ACTIVE and passive imaging at millimeter wavelengths

Transcription

1 714 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 3, MARCH 2010 High Permittivity Dielectric Rod Waveguide as an Antenna Array Element for Millimeter Waves J. Patrik Pousi, Dmitri V. Lioubtchenko, Sergey N. Dudorov, and Antti V. Räisänen, Fellow, IEEE Abstract Dielectric rod waveguide antennas of rectangular cross section have a number of advantages over conventional waveguide and horn antennas as an antenna array element. Dielectric rod waveguide antennas have relatively low cost, low losses, a broadband input match and a high packing potential. Additionally the radiation pattern of such antennas is almost frequency independent. In this paper the suitability of Sapphire rod waveguides for an antenna array is studied with simulations and prototype measurements at W band. Strong mutual coupling is observed when the elements are close to each other. Index Terms Antenna array, dielectric rod waveguide, millimeter wave. Fig. 1. Illustration of a DRW antenna array block. I. INTRODUCTION ACTIVE and passive imaging at millimeter wavelengths have gathered a lot of interest among several research groups in recent years. Such imaging systems usually require an efficient antenna array to gather the information from the object. Also other antenna applications in the millimeter wave frequency range may require antennas with high and tunable directivity, e.g., short range communication. Dielectric rod waveguide (DRW) antennas made of relatively high permittivity materials like Sapphire or Silicon require only a small cross-sectional area, mm at W band. Lower permittivity materials require a larger cross section for a good matching. For, the cross section area is mm respectively. Thus high permittivity materials have a more dense concentration of the field, which leads to a denser packing of the array and low level of sidelobes. Horizontally tapered rods offer also a low polarization cross-coupling [1]. As it has been shown in previous studies, high permittivity DRW antennas of rectangular cross section have a broadband input matching and the electromagnetic field is concentrated in the rod [2]. This can provide a low mutual coupling between the elements and a high packing potential that is often difficult to achieve in antenna arrays. So far high permittivity dielectric rod waveguide antennas with a metal waveguide feed have been fabricated and measured up to Manuscript received March 21, 2009; revised May 29, First published December 28, 2009; current version published March 03, This work was supported in part by the Academy of Finland through the Centre of Excellence program. The work of J. P. Pousi was supported in part by the Finnish Cultural Foundation, the Finnish Society of Electronics Engineers and in part by the Foundation of Emil Aaltone. The authors are with Department of Radio Science and Engineering and SMARAD Centre of Excellence, Helsinki University of Technology (TKK), FI TKK, Finland ( patrik.pousi@tkk.fi). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TAP GHz. The measurements showed also a nearly frequency independent radiation pattern over a large frequency band [3]. Single antenna elements with such a feed could be scalable up to 300 GHz or even higher by the fabrication of the antenna elements with smaller dimensions ( mm for GHz). In general an antenna array enables a larger gain and narrower beam than a single antenna element. By changing the element spacing and increasing the number of elements a wide variety of radiation patterns can be created. However, it has to be remembered that the larger is the number of elements, the more complex will be the element feeding network. Another way to create different radiation patterns is to change the signal phase between the antenna elements. By using electrically controlled phase shifters a very effective beam steering can be achieved. In this paper the suitability of high permittivity DRW antennas, made of Sapphire, for antenna arrays at W band is analyzed with simulations and prototype measurements. Anisotropy of Sapphire does not cause complications to the design in this case where the waveguide is cut along the axis of Sapphire [4]. II. DRW ANTENNA ARRAYS In the case of the higher permittivity DRW a horn feed is not required and thus valuable space is saved. Fig. 1 illustrates a 2 2 DRW antenna array fed by regular metal waveguides. The radiation pattern of an antenna array is a product of the element pattern and the array factor. The element spacing larger than one wavelength leads to grating lobes. The directivity as a function of the element spacing reaches local maxima about every [5], [6]. Grating lobes can be suppressed for example by the use of a combination of subarrays and an array amplitude tapering as it has been done in [7]. Earlier dielectric rod waveguide antennas have been studied as optimized feed elements for focal plane arrays [8]. Dielectric rods were made of polyethylene and they were fed by X/$ IEEE

