Doctoral dissertation Orthogonally-Arranged Center-Feed Single-Layer Slotted Waveguide Array Antennas for Polarization Division Duplex March 2007. Under the supervision of Professor Makoto Ando Associate Professor Jiro Hirokawa Presented by Yasuhiro Tsunemitsu Department of Electrical and Electronic Engineering Tokyo Institute of Technology
Contents Chapter 1. Introduction-------------------------------4 1.1 Microwave applications--------------------------------------------------------4 1.2 Single-layer waveguide slotted array-----------------------------------------5 1.3 Summary of this study---------------------------------------------------------5 References----------------------------------------------------------------------------6 Chapter 2. Isolation between orthogonally arranged two antennas -------9 2.1 Introductory remarks-----------------------------------------------------------9 2.2 Configuration and Simulation Model for an Array-----------------------10 2.3 Reflection and Radiation Patterns of an Array----------------------------10 2.4 Isolation Characteristics of Orthogonally Polarized Pair Arrays Arranged Side-by-Side-----------------------12 2.5 Conclusion remarks-----------------------------------------------------------12 References--------------------------------------------------------------------------13 Chapter 3. Reflection characteristics due to mutual coupling among slots -----------33 3.1 Introductory remarks----------------------------------------------------------33 3.2 Center-Feed Array Antenna--------------------------------------------------34 3.2.1 A center-feed single-layer waveguide array----------------------------------------------------34 3.2.2 A multiple way power divider with a series of cross-junctions------------------------------34 3.2.3 A radiation waveguide - linear array of reflection canceling units consisting of a slot and a wall------------------35 3.3 Reflection Characteristics----------------------------------------------------36 3.3.1 Reflection from components---------------------------------------------------------------------36-1 -
3.3.2 Overall reflection from the array-----------------------------------------------------------------36 3.4 Mechanism extraction of accumulated reflection in overall array---------------37 3.4.1 Slot mutual coupling between adjacent radiation waveguides.-----------------------------37-3.4.2 Localization of mutual coupling in the aperture as functions of illumination distribution-------------------------38 3.5 Conclusion remarks-----------------------------------------------------------39 References--------------------------------------------------------------------------40 Chapter 4. Blocking free center-feed E-to H-plane and ridged cross-junction----------------------------57 4.1 Introductory remarks----------------------------------------------------------57 4.2 Design of a Cross-Junction Power Divider -------------------------------58 4.3 SUPPRESSION OF THE SIDELOBES DUE TO BLOCKAGE OF FEED WAVEGUIDE -----------------59 4.4 An Alternating-phase fed Single-Layer Slotted Waveguide Array Fed By the Center Feed with the E- to H-Plane Cross-junctions ----------60 4.5 Conclusion remarks-----------------------------------------------------------62 References--------------------------------------------------------------------------62 Chapter 5. Aperture efficiency enhancement Reflection canceling stair ----------78 5.1 Introductory remarks----------------------------------------------------------78 5.2 Structure -----------------------------------------------------------------------79 5.3 Analysis of a linear array and full model ----------------------------------79 5.4 Conclusion remarks-----------------------------------------------------------80 References--------------------------------------------------------------------------81-2 -
Chapter 6. Slot arrays with Interchangeable slot plates -------90 6.1 Introductory remarks----------------------------------------------------------90 6.2 Structure -----------------------------------------------------------------------90 6.3 Analysis of Full Models -----------------------------------------------------91 6.4 Conclusion remarks-----------------------------------------------------------92 References--------------------------------------------------------------------------92 Chapter 7. Transmission experiments for evaluating Propagation Isolation and XPD ----101 7.1 Introductory remarks--------------------------------------------------------101 7.2 Measurements----------------------------------------------------------------101 7.3 Conclusion remarks---------------------------------------------------------101 Chapter 8. Conclusion------------------------------104 8.1 Summary of proceeding chapters------------------------------------------104 8.2 Remarks for future studies--------------------------------------------------104 Acknowledgement--------------------------------------------------------------105 List of publication--------------------------------------------------------------106-3 -
Chapter 1 Introduction 1.1 Microwave applications Fixed Wireless Access (FWA) systems in the 26 GHz band have been commercialized in Japan for high-speed Internet connections between subscribers and base stations. Compact and low cost user terminals are realized by adopting alternating phase fed single-layer waveguide slot arrays [1-1]. This unique antenna consists of two parts, a slot plate and a base plate with corrugations screwed to each other, which dispenses with electrical contact in the strict sense. All the components in the array such as power dividers and slots are designed so that the reflection is suppressed in each component; the array works in traveling wave operation for widening the bandwidth of the array even in large size and high gain applications. A Time Division Duplex (TDD) technique is adopted, and the same frequency is used for transmission and reception. To double the frequency efficiency, we proposed the system which utilizes orthogonal polarization [1-2] [1-3] [1-4]. Challenges for full frequency reuse based upon high polarization purity, which is inherent to planar slotted waveguide arrays, are now underway. This system shows an example of orthogonal arrangement of two arrays; the antenna is for transmitting while the antenna is for receiving or vice versa. An alternating phase fed array with a new feeding structure named center-feed single-layer waveguide arrays was developed for this system [1-5]. In this array, the feed waveguide is not at the end but is in the center of the array aperture; the bandwidth doubling of the array and the frequency-independent boresite beam without tilting are the two important design objectives of this array. The grating lobes due to blockage are remedied by adopting a tapered aperture illumination [1-6]. As is usual in the case of such an electrically-large structure, overall reflection can not be designed in strict sense and is assessed roughly by the simple sum of the reflections from components, which are electrically much smaller. Unfortunately, it turns out that the measured reflection is much higher than this primitive prediction and that the accumulation of reflection at the input port becomes notable as the size of the array increases. In this chapter, we diagnose the reflection in large center-feed single-layer waveguide arrays. The FEM analysis using HFSS is conducted for the whole structure -4 -
of this large array, which has more than 30 dbi of gain. Although the structure is very large in terms of the wavelength, the predicted reflection shows remarkable agreement with the experiments. The mechanism of reflection accumulation in the overall array is discussed. It is concluded that the external mutual coupling between slots in adjacent alternating phase radiation waveguides results in increased reflection at the design frequency. Amongst all, the dominant contributions are coming from strongly excited slots; for tapered illumination to produce low sidelobes, slots in the central part of the aperture cause considerable degradation. 1.2 Single-layer waveguide slotted array the structure of a center-feed single-layer waveguide array is below. The design frequency is 25.3 GHz. It has 32 (=16x2) radiation waveguides. The 32-way power divider at the center of the antenna aperture consists of 14 cross-junctions and 2 terminal-junctions; 7 cross junctions are arrayed in series on each side of the antenna input. Each cross- junction has 4 inductive posts and two ports coupling to radiation waveguides [1-8] [1-9] [1-10] [1-11] [1-12]. One radiation waveguide has 10 slots, each with a reflection canceling sidewall; the total number of slots in the array is 320 (10x32). Each part of the antenna is analyzed and designed by the method of moments [1-13] [3-14]. This antenna mechanically consists of a slotted plate and a waveguide base joined by a screw. The test antenna is fabricated for a FWA system at 25.3 GHz. 1.3 Summary of this study The dissertation investigates the design of center-feed waveguide slotted array antenna and its application for Polarization Division Duplex (PDD) system. The center-feed waveguide slot array enhances the frequency band-width in high gain range, where a conventional center-feed array suffers fro m serious efficiency reduction. The orthogonally arranged center-feed waveguide array is able to adapt to new communication system. That will realize Wireless Fiber cover all over the world in near feature with several other systems. Figure 1.1 summarizes the structure of this dissertation. The relation between general investigative topics for micrometer-wave antennas, study topics for orthogonally arranged center-feed single-layer waveguide slot antenna. At first, isolation between two antennas is reviewed in Chapter 2. Measurements and -5 -
