AbstractâA compact ultra-wideband (UWB) quasi-Yagi antenna with microstrip-to-coplanar stripline transition balun is presented in this paper. A planar balun ...
4D4-08
Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012
Compact quasi-Yagi antenna with short length microstrip-to-coplanar stripline transition Hyungsoo Park#, Jaegeun Ha#, and Jaehoon Choi# (corresponding author) #
Department of Electronics and Computer Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul, 133-791, Korea, Republic of
Abstract—A compact ultra-wideband (UWB) quasi-Yagi antenna with microstrip-to-coplanar stripline transition balun is presented in this paper. A planar balun with very short length (5.1 mm) has a 3dB back-to-back insertion loss bandwidth from 4.3 GHz to 10.8 GH. The proposed antenna has end-fire pattern and a good front-to-back ratio of 11.7 dB at 8.5 GHz. The cross polarization levels are -22 dBi and -25 dBi in the H-plane and Eplane, respectively.
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Index Terms — Balun, broadband antenna, end-fire antenna, IR-UWB, microstrip-to-coplanar stripline, quasi-Yagi.
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I. INTRODUCTION An impulse-radio ultra-wideband (IR-UWB) technology is considered as one of viable candidates for security surveillance devices and positioning systems using throughwall imaging due to the accuracy and the ability of detection at low light circumstances. However, its wide bandwidth may cause a potential interference with other communication systems. Hence, Detect and Avoid (DAA) or a limitation of specific frequencies is required by a regulation of each country [1]-[2]. In South Korea, it is allowed to be used only in limited bands from 7.2 GHz to 10.1 GHz, not an entire range of UWB band defined by FCC (3.1 GHz 10.6 GHz) unless applying advanced algorithm such as DAA. A variety of UWB antennas utilizing printed circuits have been developed. A quasi-Yagi antenna on a thin substrate is categorized as a planar dipole antenna that has a broad bandwidth, moderate gain, and end-fire beam formation as well as easy fabrication to design array antenna. These characteristics of quasi-Yagi antenna have become attractive features in the field of radar application. Another critical point of view about quasi-Yagi antennas is how to design a transition balun well. The word balun is a compound word which is made up of balanced and unbalanced and is used as transmission line transformer to turn a single-ended signal into a pair of differential signal. These transition baluns are widely found at many applications such as antennas, filters, mixers, frequency multipliers and integrated circuits. Since transition balun is used between two geometries, the overall component performance is limited by the balun structure.
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(b) (c) Fig. 1. (a) Geometry of the proposed MS-to-CPS balun. (b) Top view and (c) bottom view of the fabricated back-to-back configuration.
In the past, many kinds of transitions have been introduced in literatures [3]-[5]. A transition using a radial stub was demonstrated by providing multi-section impedance transformers [3]. Also in [4], Chiu et al. proposed a microstrip-to-coplanar stripline (MS-to-CPS) transition balun with 1dB insertion loss and 20-dB return loss over the broad bandwidth of 6.9 GHz to 12.4 GHz. In addition, a UWB transition balun using via holes was reported to have an insertion loss less than 1 dB in the frequency range from 6 GHz to over 40 GHz [5]. All of these literatures presented good performances including wide bandwidth and excellent insertion loss characteristics. However, relatively long lengths of transition section (35.3mm, 45.5 mm, and 12.7 mm in [3], [4], and [5], respectively) were required to achieve impedance matching.
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Proceedings of APMC 2012, Kaohsiung, Taiwan, Dec. 4-7, 2012
Chacracteristic Impedance (ohm)
135 130 125 120 115 110 105 0
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Fig. 2. The characteristic impedance of the Klopensteintapered CPS line for various lengths of the tapered line.
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Fig. 4. A layout of the quasi-Yagi antenna with a MS-to-CPS transition balun.
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Fig. 3. The simulated and measured scattering parameters of the proposed MS-to-CPS transition in a back-to-back configuration.
