1474 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 5, MAY 2006 Center Feed Single Layer Slotted Waveguide Array Sehyun Park, Member, IEEE, Yasuhiro Tsunemitsu, Student Member, IEEE, Jiro Hirokawa, Senior Member, IEEE, and Makoto Ando, Fellow, IEEE Abstract—A center feed configuration is newly introduced to enhance the bandwidth as well as to suppress the frequency dependent beam squinting in single layer slotted waveguide arrays. A multiple-way power divider consisting of cross-junctions is installed at the center of the aperture in an alternating-phase fed waveguide slot array to reduce the long line effect in the radiating waveguide. Each radiating slot is equipped with an inductive wall for reflection canceling, which dispenses with the beam tilting technique for suppressing cumulative reflection and assures the boresight beam. All these are equipped in single layer and are potential for low cost mass-production as is usual the case with alternating phase-fed arrays. The test antenna is fabricated at 26 GHz band for FWA system. A 30.5 dBi gain with 46% efficiency with the main beam staying in the boresight is obtained. The sidelobes of 9.5 dB associated with the aperture blockage for the center feed is suppressed to below 14.7 dB at the design frequency, by applying a genetic algorithm for controlling the slot excitation. Index Terms—Center feed, reflection canceling, waveguide arrays. I. INTRODUCTION S INGLE layer slotted waveguide arrays are attractive candidates for high-gain and high-efficiency planar antennas in millimeter-wave since the transmission loss of a hollow waveguide is quite small even in millimeter-wave band [1]. One example of alternating phase fed array is presented in Fig. 1(a). This antenna has been commercialized for FWA(Fixed Wireless Access) system of 26 GHz band in Japan. The simple structure consisting of only two parts, that is a slot plate and a grooved base plate, is suitable for mass-production with low-cost. Radiation waveguide with slots have been fed from their ends by a feed waveguide consisting of cascaded and T-junctions, in co-phase and alternating phase fed arrays, respectively [2], [3]. This end feed configuration is associated with long radiation waveguides and gives rise to narrow frequency bandwidth; the long line effect as well as the beam squinting is notable as the electrical size of arrays become larger, as is usual the case with traveling wave antennas. Fig. 1(b) shows frequency dependent main beam direction of conventional end feed waveguide array. This array consists . The main beam direcof 20-element which are spaced by tion is tilted about 1.6 [deg.] for 0.5 [GHz] frequency shift from center frequency 25.5 [GHz]. This means degradation of gain in the main beam direction. To overcome these problems, Manuscript received February 4, 2004; revised October 25, 2005 S. Park is with Samsung Advanced Institute of Technology, Kyungki-do 440600, Korea. Y. Tsunemitsu is with the Electrical and Electronic Engineering Department, Tokyo Institute of Technology, Tokyo 152-8552, Japan and also with Japan Radio Co., Ltd., Tokyo 181-8510, Japan. J. Hirokawa and M. Ando are with the Tokyo Institute of Technology, Tokyo 152-8552, Japan. Digital Object Identifier 10.1109/TAP.2006.874310 Fig. 1. End feed single layer slotted waveguide arrays. (a) End feed single layer slotted waveguide array for alternating phase (T-junction) and (b) typical beam squinting in end fed slotted waveguide arrays. the authors have proposed the center feed single layer slotted waveguide arrays as is shown in Fig. 2 [4]. By feeding the array not from its end, but from the center of the waveguide arrays, the long line effect is halved and the bandwidth enhancement is expected. The symmetrical structure of the center feed would also contribute to the main beam staying in the boresight. The key components that have been developed for this new array are 1) A cross junction with four inductive posts was designed for exciting two radiation waveguide on both sides of the feed waveguide [4]; a cascaded cross junctions composes 0018-926X/$20.00 © 2006 IEEE PARK et al.: CENTER FEED SINGLE LAYER SLOTTED WAVEGUIDE ARRAY 1475 Fig. 2. Center feed single layer slotted waveguide arrays. Fig. 3. Slot and an inductive wall. a multiple-way power divider to be installed in the center of the alternating phase fed arrays. 