Calculations are isolation between two orthogonally arranged center-feed waveguide slot array antenna. Slot array symmetry arrangement is most important for obtain high isolation over 80dB. Chapter 3 describes reflection characteristics of mutual coupling among slots. Single-layer center-feed waveguide array has a blocking area at the center of the aperture. It causes high side-lobe level and decrease aperture efficiency. So, first we adopt GA taper (almost Tailar distribution) on the amplitude illumination. This design does not account of mutual coupling in adjacent radiating waveguide slot. So, each component has a good reflection characteristics at the design frequency (almost -20dB), but combination of the components has different reflection (almost -10dB). We adopt the PEC for adjacent direction. Our study reveals almost effects of mutual coupling with slots include calculation this method. Chapter 4 describes blocking free center-feed E- to H-plane and ridged cross-junction. It can run all over the aperture. So blocking area is decrease. And reflection characteristics is wider than conventional H-plane cross-junction. Chapter 5 describes aperture efficiency enhansement:reflection canceling stair. It can improve aperture efficiency and decrease side-lobe level. Slot and reflection canceling wall s combination is one unit. And narrow wall of radiating waveguide is graduatelly change to reduce. So, radiation power from slots increase, slot offset is set to small. We confirm by simulation and measearment. Chapter 6 shows interchangeable slot plate. Waveguide slots array has relatively narrow bandwidth. Base plate is made by die-casting for mass-production. If small amout, does not good for costs. So Base plate is using common frequency range, only slot plate is change for required frequency band. It is reasonable to cover wider frequency range. Chapter 7 describes Propagation Isolation, XPD, and data transmission. Experimental environment is almost 100m, 200m and 300m distance. In the case of 100m experiments, isolation and XPD is a little change. Data transmission experiments operate using QPSK, 16QAM, and 64QAM. Max Data ratio is Tx 120Mbps and Rx 120Mbps. PDD realizes 240Mbps data ratio using 20MHz band width in 26GHz band. Chapter 8 summarizes the results of this study, and presents the future problems of the work. References [1-1] N. Goto, A waveguide-fed printed antenna, IEICE Technical Report, AP89-3, Apr. 1989. [1-2] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa and M. Ando, Polarization Isolation between Two High-gain Slotted Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-210, Sept. 2003. -6 -
[1-3] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Center Feed Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-172, Mar. 2004. [1-4] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Two Center-Feed Single-Layer Waveguide Arrays Arranged Side-by-side, IEEE AP-S Int. Symp. Dig., vol.3, pp. 2380-2383, June. 2004. [1-5] S. H. 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. [1-6] S. H. Park, A study of center feed for single layer waveguide arrays, Doctoral Dissertation, Dept. of Electrical and Electronic Eng., Tokyo Institute of Technology, 2003. [1-7] S. H. Park, J. Hirokawa and M. Ando, Analysis of a waveguide slot and a reflection-canceling inductive wall, 2003 IEEE Topical Conference on Wireless Communication Technology, Hawaii, s23p08, Oct. 2003. [1-8] S. H. Park, J. Hirokawa and M. Ando, Design of a Multiple-Way Power Divider for Center feed Single Layer Waveguide Arrays, 2003 IEEE AP-S Int. Symp. Dig., vol.2, pp. 1165-1168, June. 2003. [1-9] S. H. Park, J. Hirokawa and M. Ando, Planar Cross-Junction for the Center Feed in Single-Layer Slotted Waveguide Arrays, 2002 IEEE AP-S Int. Symp. San Antonio, Texas, Dig., vol.3, pp. 416-419, June. 2002. [1-10] S. H. Park, J. Hirokawa and M. Ando, Single-layer cross-junction power divider for the center feed in slotted waveguide arrays, IEICE Tech. Rep., EMCJ2001, vol.101, No.392, 143-147. [1-11] S. H. Park, J. Hirokawa and M. Ando, Design of a Center-Feed Multiple-Way Circuit for a Single-Layer Waveguide Array, IEICE General Conv., B-1-87, March. 2003. [1-12] S. H. Park, J. Hirokawa and M. Ando, Single-Layer Cross-Junction Power Divider for the Center Feed in Slotted Waveguide Arrays, IEICE General Conv., B-1-167, Sept. 2001. -7 -
Orthogonally arranged Center-Feed Single-Layer Slotted Waveguide Array antennas for Polarization Division Duplex Chapter 1 Introduction Center-feed alternating phase fed array Chapter 2 Isolation between orthogonally arranged two antennas Chapter 3 Reflection characteristics due to mutual coupling among slots Chapter 4 Blocking free center-feed E-to H- plane and ridged cross-junction Chapter 5 Aperture efficiency enhancement Reflection canceling stair Chapter 7 Transmission experiments for evaluating Propagation Isolation and XPD Chapter 6 Slot arrays with Interchangeable slot plates Chapter 8 Conclusion Figure 1-1. Outline of Thesis -8 -
Chapter 2 Isolation between orthogonally arranged two antennas 2.1 Introductory remarks Millimeter-wave applications [2-1] have been highlighted and intensively developed for high-speed and broadband communication due to their extensive frequency resources. To overcome serious attenuations due to rain, snow, etc., relatively short-range FWA (Fixed Wireless Access) systems in the 26GHz band are in commercial use in Japan [2-2] where extremely small size and low-cost wireless terminals have been realized. Single-layer slotted waveguide arrays [2-3] are one of the key components in this system since they have a high gain of about 32dBi, high efficiency of more than 70%, and mass producible structures. One difficulty, however, of this antenna is the relatively narrow bandwidth due to its traveling wave operation. Polarization re-use is attractive and effective for mitigating this difficulty since linear polarized slot arrays have inherently high XPD and the polarization purity does not deteriorate greatly in short range propagation. This paper demonstrates the feasibility of a challenging system where frequency is fully re-used by the use of polarization isolation only [2-4]. An FWA system with this concept is presented in Figure 1. Figure 2 presents two center-feed single-layer slotted waveguide arrays with orthogonal polarization in exactly the same frequency band for transmission and reception. In order to completely reuse the frequency two times [2-5, 2-6], approximately 100dB of transmission-reception isolation is required. A preliminary scenario is to realize this isolation by the combination of an antenna isolation of 50dB and a cross-polarization compensating algorithm circuit of 50dB. The latter dispenses with the diplexer, and the use of Microwave Integrated Circuits realizes the miniaturization and economization of equipment. This paper assesses and verifies the isolation between two pairs of arrays in orthogonal polarization by simulation using Ansoft HFSS TM (High-Frequency Structure Simulator) and measurement. We prepared two center-feed single-layer waveguide arrays [2-7] which have -9 -
boresight beams as shown in Figure 2. The arrays are arranged side-by-side in the same plane: one is for transmitting and the other is for receiving in the FWA system. Isolation of about 80 db is observed in both measurement and simulation. 2.2 Configuration and Simulation Model for an Array Figure 2-3(a) shows the structure of a center-feed single-layer waveguide array. The unique structure of the alternating-phase fed array consists of two parts: a slotted plate and a base plate with corrugations screwed to each other, which dispenses with the need for perfect electrical contact. Slots are cut in the broad wall of the rectangular waveguide [2-8 to 2-10]. This structure has a cross-junction power divider [2-11to 2-16] at the center of the array as shown in Figure 3(b) and has a stable boresight main beam. Heretofore, a beam tilting technique was used for suppression of reflection from the slot array at the antenna input [2-17]. This time, reflection canceling walls are introduced to suppress reflections from each radiating slot [2-18 to 2-20] as shown in Figure 3(c). In the FWA system, two center-feed single-layer waveguide arrays with the same structure are placed orthogonally as shown in Figure 2. Since the main beams of both antennas radiate in the same boresight direction, transmitting and receiving antennas can be installed in the same plane, and, hence, be unified. This structure has the manufacturing advantage that it can be dug from only one side of the slotted aperture. Figure 4 shows the simulated model of this antenna. The antenna size is 14.9 λ (176mm) x 18.4 λ (218mm) at the design frequency 25.3GHz. We simulated this model using HFSS. The simulation computer s specifications are given in Table 1,and the parameters used in the HFSS simulation are presented in Table 2. HFSS's adaptive mesh generation is used [2-21]. 2.3 Reflection and Radiation Patterns of an Array In order to evaluate and understand the slot coupling in the array, the analysis model of an external half space is discussed. In the design of the slots of the prototype - 10 -
array, a linear array model with an infinite ground plane, called isolated waveguide model, is considered and the mutual coupling effects between slots in adjacent radiating waveguides are neglected. The full structure simulation adopts the more realistic model as shown in Figure 2-5 (a) where the actual mutual coupling between slots in the adjacent waveguides via the half space is considered. The alternating-phase fed array is unique in that the adjacent waveguide is fed 180 degrees out-of-phase, and, if it is large enough, the external half space is well approximated by conducting metal walls that extend from the narrow walls as shown in Figure 2-5 (b). Figure 2-6 shows the calculated and measured overall reflection characteristics of this antenna. The measurements are predicted well by the simulation for model (a), though the array structure is very large and computationally heavy. As is expected from the principle of the design, the simulated result for model (b) with the metal walls also agrees with the measurements as well as the full model in (a). From the practical design point of view, the results suggest that the slot design for the reflection and the illumination control in a single radiating waveguide may be conducted by use of the linear array model with the conducting walls in (b) instead of the full array model in (a) [2-22]. In Figures 2-7 and 2-8 are presented the radiation patterns. The calculated and measured radiation patterns are almost identical. 2.4 Isolation Characteristics of Orthogonally Polarized Pair Arrays Arranged Side-by-Side Two center-feed waveguide arrays are combined side-by-side as shown in Figure 2-2. This antenna has a gain of more than 30 dbi. Figure 2-9 shows the simulated model of this antenna. The antenna size is 18.4 λ (218mm) x 33.3 λ (394mm) at the design frequency 25.3GHz. We simulated this model using HFSS, and the parameters used in the HFSS simulation are presented in Table 2-2. Figure 2-10 shows the mesh on the slotted plate, and Figure 11 presents the calculated S-parameters. Isolation (S21 and S12) between the ports of the two antennas is more than 80dB at 25.3GHz. This value is very promising for the dual polarization wireless systems proposed in Fig. 2-1. Figure 2-12 compares the measured data with the simulated data and the results support the above proposal. Next, the degradation of polarization isolation due to the offset in the arrangement -11 -