In this paper, a UWB quasi-Yagi antenna using a novel MS-to-CPS transition balun with a short length transition section is presented. To transfer radio frequency (RF) signal from microstrip to CPS, Klopenstein taper is used. The 3dB insertion loss bandwidth of the proposed MS-to-CPS transition, obtained from back-to-back configuration, ranges from 4.3 GHz to 10.8 GHz. A prototype MS-to-CPS fed quasi-Yagi antenna is manufactured and its performance is analyzed. The antenna has broad 10dB return loss bandwidth of 54.7% covering entire IR-UWB band of South Korea. II. MICROSTRIP-TO-COPLANAR STRIPLINE TRANSITION In order to evaluate the performance of the proposed MSto-CPS balun, we employ the back-to-back transition configuration as shown in Fig. 1. The balun is realized on a Taconic CER10 substrate with a relative dielectric constant of 10 and a thickness of 0.635 mm. The dimensions of the MSto-CPS balun are optimized using a finite element method (FEM) simulation tool, Ansoft HFSS ver. 13.1: Lm1 = 5.12 mm, Lm2 = 3 mm, Wm1 = 0.59 mm, Gm = 1.07 mm, Gcps = 1
mm, Lb1 = 0.38 mm, Lb2 = 0.3 mm, Lcps = 2.21 mm, Wg = 12 mm ,Wg1 = 4 mm, Wcps = 0.66 mm , and Wg2 = 1.33 mm. Section A is a 50 ohm MS line with finite ground plane and section B is the transition region between MS and CPS. In section B, the electric field of the bended MS line is coupled to the ground plane and the CPS line. In order to minimize the reflection and achieve the impedance matching within a short length of transition, a Klopenstein taper is employed [6]. Fig. 2 shows the characteristic impedance of transition section for various transition lengths. The optimized length using a full wave electromagnetic simulation tool is 5.1 mm and the characteristic impedance of the input stage connected to the MS is 108 ohm. Section C is the CPS transmission line, and its characteristic impedance is designed to have about 133 ohm to reduce the impedance mismatching. The gap between the striplines is 1 mm and the strip width is 0.66 mm. The lengths of the sections A, B and C are 4 mm, 5.1 mm, and 2.21 mm, respectively. The overall dimension of the fabricated back-to-back balun is about 12 mm in the width and 22.7 mm in the length. Fig. 3 shows the return loss and insertion loss properties of the proposed MS-to-CPS balun. The transition exhibited insertion loss of less than 3 dB and return loss of larger than 10dB over 4.6 GHz to 10.8 GHz. Especially, the bandwidth of 1dB insertion loss for the back-to-back transition was from 4.7 GHz to 8.6 GHz. The simulation and measurement results were in good agreement as shown in Fig. 3. III. ANTENNA GEOMETRY AND CHARACTERISTICS Fig. 4 shows the schematic of the proposed UWB quasiYagi antenna. The antenna is fabricated on the same substrate used for the balun and is designed for UWB band application
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transition section. The transition length defined as the length between MS and CPS was considerably shorter than that in previous literatures and the transition balun had a wide 3-dB insertion loss bandwidth ranging from 4.3 GHz to 10.8 GHz. Additionally, the proposed quasi-Yagi antenna yields end-fire radiation pattern in the frequency range of 6.1 GHz to 10.7 GHz with a peak gain of 6.3–8.1 dBi in the frequency band of interest. Therefore, the proposed antenna is well-suited for IR-UWB and surveillance devices because of the easy fabrication and compactness as well as good radiation characteristics.
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ACKNOWLEDGEMENT
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This research was supported by Samsung Inc., Korea. Fig. 5. Measured and simulated return loss characteristics of the proposed quasi-Yagi antenna. 120
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REFERENCES
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Fig. 6. Radiation pattern of the proposed antenna at 8.5 GHz (a) H-plane (yz-plane) and (b) E-plane (xy-plane).
in the frequency range from 7.2 to 10.2 GHz. The proposed antenna is composed of three components: a MS-to-CPS balun, a driver dipole, and a director. The total volume of the substrate is 30 mm x 40 mm x 0.635 mm. Each line of CPS is open-ended to form dipole arms and a director separated from the dipole by 2.4 mm enhances the antenna gain and form an end-fire radiation pattern. Fig. 5 shows the return loss characteristic of the proposed antenna. The measurement shows that the return loss is larger than 10 dB over broad bandwidth (6.1 GHz to 10.7 GHz). Also, an end-fire radiation patterns can be observed in the simulation as shown in Fig. 6. The front-to-back ratio (FBR) was 11.7 dB and the maximum cross-polarization levels were -22 dBi and -25 dBi in the Eand H-plane, respectively, at 8.5 GHz. The peak gains were 6.4 dBi, 6.3 dBi, and 8.1 dBi at 7.0 GHz, 8.5 GHz, and 10.5 GHz, respectively.
[1] FCC Report and Order for Part 15 Acceptance of Ultra Wideband (UWB) Systems from 3.1 –10.6 GHz, FCC, Washington, DC, 2002. [2] Telecommunications Technology Association (TTA), “The status of frequency utilization in WPAN physical layer and domestic frequency allocation in 900MHz”, Nov 2008. [3] W.-H. Tu and K. Chang, “Wide-band microstrip-to-coplanar stripline/slotline transitions,” IEEE Trans. Microw. Theory & Tech., vol. 54, no. 3, pp. 1084–1089, March 2006. [4] T. Chiu and Y.-S. Shen, “A broad-band transition between microstrip and coplanar stripline,” IEEE Microw. Wireless Compon. Lett., vol. 13, no. 2, pp. 66–68, February 2003. [5] Y. G. Kim, D. S. Woo, K. W. Kim, Y. K. Cho, “A new ultrawideband microstrip-to-CPS transition,” 2007 IEEE MTT-S Conference Proceedings, pp. 1563-1566, June 2007. [6] D. M. Pozar, Microwave engineering, 3rd edition, New York: J. Wiley & Sons, 2005, ch. 5.
VI. CONCLUSION An antenna using a novel compact MS-to-CPS transition balun is presented. The transition balun does not require any via holes and multiple impedance transformer as well as long
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