2 POUSI et al.: HIGH PERMITTIVITY DIELECTRIC ROD WAVEGUIDE AS AN ANTENNA ARRAY ELEMENT 715 Fig. 3. Two-element array feed designed with HFSS: metal waveguide splitter with an inductive post. Fig. 2. Simulated E plane radiation pattern of a two element DRW antenna array with =2; and 3=2 element separations at 90 GHz without mutual coupling. slotlines as the metal waveguide feeding requires a horn structure in case of low permittivity rods. Using a horn feed would require a lot of space and would lead to large element spacing. III. TWO ELEMENT ARRAY PROTOTYPE HFSS was used to simulate vertically ( plane) placed two-element DRW array with different element spacing. The difficulty in modeling the rectangular DRWs is the rapidly changing field near the waveguide corners. Earlier it has been studied by comparing simulations with measurements, that by setting, referring to the absolute difference between two iterations, a very accurate approximation of electromagnetic fields in rectangular DRWs can be obtained with HFSS [4]. The simulation tool allows calculating the pattern of an array by taking the radiation pattern of a single element. Fig. 2 presents the simulated radiation patterns at 90 GHz of two vertically placed Sapphire rods. The patterns obtained this way do not take into account the mutual coupling between the elements. If the spacing is, a narrow main beam, about 40 is obtained, but also high sidelobes appear. With spacing of one wavelength the main beam is about 60 and sidelobes are about 6 db lower. It was decided to manufacture a prototype array with element spacing of one wavelength. A. Feed System One problem in DRW antenna arrays is the difficulty to design an efficient feed system with the high packing density. In [8] a metal waveguide feed and a slotline feed were considered. There the problem with the waveguide transition was the need of a horn as the rod had a low permittivity and thus the required space of the feed was too large. A slotline feed is studied more in detail in [9] and also in [10], [11]. In the proposed transition in [8] the slotline substrate is inserted in the middle of the DRW. Insertion losses between 0.5 and 1.5 db were reported over the frequency band of GHz. Also the mutual coupling between two adjacent transitions was studied. It was estimated to be less than db when the distance between the lines is 10 mm. With high permittivity DRWs in W band the slotline feeds would be very difficult to realize mechanically. But as the high Fig. 4. Simulated H field distribution in the splitter. permittivity DRW can be well matched to a regular rectangular metal waveguide [2] also this type of DRW feed enables an antenna element separation less than.at90ghz mm. As it is not wanted to study separate channels with separate antenna elements, the array feed can be designed as a power splitter, where the power is equally divided among two waveguides. Such a splitter was studied and optimized in detail with HFSS. As the distance between the middle points of the two feeding waveguides is wanted to be only 3.3 mm, the matching of the junction becomes difficult. However, it was seen in simulations that when the corners near the junction are rounded the matching is significantly improved. Further matching improvement can be done with an inductive post [12], [13]. A post causes reflections that cancel out the reflection of the junction. If the amplitude of the second reflection is equal to the reflection of the junction and with an opposite phase, the sum reflection of the entire junction is zero. The shape of the object that causes second reflection is irrelevant, but it should have similar frequency dependence as the junction itself. Cylindrical post is often chosen as it is easy to manufacture. An optimal position and size of the post were studied with simulations. It was found out, that the radius of the post should be 0.25 mm, the height 0.2 mm and the distance from the junction wall 1.77 mm. The structure and schematic of the feed system is presented in Figs. 3, 4 and 5. In Figs. 6 8 the simulated and measured S-parameters are compared. The port 2 is located in the upper arm of the power splitter. From the results it can be concluded that the splitter performance agrees quite well with the simulated performance. The