in the pair is discussed. The second array is offset with the distance h as shown in Figure 2-9. The simulated and measured isolations for h=1 λ is also included in Fig.2-12 and are in reasonable agreement with each other. The polarization isolation is about 60-70dB and is degraded by about 10-20 db. The effects of arrangement are now discussed in more detail. We prepared two center-feed single-layer waveguide arrays which have the same structure. We measured the isolation for different values of distance d ( = 0, 1, 2, 3 λ ) and position h ( = 0, 1, 2, 3 λ ) as shown in Fig.2-13. In Figure 2-14, the measured isolation results are summarized as functions of d and h. The results indicate a serious degradation of isolation due to increasing h but an improvement in isolation due to increasing d. In order to confirm these general results qualitatively, we conducted a series of simulations. Figure 2-15 shows the full size arrays used in the simulation; isolation is evaluated for variety of distances d ( = 0, 1, 2, 3 λ ) and positions h ( = 0, 1, 2, 3 λ ). Figure 2-16 shows the results of isolation between two arrays at 25.3GHz. If the distance d is increased, the isolation improves, but, if the position h is increased, isolation degrades. Almost the same tendency as in Figure 2-14 is observed. This phenomenon can be summarized as follows. The residual cross polarization coupling between two arrays is effectively cancelled out at the receiving antenna output resulting in remarkably high isolation, due to the symmetrical structure and arrangement of the paired arrays. 2.5 Conclusion remarks We discussed the coupling characteristics of two center-feed alternating-phase fed single-layer waveguide arrays. Large-size arrays were analyzed by using HFSS. The total size of the problem is 320 x 2 = 640 slots and 33.3 λ x 18.4 λ.results of simulation and measurements exhibit good agreement. More than 80 db of isolation is achieved by the symmetrical and tight arrangement. The symmetry of the arrangement as well as the structure is the key for high isolation. These results provide the basis for the application of slot arrays to dual polarization wireless systems. Also, the effectiveness of performance simulation of large scale arrays in terms of polarization isolation and reflection indicates a promising design tool in antenna engineering. - 12 -
References [2-1] K. Sakakibara, J. Hirokawa, M. Ando and N. Goto, Single-Layer Slotted Waveguide Arrays for Millimeter Wave Applications, IEICE Trans. Commun., vol. E79-B, no.12, pp.1765-1772, Dec.1996. [2-2] 26GHz FWA http://www.jrc.co.jp/eng/product/26g_fwa/index.html [2-3] N.Goto, A waveguide-fed printed antenna, IEICE Technical Report, AP89-3, Apr. 1989. [2-4] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa and M. Ando, Polarization Isolation between Two High-gain Slotted Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-210, Sept. 2003. [2-5] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Center Feed Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-172, Mar. 2004. [2-6] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Two Center-Feed Single-Layer Waveguide Arrays Arranged Side-by-side, IEEE AP-S Int. Symp. Dig., vol.3, pp. 2380-2383, June. 2004. [2-7] S. H. Park, A study of center feed for single layer waveguide arrays, Doctoral Dissertation, Dept. of Electrical and Electronic Eng., Tokyo Institute of Technology, 2003. [2-8] R. S. Elliott and L.A. Kurtz, The Design of Small Slot Arrays, IEEE Trans. Antennas Propagat., vol. AP-26, pp.214-219, 1978. [2-9] K. Mahadevan, H. A. Auda, C. E. Smith, Analysis and Design of Planar Waveguide Slot Arrays Using Scattering Matrix Approach, ACES JOURNAL., vol.13, no.3, NOV. 1998. [2-10] R. C. Johnson and H. Jasik, Antenna Engineering Handbook, McGraw-Hill, Chap.9, 1993. [2-11] S. H. Park, J. Hirokawa and M. Ando, Single-Layer Cross-Junction Power Divider for the Center Feed in Slotted Waveguide Arrays, IEICE General Conv., B-1-167, Sept. 2001. [2-12] S. H. Park, J. Hirokawa and M. Ando, Single-layer cross-junction power divider for the center feed in slotted waveguide arrays, IEICE Tech. Rep., EMCJ2001, vol.101, No.392, 143-147. - 13 -
[2-13] S. H. Park, J. Hirokawa and M. Ando, Planar Cross-Junction for the Center Feed in Single-Layer Slotted Waveguide Arrays, 2002 IEEE AP-S Int. Symp. San Antonio, Texas, Dig., vol.3, pp. 416-419, June. 2002. [2-14] S. H. 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. [2-15] S. H. Park, J. Hirokawa and M. Ando, Design of a Center-Feed Multiple-Way Circuit for a Single-Layer Waveguide Array, IEICE General Conv., B-1-87, March. 2003. [2-16] S. H. Park, J. Hirokawa and M. Ando, Design of a Multiple-Way Power Divider for Center feed Single Layer Waveguide Arrays, 2003 IEEE AP-S Int. Symp. Dig., vol.2, pp. 1165-1168, June. 2003. [2-17] R.E.Collin and F.J.Zucker, Antenna Theory, part 1, Sec.14.8, McGraw-Hill, 1969. [2-18] S. H. Park, J. Hirokawa and M. Ando, Analysis of a waveguide slot and a reflection-canceling inductive wall, 2003 IEEE Topical Conference on Wireless Communication Technology, Hawaii, s23p08, Oct. 2003. [2-19] S. H. Park, J. Hirokawa and M. Ando, Simple Analysis of a Slot and 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. [2-20] S. H. Park, J. Hirokawa and M. Ando, Analysis of a Waveguide Slot with a Reflection-Canceling Post, IEICE Tech. Rep., AP2002, vol. 102, No.232, 31-36. [2-21] Ansoft Corporation, Ansoft HFSS manual, 2004. [2-22] Y. Tsunemitsu, S. H. Park, J. Hirokawa, M. Ando, Y. Miura, Y. Kazama and N. Goto, Reflection Characteristics of Center-Feed Single-Layer Waveguide Arrays, IEICE Trans. Commun., vol. E88-B, no.6, pp.2313-2319, June.2005. - 14 -
Base station A Receiving antenna Horizontal polarization Base station B Transmitting antenna Input=0dBm Input=0dBm 30dBi Transmitting antenna Vertical polarization free space loss 1km=-120dB 10km=-140dB 26GHz-band 30dBi Required isolation level Receiving antenna Pr=-60dBm to -80dBm P inter =-80dBm to -100dBm Figure 2-1. Dual polarization wireless system for two-times frequency reuse. - 15 -
(a) Transmitting antenna (b) Receiving antenna Figure 2-2. Orthogonally polarized slot arrays in side-by-side arrangement. - 16 -
218mm=18.4 Reflection canceling wall 176mm=14.9 Input x 10mm z y Feed waveguide Slotted plate Radiating waveguide Base plate (a) Full array 8mm 9mm 10mm 3mm z y x (b) A multiple-way power divider consisting of series of cross-junctions. - 17 -
Reflection canceling unit with slot and wall. z x y 1.3mm 8mm 6mm 3mm (c) Reflection canceling unit consisting of slot and wall. Figure 2-3. A center-feed alternating-phase fed single-layer waveguide array. - 18 -
14.9 (176mm) 0.5 (6mm) 18.4 (218mm) Figure 2-4. Full model of center feed single layer waveguide array (10*32=320slots) for simulation. - 19 -
Table 2-1. Personal computer specification. CPU Xeon 3.6GHz Memory 16GB HDD 500GB 2 OS Windows XP 64bit Edition HFSS Version 10 Table 2-2. Parameters used in HFSS simulation. Data Figure 4 Figure 9 h=0, d=0 Figure 9 h=1, d=0 Pass Number 12 10 10 Tetrahedra 829154 1104685 1084560 Delta S 0.0024275 0.0070885 0.0088132 Real Time 28h20m03s 37h23m31s 33h06m39s Memory 14GB 14GB 14GB Matrix 4953196 6541060 6421842-20 -
0.5 Radiation boundary Metal wall (a) Free space (b) Metal wall Figure 2-5. Simplified design/analysis model of external half-space above the array aperture. - 21 -
Amplitude [db] 0-5 -10-15 -20-25 -30 Cal.(Free space) -35 Cal.(Metal Wall) Mea.(Free space) -40 24.5 25 25.5 26 26.5 Frequency [GHz] Figure 2-6. Overall reflection characteristics. - 22 -
Relative Amplitude [db] 0-10 -20-30 -40 Cal. Mea. -50-90 -60-30 0 30 60 90 Angle theta [deg] Figure 2-7. H-plane radiation pattern at 25.3GHz. - 23 -
Relative Amplitude [db] 0-10 -20-30 -40 Cal. Mea. -50-90 -60-30 0 30 60 90 Angle theta [deg] Figure 2-8. E-plane radiation pattern at 25.3GHz. - 24 -
33.3 (394mm) 18.4 (218mm) h 0.5 (6mm) (a) Transmitting antenna (b) Receiving antenna Figure 2-9. Simulation model of orthogonally polarized pair arrays in symmetrical arrangement. - 25 -
33.3 (394mm) 18.4 (218mm) (a) Transmitting antenna (b) Receiving antenna Figure 2-10. Mesh on the slotted plate (HFSS). - 26 -
Amplitude [db] 0-10 -20-30 -40-50 -60-70 -80-90 S11 S12 S21 S22-100 -110 24.5 25 25.5 26 26.5 Frequency [GHz] Figure 2-11. Simulation results of reflection and isolation characteristics between two center-feed waveguide arrays. - 27 -
Amplitude [db] 0-10 -20-30 -40-50 -60-70 -80-90 S11 h=0, d=0 (Cal.) S21 h=0, d=0 (Cal.) S21 h=1, d=0 (Cal.) S11 h=0, d=0 (Mea.) S21 h=0, d=0 (Mea.) S21 h=1, d=0 (Mea.) -100-110 24.5 25 25.5 26 26.5 Frequency [GHz] Figure 2-12. Measured results of reflection and isolation characteristics between two center-feed waveguide arrays. - 28 -
h y x d (a) Transmitting antenna (b) Receiving antenna Figure 2-13. Isolation between two trial manufactured antennas arranged with distance d and position h. - 29 -
Amplitude [db] h [ ] d [ ] Figure 2-14. Position dependence of isolation at 25.3GHz (Measured). - 30 -
37.3 21.9 h y 18.4 d x (a) Transmitting antenna (b) Receiving antenna Figure 2-15. Full array model for simulation of position dependence of isolation. - 31 -
Amplitude [db] h [ ] d [ ] Figure 2-16. Position dependence of isolation between two arrays at 25.3GHz (Calculated). - 32 -
Chapter 3 Reflection characteristics due to mutual coupling among slots 3.1 Introductory remarks Fixed Wireless Access (FWA) systems in the 26 GHz band have been commercialized in Japan for high-speed Internet connections between subscribers and base stations. Compact and low cost user terminals are realized by adopting alternating phase fed single-layer waveguide slot arrays [3-1]. This unique antenna consists of two parts, a slot plate and a base plate with corrugations screwed to each other, which dispenses with electrical contact in the strict sense. All the components in the array such as power dividers and slots are designed so that the reflection is suppressed in each component; the array works in traveling wave operation for widening the bandwidth of the array even in large size and high gain applications. A Time Division Duplex (TDD) technique is adopted, and the same frequency is used for transmission and reception. To double the frequency efficiency, we proposed the system which utilizes orthogonal polarization [3-2] [3-3] [3-4]. Challenges for full frequency reuse based upon high polarization purity, which is inherent to planar slotted waveguide arrays, are now underway. Fig. 1 shows an example of orthogonal arrangement of two arrays; the antenna in (a) is for transmitting while the antenna in (b) is for receiving or vice versa. An alternating phase fed array with a new feeding structure named center-feed single-layer waveguide arrays was developed for this system [3-5]. In this array, the feed waveguide is not at the end but is in the center of the array aperture; the bandwidth doubling of the array and the frequency-independent boresite beam without tilting are the two important design objectives of this array. The grating lobes due to blockage are remedied by adopting a tapered aperture illumination [3-6]. As is usual in the case of such an electrically-large structure, overall reflection can not be designed in strict sense and is assessed roughly by the simple sum of the reflections from components, which are electrically much smaller. Unfortunately, it turns out that the measured reflection is much higher than this primitive prediction and that the accumulation of reflection at the input port becomes notable as the size of the array increases. - 33 -
