Proceedings of Asia-Pacific Microwave Conference 2006 Wideband Slotline-to-Rectangular Waveguide Transition Using Truncated Bow-Tie Antenna Ruei -Ying Fang and Chun-Long Wang Department of Electronics Engineering National Taiwan University of Science and Technology, Taipei, 10617, Taiwan Tel: 886-2-2730-1237, Fax: 886-2-2737-6424, E-mail: clw@mail.ntust.edu.tw Abstract In this paper, a slotline-to- rectangular waveguide transition using a truncated bow-tie antenna is analyzed and designed to operate in the X-band. Simulation and analysis are performed using the commercially available computer software package, High Frequency Structure Simulation (HFSS), which is based on the finite element method (FEM) technique. Our study yields a design with a return loss and insertion loss levels that can achieve better than 15dB and 0.1dB all over the X-band (8.2 12.4 GHz), respectively. Index Terms Bow-tie antenna, coplanar strip, rectangular waveguide, slotline, transition.. I. INTRODUCTION Some well-performed broadband transitions are achieved by adopting a proper antenna structure to transfer the energy between the planar circuit and the rectangular waveguide [1]-[4]. The broadband transition could be much easily achieved if a broadband antenna is adopted. Thus, we extend this brilliant idea by using a broadband truncated bow-tie antenna to attain a broadband transition. Based on the concept of broadband antenna, a new slotline-to-rectangular waveguide transition designed in the X-band (8.2-12.4 GHz) using a bow-tie antenna is proposed and introduced in this paper. To see how this idea is developed, we first consider the practical biconical antenna [5] which is made by truncating the infinite bicone at both ends. Since the three-dimensional truncated biconical antenna is not feasible in the planar circuit, a planar version of this should be adopted. The truncated bow-tie structure, which is a planar version of the truncated biconical antenna [5], is shown in Fig. 1(a), and wide bandwidth is its main advantage. Hence, this new transition can be easily realized by integrating the rectangular waveguide and the planar circuit with this truncated bow-tie antenna. With proper design, the novel slotline-to-rectangular waveguide transition can achieve an excellent performance all over the X-band. Parameters related to bandwidth, insertion loss and return loss levels of this transition are investigated and analyzed by using the commercially available computer software package, High Frequency Structure Simulation (HFSS), which is based on the finite element method (FEM) technique. Also a verification experiment is under process, which will be used to verify these simulation results and be presented at the conference. II. TRANSITION ANALYSIS AND DESIGN IN X-BAND The configuration of the transition is shown in Fig.1. A standard WR-90 waveguide with an inner dimension of 900x400mil is used as the transition waveguide having a single-mode operation in the X-band. The housing of the slotline is 400x400mil. A Rogers RT/Duroid 5880 laminate with a dielectric constant of 2.2 and a thickness of 31mil is used as the substrate of the planar circuit. The pink area in Fig. 1(a) and the area with slant lines in Fig. 1(b) represent the metal part of the planar circuit. This planar circuit consists of three parts, one is a slotline feeder, another is a quarter wavelength coplanar strip line, and the other is a truncated bow-tie antenna placed at center of the transition waveguide. The truncated bow-tie antenna is placed in such a position in order to match the electric fields of the dominant TEB10B mode of the rectangular waveguide. A. Determination of the Dimensions of Truncated Bow-Tie Antennas The design begins with the determination of dimensions of the truncated bow-tie antenna. Since the performance of the truncated bow-tie antenna has been investigated in the literature [6], we can easily choose the adequate dimension of the truncated bow-tie antenna. By observing the curves of Fig. 14(a) in this literature, we see that the flare angle of the truncated bow-tie antenna indeed affects its input impedance. Different flare angle makes different input impedance variation versus frequency. From the viewpoint of This work was supported in part under the Grand NSC 95-2221-E-011-038by National Science Council, Taiwan. Copyright 2006 IEICE
WBrB between should are and of (a) (b) Fig.1. Slotline-to-waveguide transition with truncated bow-tie antenna. (a) Threedimensional view. (b) Configuration of the planar circuit. wideband impedance matching, we have to pick up the curve resembling a constant value of input impedance with respect to the change of frequency. In other words, we need an input impedance whose behavior is frequencyindependent in order to get a better performance while integrating with the rectangular waveguide. For this reason, we choose the flare angel =70as the first design parameter. Once the flare angle of the truncated bow-tie antenna is determined, the remaining dimension for the truncated bow-tie antenna is the distance the bow-tie edge and the metal wall of the waveguide. After choosing BWBgB WBp = LBcs B= 230mil, which will be explained in