3 716 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 3, MARCH 2010 Fig. 5. Schematic structure of the feed system. Fig. 8. Simulated and measured S of the feed system. Fig. 6. Simulated and measured S of the feed system. Fig. 9. Simulated mutual coupling with different distance between the elements. Fig. 7. Simulated and measured S of the feed system. resonant frequency in the prototype is slightly shifted up from the desired 90 GHz, but still the db-bandwidth is about 10 GHz. The power division is not completely equal between the ports, because of the asymmetry caused by the matching post, but near 3 db power division is obtained, see Figs. 7 and 8. B. Mutual Coupling Mutual coupling is an important parameter in antenna array design. It can distort the adjacent channels in the array and also modify the desired radiation pattern. In simulations some effects of the mutual coupling can be seen by comparing the array radiation pattern calculated from a single antenna to a simulated two element pattern. According to HFSS simulations, it seems that the mutual coupling is not very significant when the element separation is one wavelength. In Fig. 9 one can observe that due to a small asymmetry in the feed system the main beam is slightly turned. Mutual coupling is further studied by simulating two adjacent 56 mm long sapphire rods with different vertical distances between them. All the rod ends are tapered with 6 mm tapering section and matched to a metal waveguide. The lower rod is matched to ports 1 and 2 and the upper rod to ports 3 and 4 (Fig. 10). A vertical symmetry plane was used to reduce the simulation time. When the lower rod is fed from the port 1 the power coupled to the upper rod can be seen in. This is plotted in the frequency range of GHz with five different distances between the rods in Fig. 11. The results showing the are interesting and show that the coupling is very low when the distance is over 2.3 mm. So for the antenna array operation mutual coupling is not significant. With shorter distances the coupling is stronger and one can see that at some frequencies all the power is coupled to the upper rod. For example when the distance is 0.5 mm, the is db at 80 GHz. Phenomenon is similar to the cross-talk that can occur in optical fibers, where this complete power transfer has been used in directional couplers [14]. The simulation results were

4 POUSI et al.: HIGH PERMITTIVITY DIELECTRIC ROD WAVEGUIDE AS AN ANTENNA ARRAY ELEMENT 717 Fig. 13. Simulated mutual coupling between two horizontally placed sapphire waveguides. Fig. 10. Simulation setup for mutual coupling. Fig. 14. Two element sapphire rod antenna prototype. Fig. 11. Simulated mutual coupling with different vertical distance between the elements. As the metal waveguides in the measurements include flanges, they were also added to the simulation setup. They were modelled as perfect electric conductor surfaces. Small deviations between the results are probably due to a slight tilting of the rods in the measurements. Also the small Teflon sheets around the rods in the feeds were not taken into account in the simulation model. In horizontal plane the mutual coupling is assumed to be stronger due to the stronger field concentration. Though, it has to be remembered that if the DRWs are fed with the metal waveguides, in practise the element separation is always over 3 mm if the elements are in parallel. Simulations were made also in this plane and the results are presented in Fig. 13. Cross-talk between the elements and the complete power transfer at some frequencies is clearly visible at the distances less than 1 mm. Fig. 12. Comparison between simulated and measured mutual coupling between two vertically placed sapphire waveguides. verified with measurements (Fig. 12) and the agreement between the simulations and the measurements is relatively good. IV. MEASUREMENT RESULTS A prototype antenna was built by inserting two 20 mm long Sapphire rods into the power splitter. The separation between the rod center points is 3.3 mm. The tapering section in both ends of the rod is 6 mm. Thin Teflon sheets were used to attach the rods in the middle of the metal waveguides. Fig. 14 shows the antenna prototype.