In this chapter, we diagnose the reflection in large center-feed single-layer waveguide arrays. The FEM analysis using HFSS is conducted for the whole structure of this large array, which has more than 30 dbi of gain. Although the structure is very large in terms of the wavelength, the predicted reflection shows remarkable agreement with the experiments. The mechanism of reflection accumulation in the overall array is discussed. It is concluded that the external mutual coupling between slots in adjacent alternating phase radiation waveguides results in increased reflection at the design frequency. Amongst all, the dominant contributions are coming from strongly excited slots; for tapered illumination to produce low sidelobes, slots in the central part of the aperture cause considerable degradation. 3.2 Center-Feed Array Antenna [3-5] [3-7] 3.2.1 A center-feed single-layer waveguide array Fig. 3-2 shows the structure of a center-feed single-layer waveguide array. The design frequency is 25.3 GHz. It has 32 (=16x2) radiation waveguides. The 32-way power divider at the center of the antenna aperture consists of 14 cross-junctions and 2 terminal-junctions; 7 cross junctions are arrayed in series on each side of the antenna input. Each cross- junction has 4 inductive posts and two ports coupling to radiation waveguides [3-8] [3-9] [3-10] [3-11] [3-12]. One radiation waveguide has 10 slots, each with a reflection canceling sidewall; the total number of slots in the array is 320 (10x32). Each part of the antenna is analyzed and designed by the method of moments [3-13] [3-14]. This antenna mechanically consists of a slotted plate and a waveguide base joined by a screw. The test antenna is fabricated for a FWA system at 25.3 GHz as shown in Fig. 3-1. 3.2.2 A multiple way power divider with a series of cross-junctions Fig. 3-3 (a) shows the structure of a power divider consisting of cross-junctions and Fig. 3-3 (b) shows a photo of the actual structure. This structure can divide the - 34 -
power equally to all radiation waveguides. Adjacent radiation waveguides are fed 180 degree out of phase (alternating-phase). All the windows that couple to radiation waveguides are designed to be reflection-free; the feed waveguide operates as a traveling wave waveguide. 3.2.3 A radiation waveguide - linear array of reflection canceling units consisting of a slot and a wall Fig. 3-4 (a) shows the reflection canceling units where each unit consists of a slot and a wall. The reflection from the slot is canceled by the projection in the narrow wall; the units are regarded as reflection-free. This dispenses with the beam tilting technique widely adopted for reflection suppression and realizes a main beam in the normal direction of the slotted plate. Fig. 3-4 (b) shows the photo of a reflection canceling wall. The radiation waveguides also operate as a traveling wave waveguide. One difficulty of this center feed array is that the grating lobes associated with the blocking area (width =2.1 λ ) above the feed waveguide. It results in grating lobes of about -10 db in the direction of 5.5 degree from bore-site. In order to suppress these, the slot excitation distribution is synthesized and tapered toward the edge as in Fig. 3-5. Fig. 3-6 shows the H-plane radiation patterns for this tapered illumination as compared to that for uniform illumination. The grating lobes are suppressed down to about -15 db. The gain degradation due to tapered illumination is 1 db in theory but is less than 0.2 db in measurements, where we can not account for slot mutual coupling effect in the design. [3-6]. Fig. 3-7 shows the measured gain of the prototype antenna. Antenna gain of 30.5 dbi, which meets the requirement from FWA system is obtained at 25.3 GHz. The size of this antenna is 205mm x 165mm and the aperture efficiency is 37 %; this is not high enough in terms of the potential of this types of array and should be improved in the future. - 35 -
3.3 Reflection Characteristics 3.3.1 Reflection from components Reflection characteristics are evaluated by the commercial FEM software HFSS. Before analyzing the large scale array, the reflections from key components such as the antenna input aperture, the 16-way power divider and the linear slot array, assessed in the design, are reviewed. The antenna input aperture and the 16-way power divider are modeled as shown in Fig. 3-8. Structural symmetry is taken into account to simplify the model and reduce the computational load. Calculated reflection characteristics of each component are shown in Fig. 3-9. For each component, a return loss less than -20 db is obtained at the objective frequency. Reflection from the linear array is small since the mutual coupling between slots along the radiation waveguide was taken into account in the design in this isolated environment. The overall reflection characteristic of the array is roughly predicted as the sum of the absolute value of reflections for these three components as in the same figure. A reasonable reflection of about -25 db is predicted around 25.3 GHz upon the above assumption. At this stage, the slot mutual coupling between adjacent radiation waveguides in the external half-space is neglected. 3.3.2 Overall reflection from the array The sum reflection given in 3.3.1 is a rough approximation in that the phase information as well as the mutual coupling between slots in adjacent waveguides are neglected. It may be called an isolated waveguide model. Generally, it is very difficult to simulate the reflection from electrically large, complicated structures, since not only the amplitude but also the relative phase of each component of reflection must be evaluated accurately. Modeling and meshing the entire antenna in the limited memories of personal computers is heavy and has not been discussed in the literature. But this challenging computation was conducted in spite of all these difficulties. The FEM analysis model is shown in Fig. 3-10 where due to the symmetry, only a quarter of the structure with a magnetic wall and an electric wall is considered. HFSS's adaptive mesh generation was adopted [3-15] and the mesh sizes for the Fig. 3-11 simulation models are 145,520 to 153,649 tetrahedra. It took from one to two hours and 1.7 GB - 36 -
memory to simulate each model. When the difference of the magnitude of the S-parameters between two adaptive pass (Delta S less in HFSS) is smaller than 0.01, the calculation stops. Fig. 3-11 shows the measured and calculated results of the reflection characteristics of the whole array. This experimental data is obtained using a network analyzer in an anechoic chamber. The reflection is defined and compared at the input waveguide, which is connected to the array via an input aperture. The reflection predicted for the isolated waveguide model is also included in the figure. The measured reflection is suppressed at a little bit higher frequency of 25.6 GHz and the reflection at the design frequency 25.3 GHz is as high as about 10 db. The agreement of the full structure FEM analysis and the measurement is noteworthy for such an electrically large structure. The shift of frequency as well as the level of reflection about -10 db are predicted accurately. It assures the high accuracy in fabrication and the stable operation with the simple contact by screws. At the same time, the isolated waveguide model is too rough and is not reliable. 3.4 Mechanism extraction of accumulated reflection in overall array 3.4.1 Slot mutual coupling between adjacent radiation waveguides. In order to evaluate and understand the slot coupling in the array, the analysis model of the external half space is discussed carefully. In the design of the slots of the prototype array, a linear array model with infinite ground plane, called isolated waveguide model, was considered and the mutual coupling effects between slots in adjacent radiation waveguides were neglected. The full structure simulation adopts the more realistic model as shown in Fig. 3-12 (a), where the mutual coupling via the half space was considered. This alternating phase fed array, if it is large enough, is well approximated by the model with conducting metal walls between adjacent waveguides as shown in Fig. 3-12 (b). On the other hand, the isolated waveguide model used in the design approximately corresponds to the model in Fig. 3-12 (c) with absorbing walls between adjacent waveguides. - 37 -
The FEM simulation is conducted for all three models in Fig. 3-12 (a), (b) and (c) and the reflection characteristics are shown in Fig. 3-11. As expected, the result (b) for the metal wall reasonably agrees with the full model in (a). On the other hand, the result (c) is quite different from these and is more close to the sum reflection used in the design. Therefore, it is apparent that the mutual coupling greatly affects the reflection characteristics. It suggests that the slot design accuracy would be enhanced if the full model in (a) or its approximation in (b) is used in stead of the present isolated waveguide model. 3.4.2 Localization of mutual coupling in the aperture as functions of illumination distribution In the previous section, the mutual coupling between adjacent waveguides is found to be the key factor for the accumulated reflection in alternating phase fed arrays. In this section, the slot with the dominant contribution in a linear array is localized. The overall reflection is calculated for the model with increasing number of slots in a radiation waveguide as shown in Fig. 13 (a). In every calculation, each radiation waveguide is matched or connected with an infinitely long waveguide which simulates the actual antenna operation. Fig. 14 shows overall reflection characteristics as the number of slots along the radiation waveguide is increased. The mesh sizes adopted for Fig. 14 are 96,193 to 272,317 tetrahedra. It took from one to twenty-two hours and 1.7 GB memory to simulate each model. If not for slots (zero), the reflection is similar to the sum reflection in Fig. 3-9 and 3-11. The return loss at the design frequency 25.3 GHz is considered. The reflection rapidly increases as the slot number increases; the degradation from zero to first and first to fifth is notable while that from fifth to tenth is not so clear. It seems as if the slots with lower numbers are more dominant that those with higher numbers. In order to support this observation, a complementary model was also analyzed which has the slots only from sixth to tenth slots array as is shown in Fig. 3-14. Here, the return loss is not increased drastically at the design frequency 25.3 GHz as shown in Fig. 3-14. Therefore it is confirmed that the mutual coupling of slots with lower numbers, those near the feed waveguide, greatly affects the reflection characteristics. - 38 -