2) Several types of reflection canceling element such as slot pair, slot set, circular pit [5], [6] and inductive post [7] have been designed for suppressing the cumulative slot reflection at the input port and for dispensing with the well known beam tilting technique [1]. In this paper, the above two devices are assembled to realize a center feed alternating phase fed single-layer slotted waveguide arrays, for the first time. As a reflection canceling element, a slot with an inductive wall is newly designed by the Galerkin’s method of moment [7] for simple fabrication and mass production by die-casting etc [8]. One difficulty of the center feed array is the blockage occupied with the feed waveguide in the center of the array aperture which results reduction in the efficiency as well as the high sidelobes [9], [10]; sidelobe suppression by controlling slot excitation is also conducted here by using genetic algorithm. The model array for FWA system at 26 GHz band is fabricated and the 30.5 dBi gain with 46% efficiency with the main beam staying in the boresight is obtained. The sidelobes of 9.4 dB associated with the aperture blockage for the center feed is suppressed to below 14.7 dB at the design frequency. if the beam is directed at the boresight. EM design of a slot and an inductive wall is performed by MoM with uniform line current approximation on the inductive wall surface. A choke is accommodated in the periphery of the antenna, which prevents the leakage through the gap between the slot plate and the groove structure at the periphery of the array [11]. It dispenses the electrically perfect contact and the screws are used for the simple assembly. In summary, the structure of center feed arrays has higher degree of symmetry than the conventional end feed ones. One difficulty is the blocked area in the center of the aperture occupied with the cross junctions; the typical blockage is about 2.1 free space wavelength in width. The growing sidelobes as well as the efficiency decrease should be taken into account in the design. II. STRUCTURE OF A CENTER FEED ARRAY The overall configuration of the center feed alternating phase fed single layer slotted waveguide array is explained in Fig. 2. The feed waveguide, which works as a multiple-way power divider consisting of cascade of cross-junctions, is installed at the center of the array. Each cross-junction has four inductive posts to control the amplitude and phase of the divided power [4]. An antenna input aperture is cut on the bottom at the center of the feed waveguide. The radiation waveguides are placed with a spacing of half the guide wavelength in the feed waveguide for excitation in alternating phase. The radiating shunt slots are longitudinally spaced by a half the guide wavelength while they are transversally offset on both sides of the waveguide axis. Each radiating element consists of a slot and an inductive wall which cancels out the reflection from slot; the radiation waveguide is free of cumulative reflection at the input even III. COMPONENT DESIGNS A. Reflection Canceling Slot With Inductive Wall In the design of a traveling wave array, we developed a simple design method in two steps as in [5]. The design of a radiation unit for controlling radiation and suppressing reflection comes first and then it is utilized to synthesize the array with desired illumination. This paper adopted it but the unit design is conducted in the isolated environment. An analysis model of a slot with an inductive wall is shown in Fig. 3. The longitudinal slot which is parallel with respect to -axis is adopted for linear polarization. The length and the width of the slot are and . The -position, -position and thickness of inductive wall are , and , respectively. In the analysis procedure, an electric current on the wall surface and an equivalent magnetic current on the slot aperture are assumed. The unknown expansion coefficients of basis function are determined by the Galerkin’s method of moment [7]. Then, the radiation power as well as the reflection of the slot and an inductive wall is estimated. The predicted reflection as well as the slot parameters and wall positions are presented in Fig. 4 as functions of radiation