5 718 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 3, MARCH 2010 Fig. 15. Measured S of antenna prototype compared with the simulations. Fig. 18. Measured H plane radiation pattern of the two-element antenna. Fig. 16. Simulated and measured E plane radiation pattern of the two-element antenna. Fig. 19. array. Simulated E and H plane radiation patterns of the 222 Sapphire rod agrees well with the simulation. In Fig. 17 the plane pattern of the two-element antenna is compared with the measured single element pattern. As expected the single antenna pattern covers the two-element antenna pattern. plane radiation pattern is presented in Fig. 18. Finally the radiation pattern of a 2 2 Sapphire rod waveguide antenna array was simulated. The vertical and horizontal element separations are both 3.3 mm. The results are presented in Fig. 19. One can notice that the main beam shapes nearly coincide in both and planes which can be a useful property in certain applications. Fig. 17. Measured two-element antenna E plane radiation pattern compared with the measured single element antenna radiation pattern. Reflection coefficient of the antenna was measured with HP 8510 vector network analyzer. The measurement is compared with the simulated values in Fig. 15. Measured is below db in GHz. Measurement agrees well with the simulation. Antenna radiation pattern was measured both in and plane by using the millimeter network analyzer and rotating the antenna. Measurement results are compared with the simulations made with HFSS. Measured plane radiation pattern is presented in Fig. 16. The main beam is about 60 and the pattern V. CONCLUSION In this paper the suitability of Sapphire rod waveguide as an antenna array element for W band was studied with simulations and prototype measurements. Compared to open ended metal waveguides Sapphire waveguides can provide a good and broadband input match, lower mutual coupling and a nearly frequency independent radiation pattern over a large frequency band. Therefore they would be an interesting alternative for densely packed antenna arrays. Such arrays can also provide similar radiation pattern in both and plane. Arrays could be scalable up to 300 GHz by the fabrication of the antenna elements with smaller dimensions. The challenge is the difficulty of feeding them as the metal waveguide feeding network becomes complex with several

6 POUSI et al.: HIGH PERMITTIVITY DIELECTRIC ROD WAVEGUIDE AS AN ANTENNA ARRAY ELEMENT 719 elements. The mutual coupling between the antenna elements was measured and simulated not to be significant in distances used in antenna arrays. At short distances the simulations of the mutual coupling revealed a cross-talk between the elements and a complete power transfer at some frequencies. This was also verified with the measurements. This phenomenon could be used for example in frequency selective power couplers or in monitoring the propagating power in a dielectric rod waveguides. ACKNOWLEDGMENT The power splitter for the array prototype was manufactured and measured in Elmika Co., Vilnius, Lithuania. REFERENCES [1] J. Weinzierl, J. Richter, G. Rehm, and H. Brand, Simulation and measurement of dielectric antennas at 150 GHz, in Proc. 29th Eur. Microw. Week, Munich, Germany, Oct. 1999, vol. 2, pp [2] D. Lioubtchenko, S. Dudorov, J. Mallat, J. Tuovinen, and A. V. Räisänen, Low-loss sapphire waveguide for GHz frequency range, IEEE Microw. Wireless Compon. Lett., vol. 11, no. 6, pp , [3] J. P. Pousi, D. V. Lioubtchenko, S. N. Dudorov, J. A. Mallat, and A. V. Räisänen, High permittivity dielectric rod waveguide antenna for GHz, presented at the Proc. 1st Eur. Conf. of Antennas Propag. (EuCAP06), Nice, France, Nov. 6 10, 2006, CD-ROM SP-262, paper [4] S. Dudorov, Rectangular Dielectric Waveguide and Its Optimal Transition to a Metal Waveguide, Doctoral Thesis, Radio Laboratory, Helsinki Univ. Technol., Otamedia, Finland, [5] C. A. Balanis, Antenna Theory: Analysis and Design. New York: Harper Row, [6] W. L. Stutzmann and G. A. Thiele, Antenna Theory and Design. London, U.K.: Peter Peregrius, [7] T. Sehm, A. Lehto, and A. V. Räisänen, A high-gain 58-GHz box-horn array antenna with suppressed grating lobes, IEEE Trans. Antennas Propag., vol. 47, no. 7, pp , Jul [8] J. Richter and L.-P. Schmidt, Dielectric rod antennas as optimized feed elements for focal plane arrays, in Antennas Propag. Society Int. Symp., Jul. 3 8, 2000, pp [9] J. Richter, Y. Yazici, C. Ziegler, and L.-P. Schmidt, A broadband transition between dielectric and planar waveguides at millimeter wave frequencies, in Proc. 33rd Eur. Microw. Conf., Munich, Sep. 2003, vol. 3, no. 2003, pp [10] H. Tehrani, M.