The above results are then viewed in terms of aperture illumination. This antenna adopts slot excitation tapered down as shown in Fig. 5 for suppressing side lobe levels due to blocking. Therefore, the comparison may depend upon the desired illumination; the overall reflection of an array with uniform slot excitation, though the side lobe level is high, was also conducted and is presented in Fig. 3-15. The mesh sizes for the Fig. 15 simulation models are 99,736 to 135,259 tetrahedra. As the number of slots increases, reflection gradually increases but it still remains as low as -20 db at 25.3 GHz. Furthermore, all the slots are equally contributing to the degradation; the significant difference between the importances of slots is not observed in contrast with tapered illumination. By the way, when only the first row of slots exists, the reflection characteristics are different between the uniform amplitude array in Fig. 15 and the tapered amplitude model in Fig. 3-14. This difference is related to the fact that the slot coupling coefficients assigned for the first slots depend upon the distribution. For the tapered amplitude slot excitation model, the first row of slots radiates almost 20 % of the power. The first row of slots in the uniform amplitude model, on the other hand, only radiates about 10 %. Finally, the conventional method of narrow band reflection suppression is referred to as well in Fig. 3-15. For a very narrow band system, we have only to modify the input structure, input aperture s height in this specific example, so that the reflection cancels out at the desired frequency only. Measured reflection characteristics slightly moves into the desired frequency and return loss less than 17 db is obtained at the desired frequency in the simulation. It is noted that the bandwidth is much narrower than the results of tenth in Fig. 3-15. 3.5 Conclusion remarks We discussed the reflection characteristics of center-feed, alternating phase single-layer waveguide arrays. By increasing the number of slots in the radiation waveguides, the mutual coupling effects become serious. The slots near the feed waveguide, which are strongly excited, greatly affects the reflection characteristics. Our future study is to introduce the PEC wall model into the slot design of a linear array for accurate prediction of overall reflection from alternating phase fed arrays. Uniform illumination would be advantageous in view of mutual coupling and wideband reflection suppression. - 39 -
References [3-1] N. Goto, A waveguide-fed printed antenna, IEICE Technical Report, AP89-3, Apr. 1989. [3-2] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa and M. Ando, Polarization Isolation between Two High-gain Slotted Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-210, Sept. 2003. [3-3] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Center Feed Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-172, Mar. 2004. [3-4] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Two Center-Feed Single-Layer Waveguide Arrays Arranged Side-by-side, IEEE AP-S Int. Symp. Dig., vol.3, pp. 2380-2383, June. 2004. [3-5] S. H. 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. [3-6] S. H. Park, A study of center feed for single layer waveguide arrays, Doctoral Dissertation, Dept. of Electrical and Electronic Eng., Tokyo Institute of Technology, 2003. [3-7] S. H. Park, J. Hirokawa and M. Ando, Analysis of a waveguide slot and a reflection-canceling inductive wall, 2003 IEEE Topical Conference on Wireless Communication Technology, Hawaii, s23p08, Oct. 2003. [3-8] S. H. Park, J. Hirokawa and M. Ando, Design of a Multiple-Way Power Divider for Center feed Single Layer Waveguide Arrays, 2003 IEEE AP-S Int. Symp. Dig., vol.2, pp. 1165-1168, June. 2003. [3-9] S. H. Park, J. Hirokawa and M. Ando, Planar Cross-Junction for the Center Feed in Single-Layer Slotted Waveguide Arrays, 2002 IEEE AP-S Int. Symp. San Antonio, Texas, Dig., vol.3, pp. 416-419, June. 2002. [3-10] S. H. Park, J. Hirokawa and M. Ando, Single-layer cross-junction power divider for the center feed in slotted waveguide arrays, IEICE Tech. Rep., EMCJ2001, vol.101, No.392, 143-147. [3-11] S. H. Park, J. Hirokawa and M. Ando, Design of a Center-Feed Multiple-Way Circuit for a Single-Layer Waveguide Array, IEICE General Conv., B-1-87, March. 2003. - 40 -
[3-12] S. H. Park, J. Hirokawa and M. Ando, Single-Layer Cross-Junction Power Divider for the Center Feed in Slotted Waveguide Arrays, IEICE General Conv., B-1-167, Sept. 2001. [3-13] S. H. Park, J. Hirokawa and M. Ando, Analysis of a Waveguide Slot with a Reflection-Canceling Post, IEICE Tech. Rep., AP2002, vol. 102, No.232, 31-36. [3-14] S. H. Park, J. Hirokawa and M. Ando, Simple Analysis of a Slot and 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. [3-15] Ansoft Corporation, Ansoft HFSS manual, 2004. - 41 -
z 218mm = 18.4 y E E 176mm = 15. 9 x 176mm=15.9 218mm=18.4 (a) Transmitting antenna (b) Receiving antenna Fig. 3-1 A terminal with orthogonal arrangement of arrays in dual polarization FWA systems. - 42 -
218mm=18.4 176mm=15.9 10mm z input wall Slotted plate x y Radiation waveguide Feed waveguide Fig. 3-2 A center-feed alternating phase fed single-layer waveguide array. - 43 -
8mm 10mm 9mm 3mm z y x (a) cross-junction model (b) picture Fig. 3-3 A multiple way power divider consisting of series of cross-junctions. - 44 -
Reflection canceling unit with slot and wall. z x y 1.3mm 8mm 6mm 3mm (a) Unit consisting of slot and wall (b) Picture Fig. 3-4 Reflection canceling unit consisting of slot and wall. - 45 -
Amplitude 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Cal. Exp. 0 10 20 30 40 50 60 70 80 90 100 Position from center [mm] Fig. 3-5 Synthesized aperture illumination for reduction of side lobes due to center feed waveguide blockage. - 46 -
Relative Amplitude [db] 0-10 -20-30 -40-50 Tapered amplitude (Cal.) Tapered amplitude (Exp.) Uniform amplitude(cal.) -90-60 -30 0 30 60 90 Angle [deg] Fig. 3-6 H-plane radiation pattern. Tapered slot array excitation (Cal. and Exp.)and uniform slot array excitation(cal.). - 47 -
] Gain [dbi] 31.5 30.5 31 29.5 30 28.5 29 27.5 28 26.5 27 26 Exp. 24.5 25 25.5 26 26.5 Frequency [GHz] Fig. 3-7 Measured antenna gain. - 48 -
5.9mm 8mm WR42 3mm z x y 88mm 1.9mm 1mm 3mm 0.5mm 6mm 95.37mm (a) antenna input aperture (b)16-way power divider (c) slot array Fig. 3-8 Various component models. - 49 -
0 Return Loss [db] -10-20 -30-40 -50 antenna input aperture 16-way power divider slot array Sum reflection 24.5 25 25.5 26 26.5 Frequency [GHz] Fig. 3-9 Reflection characteristics of each component. - 50 -
218mm=18 Electric wall Using symmetry Magnetic wall 176mm=15 (a) Prototype (b)whole model (c) A quarter model Fig. 3-10 Whole analysis model. (center-feed single-layer waveguide arrays) - 51 -
Return Loss [db] 0-10 -20-30 -40-50 24.5 25 25.5 26 26.5 Frequency [GHz] Fig. 3-11 Overall reflection characteristics. Exp. Sum reflection (cal.) Free space (cal.) Metal wall (cal.) Absober (cal.) - 52 -
0.5 Radiation boundary (a) Free space (b) Metal wall (c) Absorber Fig. 3-12 Model of external half-space above the array aperture. (In HFSS, metal wall is conductor with bulk conductivity of 1e+30 Siemens/m. Absorber is expressed by the second-order radiation boundary condition). - 53 -
Tenth Tenth Fifth sixth First (a) Number of slots increase (b) Only sixth to tenth slots array Fig. 3-13 Various models for external slot coupling simulation. - 54 -
Return Loss [db] 0-10 -20-30 -40-50 Zero First Fifth Tenth Only 6th to 10th 24.5 25 25.5 26 26.5 Frequency [GHz] Fig. 3-14 Reflection as function of number of slots which are tapered amplitude slot excitation. (Cal.) The condition for convergence is Delta S less than 0.02-55 -
Return Loss [db] 0-10 -20-30 -40-50 Zero(cal.) First(cal.) Fifth(cal.) Tenth(cal.) Narrow band matching(mea.) 24.5 25 25.5 26 26.5 Frequency [GHz] Fig. 3-15 Reflection as function of number of slots which have uniform amplitude of slot excitation.(cal.) Narrow band matching (Exp.) is conducted for tapered illumination by changing the aperture height from 5.9 mm to 5.0 mm. Delta S less than 0.02. - 56 -
Chapter 4 Blocking free center-feed E-to H-plane and ridged cross-junction 4.1 Introductory remarks Alternating-phase fed single-layer waveguide slotted arrays [4-1] realize compact and low-cost antenna. Fixed Wireless Access (FWA) systems in the 26GHz band have been commercialized in Japan for high-speed Internet connections between subscribers and base stations [4-2]. This antenna has a cascaded T-junction multiple-way power divider as a feed waveguide. This feed waveguide is located at the end of the antenna aperture. So, the slot array has relatively narrow band width due to the long line effect. And the main beam is tilted by frequency change. The authors have developed center-feed single-layer slotted waveguide array [4-3]. This antenna has an H-plane cross-junction multiple way power dividers at the center [4-4] [4-8] for feeding the radiating waveguides on both sides. The feed waveguide lies in the same layer as the radiating waveguide [4-9] and the array is still in single layer. But, H-plane cross-junctions have two problems. Firstly, the center feed waveguide array has large blocking area in the aperture. It causes high side lobes. Secondly, each cross-junction has 4 inductive posts which should be installed separately after die casting the base plate; this process is not suitable for mass production. This chapter introduces a new structure named E- to H-plane cross-junction for reducing the blocking area and lowering the sidelobe levels. A multiple way power dividers consisting of series of E- to H-plane cross junctions is designed for the use in the alternating-phase fed single-layer waveguide array. The feed waveguide has its narrow wall in the array aperture, which causes blocking. Since this area is smaller than the broadwall blocking associated with the H-plane cross junctions, the sidelobes are reduced. Fig.4-1 shows single-layer waveguide arrays with three kinds of feeding structures, - 57 -