power. The other parameters are given as follows: , , and . The longitudinal slot with resonant length of 5.8 mm is used. For wide range of coupling power, the low reflection below 30 dB is realized 1476 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 5, MAY 2006 Fig. 4. Slot and an inductive wall parameters for reflection canceling. Fig. 5. Fig. 6. Suppressed sidelobe level by using genetic algorithm. Efficiency and sidelobe level as function of antenna size. Fig. 7. Slot excitation determined by genetic algorithm. by varying the wall location. In the later discussion of sidelobe level control, Fig. 4 provides the reflection canceling element design for various radiation power of slot, specified in the array design. B. Suppression of Sidelobes Due to Aperture Blockage by Center Feed Waveguide A center feed waveguide arrays has typical blockage region at the center of the aperture with the width of 2.1 free space wavelength. This give rises to high side lobe levels. Fig. 5 shows the prediction of antenna efficiency and the sidelobe levels as functions of array size. A square aperture and magnetic dipole source model is used in the calculation. The sidelobe of a center feed waveguide arrays mainly depends on the ratio of array size and blockage region. It is required to increase the number of slot for some application requiring lower sidelobe. The efficiency degradation as well as the sidelobe level growth is associated with the blocking area ratio. For example, the aperture in 26 GHz band has the initial first side lobe level of 13 dB for 20-slot design of uniform illumination is increased to 9.4 dB with 2.1 free space wavelength blockage as shown in Fig. 5. Here, the first side lobe level is suppressed by using genetic algorithm [12]. In this step, only the excitation amplitudes are optimized for half side array due to symmetry structure. The genetic algorithm optimized a 10-element array with 6-bit amplitude accuracy. The maximum relative-sidelobe level is used for cost function [13], [14]. Sometimes, GA give us unacceptable slot illumination with large fluctuation even though sidelobe level is suppressed enoughly. To avoid this, the initial value of slot excitation amplitude set to be taylor distribution of 20 dB sidelobe level. The parametric range of amplitude has dynamic range 0.3 compare to taylor distribution. Fig. 6 shows the resulting H-plane radiation pattern after GA optimization compare to uniform illumination. The radiation pattern has a reduced sidelobe level of 16 dB though the inevitable gain degradation due to tapered illumination about 1 dB is observed. The optimized excitation amplitude is given in Fig. 7. Fig. 8 shows the slot and inductive wall parameters to realize desired excitation amplitude and reflection canceling. The end-most slot is specially designed as a “matching slot” which radiates all the residual power with the reflection suppressed [15], [16]. PARK et al.: CENTER FEED SINGLE LAYER SLOTTED WAVEGUIDE ARRAY 1477 Fig. 11. Near field distribution of the aperture field at 25.5 GHz. Fig. 8. Slot parameters for desired slot coupling. as 10 dB at the design frequency 25.3 GHz. It also includes the result by HFSS simulator applied for 1/4 structure of the full array based upon the symmetry. The reason for this degradation of return loss was intensively discussed numerically and experimentally by authors [17]. The mutual coupling of inner-most slots, which are strongly excited according to GA design, has been identified as the source of reflection. However, they are remarkable agreement though the structure is very large and complicated. This implies that the fabrication of parts, the slot plate, the base plate and the posts in cross junctions, as well as the contact by the choke with simple screw is accurate and works well. Fig. 9. Fabricated model antenna. B. Near Field Distribution Fig. 10. Reflection at the feed point of the antenna. IV. EXPERIMENTAL RESULTS A. Reflection A test antenna of the center feed waveguide arrays is designed and fabricated at 26 GHz band as shown in Fig. 9. The antenna consists of two sets of radiation waveguides, one on each side of the feed waveguide. Each set consists of 16 radiation waveguides, and each radiation waveguide has 10 slots. The aperture area is around 160 180 mm. Fig. 