-Y. Li, and K. Chang, Broadband microstrip to dielectric image line transitions, IEEE Microw. Guided Wave Lett., vol. 10, no. 10, pp , Oct [11] J. Tang and K. Wu, Co-layered integration and interconnect of planar circuits and nonradiative dielectric (NRD) waveguide, IEEE Trans. Microw. Theory Tech., vol. 48, no. 4, pp , Apr [12] T. Sehm, A. Lehto, and A. V. Räisänen, Matching of a rectangular waveguide T junction with unequal power division, Microw. Opt. Technol. Lett., vol. 14, no. 3, pp , Feb [13] J. Hirokawa, K. Sakurai, M. Ando, and N. Goto, An analysis of a waveguide T junction with an inductive post, IEEE Trans. Microw. Theory Tech., vol. 39, no. 3, pp , [14] A. W. Snyder, Optical Waveguide Theory. New York: Academic Press, J. Patrik Pousi was born in Vantaa, Finland, in August He received the Master of Science (Tech.) and Licentiate of Science (Tech.) degrees in electrical engineering from Helsinki University of Technology (TKK), Espoo, Finland, in 2003 and 2006, respectively, where he is currently working towards the Doctor of Science (Tech.) degree. Since 2004, he has been a Research Engineer with the Department of Radio Science and Engineering, TKK. His current research interests include active and passive dielectric rod waveguide components for millimeter wavelengths. Dmitri V. Lioubtchenko was born in Gorky, Russia, in May He received the B.S. and M.S. degrees and the Ph.D. degree in applied physics and mathematics from Moscow Institute of Physics and Technology, in 1993, 1994, and 1998, respectively. From 1994 to 1997, he was a Researcher in the Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow. From 1997 to 1998, he was a visiting researcher at the University of Liverpool, U.K. In 1998, he joined the Department of Radio Science and Engineering, Helsinki University of Technology (TKK), Finland, where he is currently an Academy Research Fellow. His research interests and experience cover various topics including investigations of new materials for millimeter wave, microwave and optoelectronic applications particularly, on the development of active and passive dielectric waveguides for the frequency above 100 GHz. Sergey N. Dudorov was born in the Kirov region, Russia, in May From September 1992 to June 1998, he studied at the Moscow Institute of Physics and Technology, where he received the Master of Science degree in applied physics and mathematics in June His thesis topic was Investigation of dielectric waveguides and devices based on them. He received the Licentiate degree and the Doctor of Science in Technology degree from the Helsinki University of Technology (TKK), Espoo, Finland, in 2001 and 2002, respectively, and the Candidate of Science degree from the Moscow Institute of Physics and Technology. His dissertation title was Rectangular dielectric waveguide and its optimal transition to a metal waveguide. In November 1998, he joined the Department of Radio Science and Engineering, Helsinki University of Technology (TKK), Finland, where he is currently a Postdoctoral Researcher. His research activities are focused on the dielectric property measurements in application to development of new devices for millimeter and microwave applications based on the dielectric waveguides. Antti V. Räisänen (S 76 M 81 SM 85 F 94) received the Doctor of Science (Tech.) degree in electrical engineering from the Helsinki University of Technology (TKK), Espoo, Finland, in He was appointed to the Professor Chair of Radio Engineering at TKK in 1989, after holding the same position pro tem in 1985 and He has held visiting scientist and professor positions at the Five College Radio Astronomy Observatory (FCRAO) and UMass, Amherst ( , 80, 81), at Chalmers University of Technology, Göteborg, Sweden (1983), at the Department of Physics, UC Berkeley ( ), at JPL Caltech, Pasadena ( ), and at Paris Observatory and University of Paris 6 ( ). Currently, he is supervising research in millimeter-wave components, antennas, receivers, microwave measurements, etc. at TKK Dept. Radio Science and Engineering and MilliLab (Millimetre Wave Laboratory of Finland ESA External Laboratory). He leads the Centre of Smart Radios and Wireless Research (SMARAD) at TKK, which obtained the national status of CoE in Research for and Currently he is also Head of TKK Dept. of Radio Science and Engineering. He has authored and coauthored some 400 scientific or technical papers and six books, e.g., Radio Engineering for Wireless Communication and Sensor Applications (Artech House, 2003). Dr. Räisänen is a Fellow of the IEEE since 1994 and Fellow of AMTA since He has been Conference Chairman of several international microwave and millimeter wave conferences including the European Microwave Conference in In 1997, he was elected the Vice-Rector of TKK for the period of He served as an Associate Editor of the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES from 2002 to He is a member of the Board of Directors of the European Microwave Association (EuMA) for and He is also the Chair of the Board of Directors of MilliLab.