that is (a) conventional end-feed, (b) H-plane cross junction center feed and (c) new center-feed named as E-to H-plane cross junction center feed. In (b), the broad wall of the feed waveguide is embedded on the slotted aperture, which has a large blocking area causing large side-lobes of the slot array. In a novel structure (c) on the other hand, the narrow wall is embedded in the aperture and results in reduction of the blocking area and the sidelobe levels. The center feed array antennas would be the key devices for the future Fixed Wireless Access (FWA) systems using a dual polarization system [4-10] [4-13]. The Finite Element Method (FEM) analysis is conducted using Ansoft HFSS TM (High-Frequency Structure Simulator) for each component as well as the full structure of the array, which has more than 31.5dBi of gain. 4.2 Design of a Cross-Junction Power Divider Fig. 4-2 compares two kinds of the center feed cross-junction multi-way power dividers; that is H-plane and E- to H-plane ones; only half of the feeding waveguide is illustrated. Fig. 4-3(a) shows the unit H-plane cross junction. It has 4 inductive posts to control the division to two radiating waveguides and to suppress the reflection [4-14]. An additional manufacturing process to install the posts is required after the fabrication of the grooved feed waveguide. Fig. 4-4(a) shows the newly proposed E- to H-plane cross-junction [4-15] for the feed waveguide in Fig. 4-1(c) and Fig. 4-2(b). It has a wall at the bottom of the feed waveguide to suppress the reflection and two windows on each broadwall to control divided power into the two radiating waveguides. This structure can be produced by diecasting in contrast with the H-plane cross junctions with the posts. The power division is controlled by the width Lw of the coupling windows. The reflection is controlled by the height Wh of the wall and its position Wp from the center of the cross-junction. It is suppressed below -30dB at 25.3GHz in the design. The broad wall width of the feed waveguide is set 7.2mm so that the spacing of the radiating waveguide is half of the guide wavelength of the feed waveguide. The narrow wall of the feed waveguide is chosen to be 3.6mm, which is half of the broad wall width. Seven cross junctions are cascaded to realize the uniform amplitude and alternating-phase between the adjacent radiating waveguides (180deg out of phase). The cross junctions are designed so that the coupling may be set as S31 2 =(S41 2 )=1/14, 1/12,.1/6, 1/4 and - 58 -
1/2. For the cross junction #6 (S31 2 =S41 2 =1/4), the values of Lw, Wh, and Wp are shown in Fig. 4-4. When Lw increases, Wh increases for reflection canceling due to larger reflection by the coupling windows. Fig. 4-5 compares the reflection characteristics of this E- to H-plane cross-junction with the conventional H-plane one for (S31 2 =S41 2 =1/4). The E to H-plane cross-junction has reflection below -20dB in a bandwidth wider than the H-plane cross-junction. 4.3 SUPPRESSION OF THE SIDELOBES DUE TO BLOCKAGE OF FEED WAVEGUIDE Fig. 4-6 shows the center feed linear slot array model using two types of cross-junction power dividers. Slots are cut in the broad wall of the rectangular waveguides on both sides of the cross junction [4-16]. Each radiating waveguide has 10 slots, each of which has a reflection-canceling inductive side wall [4-17] [4-19]. Uniform illumination is designed based upon the method [*] where the slot and the cross junction is designed independently. Since the phase of the divided power is the same (or 180deg shifted) in two waveguides on both sides of the (a) H-plane (or (b) E- to H-plane) junction, the slot arrangement is (a) symmetrical (or (b) anti-symmetrical) with respect to Y=0 as are indicated in the figures. Sidelobe levels in H-plane will vary with the blocking due to the feed waveguide or the spacing d between two innermost slots. The sidelobe reduction effects of E- to H-plane cross junction is simulated by FEM where the EM coupling between slots and the cross junction is fully taken into account. In general, smaller d gives lower sidelobes. The dynamic range for the spacing d depends upon the physical dimension of the feed waveguide occupying the aperture; the minimum value for d is smaller for the E- to H-plane cross junctions. Moreover, the EM coupling affects the resultant sidelobe levels. The first sidelobe levels in the H-plane radiation pattern are predicted and plotted in Fig.4-6 for H-plane and E- to H-Plane cross junctions, as functions of spacing d, measured between two middle points of the innermost slots. The lowest sidelobe of -13 db is obtained for d=1.0 λ in E- to H-plane cross - 59 -
junction. On the other hand, the sidelobes are not lower than -10 db obtained for d=1.6-2 λ in H-plane cross-junction. It is observed that EM coupling between slot and feed waveguide is relatively small for E- to H-plane cross junctions and about 2mm separation between the slot and the feed waveguide is enough; the reduction of blocking directly contributes the sidelobe reduction. The reflection characteristics of one cross-junction and two radiating waveguides defined at the input port are simulated and included in Fig.4-5. The results for the cross junction without radiating slots are slightly perturbed by the reflection and/or coupling of slots, but almost similar reflection is observed. 4.4 An Alternating-phase fed Single-Layer Slotted Waveguide Array Fed By the Center Feed With the E- to H-Plane Cross-junctions In [4-21] [4-23] An alternating phase fed array is fabricated using the center feed with E- to H-plane cross junctions as is shown in Fig. 4-10. Fig.11 shows the front view of the center feed single-layer waveguide array consisting of the base plate and a slot plate. It has 32(=16x2) radiating waveguides. Each radiating waveguide has 10 reflection-canceling units, which consists of a slot and the inductive side wall. The total number of the slots in the array is 320 (10x32). The slots are designed for uniform aperture illumination. The adjacent radiating waveguide is alternating-phase fed and the slot plate and base plate does not need electrical contact in principle. The slot plate and the grooved waveguide plate are fixed by only screws at the aperture periphery. The design frequency is 25.3GHz. Fig. 12 shows the measured and calculated results of the reflection characteristics of the whole array. This measurement data is obtained using a network analyzer in an anechoic chamber. The FEM analysis model for such an large array is challenging; the symmetry for H-plane and E- to H-plane cross junction arrays are utilized and only quarter and the half of the full structure are analyzed for H-plane and E- to H-plane cross junction arrays, respectively. HFSS s adaptive mesh generation - 60 -
was adopted [4-24] and the mesh sizes for the simulation in Fig.4-10 are 201,030 to 427,645 tetrahedra. It took from 5.5 to 11.5 hours and 3.2 to 7.41 GB memory to simulate each model in discrete sweep. The difference of the magnitude of the S-parameters is among two adaptive passes (Delta S in HFSS) smaller than 0.01. Pass number is set to 12 for calculation to the end. For E- to H-plane cross junction array, the measurements and the simulation shows the reasonable agreement. The reflection at 25.3GHz is less than -20dB. The results for the conventional H-plane cross junction array is also showing the reasonable agreement. The advantage or the lower reflection of E- to H-plane cross junctions is not clear in the alternating phase fed 2-D array. The external coupling between slots on the adjacent waveguides may be the factor of the enhanced reflection in comparison with the results for one radiating waveguide presented in Fig.4-5. As shown in Fig. 4-14(a), the grating lobes due to blockage are remedied by adopting a tapered aperture illumination in the case of H-plane cross-junction model [4-25]. The GA taper is similar to the Taylor taper. Uniform aperture illumination is shown in Fig 4-14(b). Fig. 4-11 shows the radiation pattern of E- to H-plane cross-junction model antenna. The measured and the calculated results are good agreement. In the radiation pattern of H-plane first sidelobe level is less than -13.7dB. Fig. 12 shows the radiation pattern of the H-plane cross-junction array. The measured and the calculated results are in good agreement. The sidelobe is as high as -11 db due to the blockage. Authors already introduced tapered amplitude illumination for reducing the sidelobe levels, at the cost of aperture efficiency reduction. Fig. 4-13 presents the H-plane radiation patterns and the sidelobe lower than -15dB was realized. Fig. 4-14 shows the gain characteristics of the calculated and measured gain of the H-plane cross junction array and the E-to H-plane cross-junction arrays both with uniform illumination. Antenna gain of 31.5 dbi is obtained at 25.3 GHz. The E- to H-plane model antenna size is 206mm x 168mm and the aperture efficiency is almost 50 %. Almost the same gain was observed for H-plane and the E- to H-plane cross junction arrays, though the sidelobe levels are lower for the latter. The gain of the array for taper aperture illumination is also compared in Fig. 4-15. The low side level was achieved by the taper illumination but the gain is reduced by about 1dB at the same time. - 61 -
4.5 Conclusion remarks We discuss the new cross-junction power divider for center feed with small aperture blocking. Additional manufacturing process to install the posts is not required after fabrication of the grooved feed waveguide. So this antenna is good for mass production. The alternating phase fed array with the E- to H-plane cross junctions has the reduced sidelobes of -13 db comparable to uniform illumination without blockage. Our future study includes to improve bandwidth of the array to the level of the unit E- to H-plane cross junction. References [4-1]N. Goto, A waveguide-fed printed antenna, IEICE Technical Report, AP89-3, Apr. 1989. [4-2]26GHz FWA http://www.jrc.co.jp/eng/product/26g_fwa/index.html [4-3]S. H. Park, Y. Tsunemitsu, J. Hirokawa, M. Ando, Center Feed Single Layer Slotted Waveguide Array, IEEE Trans. Antennas Propag., vol. 54, no5, pp. 1474-1480, May 2006. [4-4]S. H. 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. [4-5]S. H. Park, J. Hirokawa and M. Ando, Design of a Multiple-Way Power Divider for Center feed Single Layer Waveguide Arrays, 2003 IEEE AP-S Int. Symp. Dig., vol.2, pp. 1165-1168, June. 2003. [4-6]S. H. Park, J. Hirokawa and M. Ando, Planar Cross-Junction for the Center Feed in Single-Layer Slotted Waveguide Arrays, 2002 IEEE AP-S Int. Symp. San Antonio, Texas, Dig., vol.3, pp. 416-419, June. 2002. [4-7]S. H. Park, J. Hirokawa and M. Ando, Single-layer cross-junction power divider for the center feed in slotted waveguide arrays, IEICE Tech. Rep., EMCJ2001, vol.101, No.392, 143-147. [4-8]S. H. Park, J. Hirokawa and M. Ando, Design of a Center-Feed Multiple-Way Circuit for a Single-Layer Waveguide Array, IEICE General Conv., B-1-87, March. 2003. - 62 -