10 shows the frequency dependence of the overall reflection at the input port. It is as high Fig. 11 shows the two-dimensional amplitude distribution at 25.5 GHz. The feed waveguide with cascaded cross junctions is placed at the center along the vertical axis. As is expected, weak amplitude is observed at the center of the arrays because there is no slot above on the feed waveguide. The one-dimensional amplitude and phase distribution along the feed waveguide, measured at the center of the antenna, is shown in Fig. 12(a) and (b), respectively. Uniform amplitude and phase distributions with small ripples are observed at 25.5 GHz. These confirm the desired operation of the cross-junction feed waveguide. The frequency characteristics are slightly shifted to higher than design one of 25.3 GHz. Figs. 13(a) and (b) shows those along the radiation waveguides. The amplitude has the blocking at the center and is tapered down toward the aperture end following the design as shown in Fig. 7 for sidelobe suppression, while the uniform phase is observed. C. Radiation Pattern Fig. 14 shows the measured H-plane pattern for 16 dB sidelobe suppression design. This pattern is associated with the excitation of the radiating slot array. The maximum sidelobe of measured result is 14.7 dB at design frequency 25.3 GHz. It is slightly higher than calculated one of 16 dB but reasonable agreement was obtained. The measured radiation pattern in E-plane coincides with that for uniform illumination with the sidelobe 13 dB as shown in Fig. 15. As an important advantage of the center feed array confirmed in these figures, the 1478 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 5, MAY 2006 Fig. 12. Amplitude and phase distribution in parallel with the feed waveguide. (a) Amplitude and (b) phase. Fig. 13. Amplitude and phase distribution in perpendicular with the feed waveguide. (a) Amplitude and (b) phase. main beam direction in E- and H-plane stays in the boresight in wide frequency range and is not squinted with the frequency. Generally, slot array has very high polarization purity. This advantage is independent of feed structure. The measured XPD of center feed waveguide array is more than 45 dB as shown in Fig. 16. D. Gain and Efficiency Fig. 17 shows the frequency characteristics of the gain and efficiency of the antenna. The measured gain is compared with the end feed waveguide array for commercial use of FWA system. The enhancement of frequency bandwidth is observed as expected. The maximum gain is 30.5 dBi which correspond 46% efficiency for the aperture size. This is a bit lower than the end feed waveguide arrays. The degradation of the gain and efficiency comes from the blockage region and the aperture tapering for sidelobe suppression. These are roughly estimated as aperture blockage 0.5 dB, tapered illumination 1 dB, return loss 0.5 dB, and the slot excitation (amplitude/phase) error due to mutual coupling 0.5 dB. The former two are inherent to this specific design of the array while the latter two may be reduced by the advanced slot design taking the mutual coupling effects. Fig. 14. Radiation pattern in the yz -plane (H-plane). V. CONCLUSION We have proposed a center feed single layer slotted waveguide array. The main advantage of this antenna is a wide bandwidth and the stable boresight main beam. The main beam direction in H-plane is not squinted for frequency change. PARK et al.: CENTER FEED SINGLE LAYER SLOTTED WAVEGUIDE ARRAY 1479 to 46% efficiency is obtained. The inductive posts will be replaced with walls to simplify fabrication in the near future. The advanced slot design taking the mutual coupling effects into account, should be developed for gain enhancement as well as input VSWR improvement. REFERENCES Fig. 15. Radiation pattern in the xz -plane (E-plane). Fig. 16. Measured cross polarization (E-plane). Fig. 17. Gain and efficiency of the antenna. A cross-junction is used for multiple-way power divider. A reflection canceling slot with inductive walls is introduced to escape from the beam tilting technique and a waveguide arrays with boresight main beam is realized. The sidelobe which is a bit higher than conventional end feed waveguide array is suppressed by genetic algorithm. The maximum sidelobe is suppressed below 14.7 dB. A gain of 