the following sections, simulation results of the scattering parameters for different widths WBrB shown in Fig.2. As shown in Fig. 2, as the bow-tie edge approaches too close to the waveguide wall, the capacitance between the edge of the truncated bow-tie antenna and the waveguide wall will be enhanced significantly. Thus, the truncated bowtie antenna will suffer from the enhanced capacitance that leads to impedance mismatch and therefore inefficient radiation. Fig. 2 agrees with our thought in that it shows that narrower width between border of the truncated bow-tie antenna and the waveguide wall provokes performance worse. Thus, the adequate value for WBr Bshould be chosen as WBr B= 52mil. B. Impedance Matching Up to now, the dimensions of the truncated bow-tie antenna are determined. Next, a quarter wavelength coplanar strip line is designed to match the impedances of the slot line and the truncated bow-tie antenna. Two dimensions including the length LBCSB and the width WBpB the coplanar strip are investigated to be determined. First, simulation results of the scattering parameters by varying the lengths of the coplanar strip (CPS), LBcs,B are shown in Fig. 3. From this figure, we see that a longer CPS length makes a lower resonant frequency and the return loss level is insensitive to the variation of this parameter. Thus, in order to achieve a broadband transition, the CPS length LBcsB should be chosen as LBcs B= 230mil corresponding to the quarter wavelength BgB/4 of the CPS at 10GHz. Secondly, simulation results of the scattering parameters for different widths of the CPS line WBpB are shown in Fig. 4. Since the length LBcsB is designed to be a quarter wavelength at f=10ghz, there exists an characteristic impedance of the CPS that will make the impedances of the slot line and truncated bow-tie antenna to be matched at this frequency. As can be see from the figure, the width of CPS WBpB be chosen as WBpB=10mil to realize this characteristic impedance. Also, we observe that the return loss level is insensitive to the widths of the CPS line. In other words, this means the return loss level is insensitive to the characteristic impedance of the CPS line.
as as Fig. 2 The return and insertion losses versus frequency with WBrB a parameter. =70, L=330mil, WBgB=WBpB=10mil, and LBcsB=230mil. Fig. 4 The return and insertion losses versus frequency with CPS width WBPB a parameter. =70, L=330mil, WBgB=10mil, and WBrB=52mil, and LBcsB=230mil. Fig. 3 The return and insertion losses versus frequency with CPS length LBcsB as a parameter. =70, L=330mil, WBg B= WBp B= 10mil, and WBr B= 52mil. Fig. 5 The return and insertion losses versus frequency for the optimal transition parameters with =70, L=330mil, WBgB=WBpB=10mil, D=31mil, WBrB=52mil, LBcsB=230milBB According to the above design procedures, the optimal geometric dimensions of the transition are determined as WBrB=52mil, LBcsB=230mil, and WBpB=10mil. The return loss and insertion loss of this optimal geometry is shown to achieve better than 15dB and 0.1dB all over the X-band as shown infig. 5. IV. CONCLUSION A novel slotline-to-rectangular waveguide transition based on the idea of bow-tie antenna is proposed and simulated by the commercially available software, Ansoft HFSS. After properly investigating the transition parameters, an optimal wideband performance is obtained. Simulation results demonstrate that an excellent performance is achieved with return loss and insertion loss better than 15dB and 0.1dB all over the X-band (8.2 12.4 GHz), respectively. ACKNOWLEDGEMENT We sincerely thank Mr. Chou-Wei Wang and Mr. Wei-Shan Wang for helpful discussions and suggestions on the simulation and the measurement. We also thank Wireless Communications & Applied Electromagnetic LAB, National Taiwan University of Science and Technology for providing the simulation environment of Ansoft HFSS and the measurement instruments. Also, we thank Prof. Ruey-Beei Wu and Prof. Powen Hsu, National Taiwan University, for providing us the waveguide components and HP X11644A calibration kit.
P ed., REFERENCES [1] N. Kaneda, Y. Qian, and T. Itoh, A broadband CPW-to-waveguide transition using quasi-yagi antenna, in Proc. IEEE MTT-S Int. Microwave Symp. Dig., 2000, pp. 617 620. [2] T.-H. Lin and R.-B. Wu, CPW to waveguide transition with tapered slotline probe, IEEE Microwave Wireless Comp. Lett., vol. 11, pp. 314 316, Jul. 2001. [3] T. H. Lin, Planar circuit to waveguide transition with tapered CPS probe, PhD. dissertation, Nat. Taiwan Univ., Taipei, Taiwan, R.O.C., Jun. 2001. [4] C.-F. Hung, A.-S. Liu, C.-L. Wang, and R.-B. Wu, A broadband conductor backed CPW to waveguide transition realized on high dielectric constant substrate, in Proc. Asia Pacific Microwave Conf., 2003, pp. 1038 1041. [5] Warren L. Stutzman and Gary A. Thiele, Antenna nd Theory and Design, 2P John Wiley & Sons, 1998. [6] Disala Uduwawala, Martin Norgren, Peter Fuks, and Aruna W. Gunawardena, A deep parametric study of resistor-loaded bow-tie antennas for groundpenetrating radar applications using FDTD, IEEE Transactions on Geoscience and Remote Sensing, vol. 42, no. 4, Apr. 2004.
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