[4-9]S. H. Park, A study of center feed for single layer waveguide arrays, Doctoral Dissertation, Dept. of Electrical and Electronic Eng., Tokyo Institute of Technology, 2003. [4-10]Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa and M. Ando, Polarization Isolation between Two High-gain Slotted Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-210, Sept. 2003. [4-11]Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Center Feed Waveguide Arrays Arranged Side-by-side, IEICE General Conv., B-1-172, Mar. 2004. [4-12]Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Two Center-Feed Single-Layer Waveguide Arrays Arranged Side-by-side, IEEE AP-S Int. Symp. Dig., vol.3, pp. 2380-2383, June. 2004. [4-13]Y. Tsunemitsu, J. Hirokawa and M. Ando, Polarization and Isolation between Two Slotted Waveguide Arrays Arranged Side-by-side and Fed by Two Center Feeds comprised of E-plane to H-plane Cross-Junction Power Dividers, IEICE General Conv., B-1-102, Sept. 2005. [4-14]S. H. Park, J. Hirokawa and M. Ando, Single-Layer Cross-Junction Power Divider for the Center Feed in Slotted Waveguide Arrays, IEICE General Conv., B-1-167, Sept. 2001. [4-15]Y. Tsunemitsu, J. Hirokawa and M. Ando, Cross Junction Power Divider for the Center Feed in Slotted Waveguide Arrays, IEICE General Conv., B-1-103, Sept. 2004. [4-16]R. S. Elliott and L.A. Kurtz, The Design of Small Slot Arrays, IEEE Trans. Antennas Propagat., vol. AP-26, pp.214-219, 1978. [4-17]S. H. Park, J. Hirokawa and M. Ando, Analysis of a waveguide slot and a reflection-canceling inductive wall, 2003 IEEE Topical Conference on Wireless Communication Technology, Hawaii, s23p08, Oct. 2003. [4-18]S. H. Park, J. Hirokawa and M. Ando, Analysis of a Waveguide Slot with a Reflection-Canceling Post, IEICE Tech. Rep., AP2002, vol. 102, No.232, 31-36. [4-19]S. H. Park, J. Hirokawa and M. Ando, Simple Analysis of a Slot and 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. [4-20]Y. Tsunemitsu, J. Hirokawa and M. Ando, An E-plane to H-plane Cross-Junction and the Multiple-way Power Divider for the Center Feed in an Alternating-Phase Fed Single-Layer Slotted Waveguide Array, IEICE Tech. Rep., AP2005-6, pp29-34, April 2005. - 63 -
[4-21]Y. Tsunemitsu, J. Hirokawa and M. Ando, "Characteristics of E- to H-Plane Multiple Way Power Divider for the center feed in alternating phase fed Single-Layer Slotted Waveguide Array," 2005 INTERNATIONAL SYMPOSIUM ON ANTENNAS AND PROPAGATION (ISAP2005), FD3-3, Vol.3, pp.1185-1188, Seoul, KOREA, August 3-5, 2005. [4-22]Y. Tsunemitsu, J. Hirokawa and M. Ando, A Center Feed comprised of E-plane to H-plane Cross-Junction Power Dividers in a Slotted Waveguide Array, IEICE General Conv., B-1-214, Mar. 2005. [4-23]Y. Tsunemitsu, J. Hirokawa and M. Ando, "Center-Feed comprised of E to H-plane Cross-Junctions in an Alternating-Phase Fed Single-Layer Slotted Waveguide Array," IEEE AP-S International Symposium and USNC/URSI National Radio Science Meeting, Session:P65.6, Vol.3A, pp.716-719, Washington DC, USA, July 3-8, 2005. [4-24]Ansoft Corporation, Ansoft HFSS manual, 2004. [4-25]Y. Tsunemitsu, S. H. Park, J. Hirokawa, M. Ando, Y. Miura, Y. Kazama and N. Goto, Reflection Characteristics of Center-Feed Single-Layer Waveguide Arrays, IEICE Trans. Commun., vol. E88-B, no.6, pp.2313-2319, June.2005. - 64 -
z large small x y Feed waveguide (a) T-junction (b) H-plane cross-junction (c) E-to H-plane cross-junction Fig. 4-1. Three different types of feed waveguides for single-layer slotted waveguide arrays - 65 -
9mm Feed waveguide 3.6mm Inductive post (r=0.5mm) Feed waveguide 3mm 3mm Radiating waveguide (a) H-plane model Fig. 4-2. 3mm Reflection canceling wall Radiating waveguide 7.2mm (b) E-to H-plane model Structure of multi-way power divider - 66 -
Feed waveguide Radiating waveguide Coupling window Port 1 Port 3 Feed waveguide Post Port 1 3mm 3mm Port 4 Port 4 z 8mm x y 9mm Port 2 8mm Radiating waveguide Lw Wp Wh Wall Port 3 7.2mm Port 2 3.6mm (a) H-plane cross-junction (b) E- to H-plane cross-junction Fig. 4-3. Structure of the unit of cross-junction - 67 -
Wh and Wp [mm] 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Wh Wp Divided power 4.5 5 5.5 6 6.5 7 Lw [mm] 50 45 40 35 30 25 20 15 10 5 0 Divided power [%] Fig. 4-4. Design parameters of wall position (Wp) and wall height (Wh) versus length of coupling window (Lw). S11 is less than -30dB at 25.3GHz. - 68 -
Reflection (db) 0-10 -20-30 -40 (A H-plane cross junction) (An E-to H-plane cross junction) d=1.6 (H-plane cross junction with slot) d=1.0 (E-to H-plane cross junction with slot) -50 24.5 25 25.5 26 26.5 Frequency (GHz) Fig. 4-5 Those are the reflection characteristics of each cross-junction power divider (Cal.). These are one of examples to design a -6dB distribution to each radiating waveguide. In the case of the slot distance d is changed, almost reflection characteristics does not change. - 69 -
Radiating waveguide Feed waveguide Input d Output d Input Output Distance of two first slots Radiating waveguide (a) H-plane cross-junction (b) E- to H- plane cross-junction Fig. 4-6 Linear slot array models for evaluate first side-love level in H-plane Radiation pattern. - 70 -
Relative Amplitude (db) 0-5 H-plane -10-15 d=2.1λ d=1.6λ d=1.3λ d=1.1λ d=1.0λ E-to H-plane -20-10 -8-6 -4-2 0 2 4 6 8 10 Angle (deg) Fig. 4-7 The first side-lobe level in H-plane radiation pattern (Cal.) changing distance d. This model is adopted the H-plane and the E-to H-plane cross-junction power divider. - 71 -
Slotted plate Radiating waveguide Feed waveguide Fig. 4-8 Photo of the single-layer waveguide slot array which is adopted E-to H-plane cross-junction multiple way power divider. - 72 -
203mm 165.5mm z x y (a) Base plate (b) Slotted plate Fig. 4-9 Photo of the E-to H-plane center feed waveguide slot array antenna - 73 -
Reflection (db) 0-10 -20 H-plane -30 (Cal.) (Cal.) -40 (Mea.) E-to H-plane (Mea.) -50 24.5 25 25.5 26 26.5 Frequency (GHz) Fig. 4-10 Reflection characteristics of whole antenna. - 74 -
Relative Amplitude (db) 0-10 -20-30 -40 Cal. Mea. -50-90 -60-30 0 30 60 90 Angle (deg) Fig. 4-11 H-plane Radiation pattern in uniform aperture illumination using E-to H-plane cross-junction. (f=25.3ghz) - 75 -
Relative Amplitude (db) 0-10 -20-30 -40 Cal. Mea. -50-90 -60-30 0 30 60 90 Angle (deg) (a) Uniform illumination Relative Amplitude (db) 0-10 -20-30 -40 Cal. Mea. -50-90 -60-30 0 30 60 90 Angle (deg) (b) GA Tapered illumination Fig. 4-12 Radiation pattern using H-plane cross-junction power divider for antenna. (f=25.3ghz) - 76 -
Gain (dbi) 33.0 32.5 32.0 31.5 31.0 30.5 30.0 29.5 29.0 28.5 28.0 27.5 27.0 26.5 26.0 H-plane (Cal.) (Cal.) (Mea.) (Mea.) E-to H-plane 24.5 25 25.5 26 26.5 Frequency (GHz) (a) Comparison with the H-plane cross coupler alternating-phase fed single-layer waveguide arrays. The characteristics are confirmed by calculation and measurement. Gain (dbi) 33.0 32.5 32.0 31.5 31.0 30.5 30.0 29.5 29.0 28.5 28.0 27.5 27.0 26.5 26.0 E-to H-plane (Uniform) H-plane GA (Cal.) (Cal.) (Mea.) (Mea.) 24.5 25 25.5 26 26.5 Frequency (GHz) (b) Comparison with the H-plane cross coupler with GA tapered illumination. Fig. 4-13 Gain characteristics of each antennas - 77 -
Chapter 5 Aperture efficiency enhancement Reflection canceling stair 5.1 Introductory remarks Single-layer slotted waveguide arrays [5-1] are developed for millimeter wave application. An alternating phase fed array, which is one type, consists of a slotted plate and a waveguide base joined by a screw and is easy to fabricate. A new feeding structure for single-layer arrays which is called a center-feed [5-2] [5-4] has been developed. This type of antenna has two important design advantages: the array bandwidth is doubled and the direction of the boresight beam is frequency-independent. On the other hand, the aperture efficiency is relatively low, about 57% in the calculation. The first reason for low efficiency is that a unit element in the array, which consists of a slot with reflection canceling inductive wall [5-5], produces a transmission phase advance; so the slot spacing is large and the number of slots is relatively small. The second reason is that the slots are staggered about the center of the radiation waveguide broad wall with increasing offsets towards the waveguide end for compensating the decreasing energy. In the alternating phase-fed planar array, the slot offset create high side-lobes in phi=45 degree plane [5-6]. To solve the latter problem, radiating waveguides were staggered so that the slot offset may be decreased in [5-7]. In this chapter, we propose a new slotted waveguide array for solving the above two problems. A novel reflection canceling unit consisting of a slot and a stair beneath on the broad wall of the waveguide is proposed. It brings about the transmission phase delay resulting in dense slot arrangement while the increase of slot coupling toward the waveguide end dispenses with the increasing slot offset. The improvement of the array characteristics are predicted. - 78 -
5.2 Structure Figure 5-1 shows the structure of a slot array with reflection canceling elements. In the conventional model shown in Figure 5-1(a), the reflection from a slot is canceled out by an inductive wall. Figure 5-1(b) show the proposed stair model. The reflected wave from a slot is canceled by the reflected wave from the step which has the same amplitude and opposite phase as that from the slot. Figure 2 shows the reflection characteristics of the stair model. The design frequency is 25.3GHz. We design one unit (slot+step) with 10% radiation to be used in ten-slot linear array with uniform amplitude. Reflection ( S11 ) and transmission ( S21 ) characteristics are evaluated by the commercial FEM software HFSSTM (High-Frequency Structure Simulator). First, the resonant slot is designed by adjusting the slot length and the slot offset from the center of broad wall in the radiating waveguide. In this case, the slot length is 5.8mm and the slot offset is 0.41mm. The broad wall width is 8mm and the narrow wall height is 3mm in the radiating waveguide. The first slot (10% radiation) reflection is about -25.9dB. Second, we design a step in the floor of the radiating waveguide that has same amplitude of reflection as the slot as shown in Figure 5-2. This step does not affect waveguide length. On the other hand, the conventional model with an inductive wall has a slot interval that is wider than the proposed model. Finally, the step position is adjusted to produce a reflection 180 degrees out of phase from that of the slot as shown in Figure 3. The step position from the center of slot is 4.8mm and the step height is 0.3mm. Next, the second slot is designed by the same procedure. It is important that the narrow wall height changes from 3.0mm to 2.7mm. Because the radiating waveguide height is reduced but the power input is the same, radiation power from the slot is increased. Therefore, the slot offset is reduced over that of the conventional model. This stair model structure can be fabricated with the grooved radiating waveguide. This structure still has the manufacturing advantage that it can be dug only from the side of the slotted aperture. 5.2 Analysis of a linear array and full model We design a uniform amplitude ten-slot linear array for a planer antenna. Each unit of a slot and a step has a reflection less than -20dB at the design frequency - 79 -
