern(dB) Radiation -15 -15 for providing the CST Training Center (Northeast China

-20 -20 Region) at our university with a free package of CST MWS

-25 -25 software. -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 Theta(degree) Fig. 3. Comparison scanning beams of transmitarray with References proposed design and traditional design. [1] Y. Arima and A. Hirose, "Performance Dependence on System The traditional transmitarray antenna is based on the Parameters in Millimeter-Wave Active Imaging Based on Complex- principle of parabolic phase compensation. In the beam Valued Neural Networks to Classify Complex Texture," IEEE Access, vol. 5, pp. 22927-22939, September 2017. scanning method with movable feed, the phase error of the [2] N. Landsberg and E. Socher, "Design and Measurements of 100 GHz aperture will be increased with wide scan angles, resulting Reflectarray and Transmitarray Active Antenna Cells," IEEE in the loss of the gain and higher side lobe level. Transactions on Antennas and Propagation, vol. 65, no. 12, pp. 6986-6997, December 2017. The loss of gain is reduced through the four focuses [3] P. Nayeri, F. Yang and A. Z. Elsherbeni, "Bifocal Design and design of the reflecting surface. What’s more, the staggered Aperture Phase Optimizations of Reflectarray Antennas for Wide- arrangement of elements can further effectively restrain the Angle Beam Scanning Performance," IEEE Transactions on Antennas and Propagation, vol. 61, no. 9, pp. 4588-4597, September side lobe level of the array during wide-angle beam 2013. scanning.

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