30.5 dBi corresponding [1] J. Hirokawa, M. Ando, N. Goto, N. Takahashi, T. Ojima, and M. Uematsu, “A single-layer slotted leaky waveguide array antenna for mobile reception of direct broadcast from satellite,” IEEE Trans. Veh. Tech., vol. 44, no. 4, pp. 749–755, Nov. 1995. [2] J. Hirokawa, M. Ando, and N. Goto, “A waveguide  -junction with an inductive post,” IEICE Trans. Trans. Electron., vol. E75-C, no. 3, pp. 348–351, Mar. 1992. [3] N. Goto, “A waveguide-fed printed antenna,” IEICE Tech. Rep., pp. 17–21, Apr. 1989. [4] S. Park, J. Hirokawa, and M. Ando, “A planar cross-junction power divider for the center feed in single-layer slotted waveguide arrays,” IEICE Trans. Commun., vol. E85-B, no. 11, pp. 2476–2481, Nov. 2002. [5] K. Sakakibara, J. Hirokawa, M. Ando, and N. Goto, “A linearly-polarized slotted waveguide array using reflection canceling slot pairs,” IEICE Trans. Commun., vol. E77-B, no. 4, pp. 511–518, Apr. 1994. [6] J. Sato, J. Hirokawa, and M. Ando, “Reflection-canceling of slotted waveguide antenna by using a circular pit,” in IEEE Antennas Propagation Soc. Symp. Dig., 1998, pp. 1706–1709. [7] S. Park, J. Hirokawa, and M. Ando, “Simple analysis of a slot with a reflection-canceling post in a rectangular waveguide using only the axial uniform currents on the post surface,” IEICE Trans. Commun., vol. E86-B, no. 8, pp. 2482–2487, Aug. 2003. [8] , “Analysis of a waveguide slot and a reflection-canceling inductive wall,” presented at the IEEE Topical Conf. Wireless Communication Technology, Session 23, , Oct. 15–17, 2003, p. 8. [9] A. C. Ludwig, “Low sidelobe aperture distribution for blocked and unblocked circular apertures,” IEEE Trans. Antennas Propag., vol. AP-30, no. 5, pp. 933–946, Sep. 1982. [10] N. Goto and F. Watanabe, “The optimum aperture efficiency of cassegrain antennas with a specified sidelobe level,” Trans. IECE, vol. 61-B, no. 5, pp. 321–326, May 1978. [11] Y. Kimura, T. Hirano, J. Hirokawa, and M. Ando, “Alternating-phase fed single-layer slotted waveguide arrays with chokes dispensing with narrow wall contacts,” Proc. Inst. Elect. Eng. -Microwave, Antenna and Propagation, vol. 148, no. 5, pp. 295–301, May 2001. [12] R. L. Haupt, “An introduction to genetic algorithm for electromagnetics,” IEEE Antennas Propag. Mag., vol. 37, no. 2, Apr. 1995. , “Thinned arrays using genetic algorithm,” IEEE Trans. Antennas [13] Propag., vol. 42, pp. 993–999, Jul. 1994. [14] M. Shimizu, “Determining the excitation coefficient of an array using genetic algorithm,” in IEEE Trans. Antennas Propag. Int. Symp., vol. 1, Jun. 1994, pp. 530–533. [15] K. Sakakibara, J. Hirokawa, M. Ando, and N. Goto, “Periodic boundary condition for evaluation of external mutual couplings in a slotted waveguide array,” IEICE Trans. Commun., vol. E79-B, no. 8, pp. 1156–1164, Aug. 1996. [16] Y. Kimura, K. Fukazawa, J. Hirokawa, M. Ando, and N. Goto, “Low sidelobe single-layer slotted waveguide arrays at 76 GHz band,” IEICE Trans. Commun., vol. E84-B, no. 9, pp. 2377–2386, Sep. 2001. [17] Y. Tsunemitsu, S. Park, J. Hirokawa, M. Ando, Y. Miura, Y. Kazama, and N. Goto, “Reflection characteristics of center-feed single-layer wavegudie arrays,” IEICE Trans. Commun., vol. E88-B, no. 6, pp. 2313–2319, Jun. 2005. Sehyun Park (S’01–M’04) was born in Kyeongnam, Korea, on February 2, 1973. He received the B.S. degree in control and instrumentation engineering and the M.S. degree in radio sciences and engineering from Korea Maritime University, Busan, Korea, and the D.E. degree in electrical engineering from the Tokyo Institute of Technology, Tokyo, Japan in 2003. From 2003 to 2004, he was Postdoctoral Research Fellow at the Tokyo Institute of Technology. He is currently a Senior Research Engineer at the Samsung Advanced Institute of Technology, Kyungki-do, Korea. 