25.3GHz. Figure 5-4 shows the cross-section of the radiating waveguide. The reflection canceling steps are similar to stairs. This array works in the traveling-wave operation for widening the bandwidth of the array, even for large size and high gain applications. The reflection characteristics of the proposed model (stair model) and the conventional model (inductive wall model) are approximately -14.6 db and -24.9 db, respectively, at the design frequency as shown in Figure 5-5. On the other hand, reflection characteristics of a slot only model is -4.8 db. The half waveguide length interval between slots in the three models (only slots model, stair model and inductive wall model) produces a boresight main beam. Figure 6 shows the H-plane radiation pattern. We compare the conventional model array and the proposed model array to confirm both the suppression of the sidelobes in phi=45 degree radiation pattern and the improvement in aperture efficiency. This structure has a cross-junction power divider [5-8] at the center of the array and has a stable boresight main beam. Each radiating waveguide is connected to the feed waveguide. One radiating waveguide has a ten-slot slot array. The full model has 320 slots. Figure 5-7 shows the phi=45 degree radiation pattern. The sidelobes at theta plus-minus 60 degree are less than -20dB in the proposed model because the slot offset is reduced over that of the conventional model. Also, because the slot interval is reduced compared with the conventional model, the antenna size is reduced. Hence, the aperture efficiency improves from 57% (inductive wall model) to 75% (stair model) as shown in Figure 5-8. 5.3 Conclusion remarks We have proposed a single-layer slotted waveguide array with reflection canceling stairs. Each unit which consists of a slot and a step has a total reflection less than -20dB. In addition, this structure decreases the slot offset from the center of broad wall in the radiating waveguide. Reduction of sidelobes as well as enhanced aperture efficiency are confirmed by simulation. - 80 -
References [1] N. Goto, A waveguide-fed printed antenna, IEICE Technical Report, AP89-3, Apr. 1989. [2] S. H. Park, A study of center feed for single layer waveguide arrays, Doctoral Dissertation, Dept. of Electrical and Electronic Eng., Tokyo Institute of Technology, 2003. [3] Y. Tsunemitsu, Y. Miura, Y. Kazama, S. H. Park, J. Hirokawa, M. Ando and N. Goto, Polarization Isolation between Two Center-Feed Single-Layer Waveguide Arrays Arranged Side-by-side, IEEE AP-S Int. Symp. Dig., Vol.3, pp.2380-2383, June. 2004. [4] Y. Tsunemitsu, J. Hirokawa and M. Ando, "Center-Feed comprised of E to H-plane Cross-Junctions in an Alternating-Phase Fed Single-Layer Slotted Waveguide Array," IEEE AP-S Int. Symp. Dig., Vol.3A, pp.716-719, July 2005. [5] S. H. Park, J. Hirokawa, M. Ando, Simple Analysis of a Slot and 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. [6] N. Goto, Radiation Patterns of A Waveguide-Fed Printed Antenna, IEICE General Conv., B-1-82, Sept. 2002 [7] H. Tanaka, Y. Kimura, M. Haneishi, Alternating-phase fed single-layer slotted waveguide arrays with linearly arrayed slots, IEICE General Conv., B-1-88, Mar. 2003 [8] S. H. Park, J. Hirokawa and M. Ando, Design of a Multiple-Way Power Divider for Center-feed Single-Layer Waveguide Arrays, IEEE AP-S Int. Symp. Dig., Vol.2, pp.1165-1168, June 2003. - 81 -
z b (a)unit with Inductive wall (b) Unit with a Stair Figure 5-1: Structure of units in the slot array. x y - 82 -
Reflection [db] 0-10 -20-30 -40-50 Only slot Only single step Slot with single step -60 24.5 25 25.5 26 26.5 Frequency [GHz] Figure 5-2: Reflection from one unit for 10% radiation. (Cal.) - 83 -
0-10 Reflection [db] -20-30 -40-50 -60 0 1 2 3 4 5 6 Step position from slot [mm] Figure 5-3: Reflection for various distance of step from slot. (Cal.) - 84 -
Slot b = height of narrow wall Step Figure 5-4: Cross-section of a linear array. - 85 -
Reflection [db] 0-5 -10-15 -20-25 Only slot model Stair model Inductive wall model -30 24.5 25 25.5 26 26.5 Frequency [GHz] Figure 5-5: Reflection characteristics of a ten-slot linear array (Cal.) - 86 -
Relative Amplitude [db] 0-10 -20-30 -40 Stair model Inductive wall model -50-90 -60-30 0 30 60 90 Angle theta [deg] Figure 5-6: H-plane radiation (Cal.) - 87 -
Relative Amplitude [db] 0-10 -20-30 -40 Stair model Inductive wall model -50-90 -60-30 0 30 60 90 Angle theta [deg] Figure 5-7: Radiation pattern in phi=45degree (Cal.) - 88 -
Directive Gain [db] 34.0 33.5 33.0 32.5 32.0 31.5 31.0 Stair model Inductive wall model 80% 60% 70% 50% 24.5 25 25.5 26 Frequency [GHz] Figure 5-8: Directive gain and aperture efficiency (Cal.) - 89 -
Chapter 6 Slot arrays with Interchangeable slot plates 6.1 Introductory remarks An alternating phase fed single-layer slotted waveguide array [6-1] has been developed for millimeter wave and microwave applications [6-2]. This antenna consists of a slotted plate and a waveguide base plate joined by a screw which dispenses with electrical contact in the strict sense. The waveguide base and the slot plate are mass produced by die casting and punching, respectively. Unfortunately this array has a relatively narrow bandwidth. Fixed Wireless Access systems in the 26 GHz band [6-3] uses a hundred or so MHz of bandwidth for each carrier. Until now, different arrays were designed for each carrier with different channels, though manufacturing base plates by die casting needs quite a large volume of orders due to cost requirements. On the other hand, the slot plate can be produced cost effectively even in small numbers. If the waveguide base can be commonly used and only slot plates interchanged for individual frequencies, every channel can be covered with reasonable cost. This chapter discusses the possibility of the diversion of the waveguide base plate for frequencies different from the designed ones; multi-frequency operation is computationally evaluated by the Finite Element Method (FEM) with Ansoft HFSS TM full structure simulation of the array. We compare the results of calculation and measurements. 6.2 Structure Fig.6-1 shows the alternating-phase fed single layer slotted waveguide array. This antenna is made from two components: the slotted plate and the waveguide base. The radiating waveguides and feed waveguide are in the same layer. The radiating waveguides are fed in alternation, so electrical contact is not needed between the slotted plate and the waveguide base. This feature makes mass production easy for commercial - 90 -
use. Unfortunately, this antenna has a relatively narrow bandwidth due to line effects. We tried to improve its bandwidth to reduce the series slots and the feed waveguide power dividers [6-4] [6--6] as shown in Fig.2. We propose interchangeable slot plates for multiple frequency operation. Fig. 6-3 shows the structure of 3 different slot plates for 3 design frequencies (slot plate #1 designed at 24.85GHz, slot plate #2 designed at 25.3GHz, and slot plate #3 designed at 25.75GHz) and a base plate designed at 25.3GHz. The usual slot array start position line is shown in Fig. 3(b). On the other hand, slot plate #1 and slot plate #3 have different start position lines for each slot array adjusted by the waveguide length. Slots are designed, and each slot plate and waveguide base is joined by a screw. 6.3 Analysis of Full Models [7] First, we simulate model with no slots that consists of a input aperture, a feed waveguide, and radiating waveguides. To obtain the estimation in the case of the frequency change of the feed waveguide phase, we set ports #2 to #9 at the ends of the radiating waveguides. The input aperture is port #1. This antenna is symmetric with respect to the center, so we can use electric walls to reduce the computation requirements. The amplitude characteristics of the reflection and the divided power are shown in Fig.6-4. The design frequency is 25.3GHz. This frequency band is used for Fixed Wireless Access in Japan. The reflection is less than -20dB. Almost uniform divided amplitude is achieved. The phase characteristics of the divided power for each radiating waveguide are shown in Fig.6-5. Adjacent radiating waveguides are almost 180 degrees out of phase at design frequency. We design the slot plate with uniform illumination to obtain high gain. Fig. 6 shows the reflection characteristics of each frequency designed slotted plate. The calculated and measured results are in good agreement. The radiation pattern in H-plane is shown in Fig.7. The main beam is tilted toward feed waveguide 4.6 to 4.9degree each model to cancel the reflection from each slots [6-8]. The -3dB beam width is 4 to 4.2 degree each model. Fig. 8 shows the calculated and measured results of gain characteristics using HFSS TM (FEM analysis). The antenna gain using the slot plate #2 decreases 0.83dB (at 24.85GHz) and 2.62dB (at 25.75GHz) respectively. When interchanging slot plates #1-91 -
or #3, the antenna gain decreases only 0.12dB (at 24.85GHz or at 25.75GHz) in calculation. Measurements results indicate almost the same tendency. 6.4 Conclusion remarks We have proposed an alternating phase fed single-layer slot array with interchangeable slot plates for multi-frequency operation. The characteristics are confirmed by simulations and measurements. References [6-1] N. Goto, A waveguide-fed printed antenna, IEICE Technical Report, AP89-3, Apr. 1989. [6-2] Y. Kimura, Y. Miura, J. Hirokawa and M. Ando, "A LOW-COST AND COMPACT WIRELESS TERMINAL WITH AN ALTERNATING PHASE FED SINGLE-LAYER WAVEGUIDE ARRAY FOR 26GHz FIXED WIRELESS ACCESS SYSTEMS," JINA, Nov. 2002. [6-3] 26GHz FWA http://www.jrc.co.jp/eng/product/26g_fwa/index.html [6-4] S. H. Park, Y. Tsunemitsu, J. Hirokawa, M. Ando, Center Feed Single Layer Slotted Waveguide Array, IEEE Trans. Antennas Propag., vol. 54, no5, pp. 1474-1480, May 2006. [6-5] Y. Tsunemitsu, J. Hirokawa, and M. Ando, "Characteristics of E- to H-Plane Multiple Way Power Divider for the Center Feed in Alternating Phase Fed Single-Layer Slotted Waveguide Array," 2005 INTERNATIONAL SYMPOSIUM ON ANTENNAS AND PROPAGATION (ISAP2005), FD3-3, Vol.3, pp.1185-1188, Seoul, KOREA, August 3-5, 2005. [6-6] M. Zhang, J. Hirokawa, and M. Ando, "A Three-Way Divider for Partially-Corporate Feed in an Alternating Phase-Fed Single-Layer Slotted Waveguide Array," IEICE Trans. Commun., Vol.E88-B, No.11, pp.4339-4345, Nov. 2005. [6-7] Y. Tsunemitsu, S. H. Park, J. Hirokawa, M. Ando, Y. Miura, Y. Kazama, and N. Goto, Reflection Characteristics of Center-Feed Single-Layer Waveguide Arrays, IEICE Trans. Commun., vol. E88-B, no.6, pp.2313-2319, June.2005. [6-8] R.E.Collin and F.J.Zucker, Antenna Theory, part 1, Sec.14.8, McGraw-Hill, 1969. - 92 -
Figure 6-1. Structure of an alternating-phase fed single-layer slotted waveguide array - 93 -