7 易迪拓培训 专注于微波 射频 天线设计人才的培养网址 : 射频和天线设计培训课程推荐 易迪拓培训 ( 由数名来自于研发第一线的资深工程师发起成立, 致力并专注于微波 射频 天线设计研发人才的培养 ; 我们于 2006 年整合合并微波 EDA 网 ( 现已发展成为国内最大的微波射频和天线设计人才培养基地, 成功推出多套微波射频以及天线设计经典培训课程和 ADS HFSS 等专业软件使用培训课程, 广受客户好评 ; 并先后与人民邮电出版社 电子工业出版社合作出版了多本专业图书, 帮助数万名工程师提升了专业技术能力 客户遍布中兴通讯 研通高频 埃威航电 国人通信等多家国内知名公司, 以及台湾工业技术研究院 永业科技 全一电子等多家台湾地区企业 易迪拓培训课程列表 : 射频工程师养成培训课程套装该套装精选了射频专业基础培训课程 射频仿真设计培训课程和射频电路测量培训课程三个类别共 30 门视频培训课程和 3 本图书教材 ; 旨在引领学员全面学习一个射频工程师需要熟悉 理解和掌握的专业知识和研发设计能力 通过套装的学习, 能够让学员完全达到和胜任一个合格的射频工程师的要求 课程网址 : ADS 学习培训课程套装该套装是迄今国内最全面 最权威的 ADS 培训教程, 共包含 10 门 ADS 学习培训课程 课程是由具有多年 ADS 使用经验的微波射频与通信系统设计领域资深专家讲解, 并多结合设计实例, 由浅入深 详细而又全面地讲解了 ADS 在微波射频电路设计 通信系统设计和电磁仿真设计方面的内容 能让您在最短的时间内学会使用 ADS, 迅速提升个人技术能力, 把 ADS 真正应用到实际研发工作中去, 成为 ADS 设计专家... 课程网址 : HFSS 学习培训课程套装该套课程套装包含了本站全部 HFSS 培训课程, 是迄今国内最全面 最专业的 HFSS 培训教程套装, 可以帮助您从零开始, 全面深入学习 HFSS 的各项功能和在多个方面的工程应用 购买套装, 更可超值赠送 3 个月免费学习答疑, 随时解答您学习过程中遇到的棘手问题, 让您的 HFSS 学习更加轻松顺畅 课程网址 : `

8 易迪拓培训 专注于微波 射频 天线设计人才的培养网址 : CST 学习培训课程套装该培训套装由易迪拓培训联合微波 EDA 网共同推出, 是最全面 系统 专业的 CST 微波工作室培训课程套装, 所有课程都由经验丰富的专家授课, 视频教学, 可以帮助您从零开始, 全面系统地学习 CST 微波工作的各项功能及其在微波射频 天线设计等领域的设计应用 且购买该套装, 还可超值赠送 3 个月免费学习答疑 课程网址 : HFSS 天线设计培训课程套装套装包含 6 门视频课程和 1 本图书, 课程从基础讲起, 内容由浅入深, 理论介绍和实际操作讲解相结合, 全面系统的讲解了 HFSS 天线设计的全过程 是国内最全面 最专业的 HFSS 天线设计课程, 可以帮助您快速学习掌握如何使用 HFSS 设计天线, 让天线设计不再难 课程网址 : MHz NFC/RFID 线圈天线设计培训课程套装套装包含 4 门视频培训课程, 培训将 13.56MHz 线圈天线设计原理和仿真设计实践相结合, 全面系统地讲解了 13.56MHz 线圈天线的工作原理 设计方法 设计考量以及使用 HFSS 和 CST 仿真分析线圈天线的具体操作, 同时还介绍了 13.56MHz 线圈天线匹配电路的设计和调试 通过该套课程的学习, 可以帮助您快速学习掌握 13.56MHz 线圈天线及其匹配电路的原理 设计和调试 详情浏览 : 我们的课程优势 : 成立于 2004 年,10 多年丰富的行业经验, 一直致力并专注于微波射频和天线设计工程师的培养, 更了解该行业对人才的要求 经验丰富的一线资深工程师讲授, 结合实际工程案例, 直观 实用 易学 联系我们 : 易迪拓培训官网 : 微波 EDA 网 : 官方淘宝店 : 专注于微波 射频 天线设计人才的培养易迪拓培训官方网址 : 淘宝网店 :