1480 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 5, MAY 2006 Yasuhiro Tsunemitsu (S’05) was born in Kanagawa, Japan, on September 1, 1976. He received the B.S. degree in electrical engineering from Takushoku University, Tokyo, Japan, in 2000, and the M.S. degree in electrical and electronic engineering from Yokohama National University, Yokohama, Japan, in 2002. He is currently working toward the D.E. degree at Tokyo Institute of Technology, Tokyo. He works at Japan Radio Co., Ltd. His current research interests are in slotted waveguide array antenna. Mr. Tsunemitsu is a member of the Institute of Electronics, Information and Communication Engineers (IEICE), Japan and the Applied Computational Electromagnetics Society (ACES). Jiro Hirokawa (S’89–M’90–SM’03) was born in Tokyo, Japan, on May 8, 1965. He received the B.S., M.S., and D.E. degrees in electrical and electronic engineering from the Tokyo Institute of Technology (Tokyo Tech.), Tokyo, Japan, in 1988, 1990, and 1994, respectively. He was a Research Associate from 1990 to 1996, and is currently an Associate Professor at Tokyo Tech. From 1994 to 1995, he was with the antenna group at Chalmers University of Technology, Gothenburg, Sweden, as a Postdoctoral Fellow, on leave from Tokyo Tech. His research area has been in analyzes of slotted waveguide array antennas. Dr. Hirokawa is a Member of the Institute of Electronics, Information and Communication Engineers (IEICE), Japan. He received an IEEE AP-S Tokyo Chapter Young Engineer Award in 1991, a Young Engineer Award from IEICE in 1996, a Tokyo Tech. Award for Challenging Research in 2003, and a Young Scientist Award from the Minister of Education, Cultures, Sports, Science and Technology of Japan in 2005. View publication stats Makoto Ando (SM’01–F’03) was born in Hokkaido, Japan, on February 16, 1952. He received the B.S., M.S., and D.E. degrees in electrical engineering from Tokyo Institute of Technology, Tokyo, Japan, in 1974, 1976, and 1979, respectively. From 1979 to 1983, he was with Yokosuka Electrical Communication Laboratory, NTT, and was engaged in development of antennas for satellite communication. He was a Research Associate with Tokyo Institute of Technology from 1983 to 1985, and is currently a Professor. His main interests have been high-frequency diffraction theory such as physical optics and geometrical theory of diffraction. His research also covers the design of reflector antennas and waveguide planar arrays for DBS and VSAT. His latest interest includes the design of high-gain millimeter-wave antennas. Dr. Ando is a Member of the Institution of Electrical Engineers (IEE) London, U.K., and the Institute of Electronics, Information and Communication Engineers (IEICE), Japan. He has been a Member of the Scientific Council for the Antenna Centre of Excellence (ACE) in EU’s Sixth Framework Programme for research a network of excellence since 2004. He received the Young Engineers Award of the Institute of Electronics, Information and Communication Engineers (IEICE), Japan, in 1981, the Fifth Telecom Systems Award in 1990, the Eighth Inoue Prize for Science in 1992, the Achievement Award and the Paper Award from IEICE Japan in 1993, and the Meritorious Award on Radio, the Minister of Public Management, Home Affairs, Posts and Telecommunications in 2004. He has been the Vice-Chair and Chair of Commission B of the International Union of Radio Science (URSI), for 1999 to 2002 and 2002 to 2005, respectively. He is the Chairman of the Board of Association of Radio Industries and Businesses (ARIB) in 2004. He served as the Chairman of the Technical Program Committee of the International Symposium on Antennas and Propagation (ISAP) in 2000, the Technical Program Co-Chair for the 2003 IEEE Topical Conference on Wireless Communication Technology, the Vice-Chair of ISAP 2004, the Chair of 2004 URSI International Symposium on Electromagnetic Theory, and the Co-Chair of 2005 IEEE ACES International Conference on Wireless Communications and Applied Computational Electromagnetics. He is the Chair of the IEICE Electronic Society’s Technical Group of Electromagnetic Theory for 2004 to 2005 and the IEICE Communication Society’s Technical Group of Antennas and Propagation for 2005 to 2007. He is a Member of the Administrative Committee of the IEEE Antennas and Propagation Society for 2004 to 2006. He served as the Guest Editor-in-Chief of the Special Issue on Innovation in Antennas and Propagation for Expanding Radio Systems in IEICE Transactions on Communications in 2001 and the Special Issue on Wave Technologies for Wireless and Optical Communications in IEICE Transactions on Electronics in 2004. He has also been appointed Guest Editor-in-Chief for several special issues in Radio Science and IEICE Transactions on Electronics in 2005.