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DUAL-BAND SICL BRANCH-LINE COUPLER Qi Wu, Haiming Wang, Chen Yu, Xiaowei Zhang, and Wei Hong State Key Laboratory of Millimeter Waves, Southeast University, Nanjing, China; Corresponding author: hmwang@seu.edu.cn Received 17 October 2014 ABSTRACT: A dual-band substrate integrated coaxial line (SICL) 3-dB branch-line coupler and its compact realization are proposed in this let- ter. The SICL coupler with the compact structure shows a comparable performance and great size reduction compared with the conventional structure. The simulated and measured results match very well. V C 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1246–1249, 2015; View this article online at wileyonlinelibrary.com. DOI 10.1002/ mop.29062 Key words: dual-band; branch-line coupler; substrate integrated coaxial line; compact structure 1. INTRODUCTION As an important passive microwave component, the branch-line coupler with the characteristics of compact size, broadband or multiple-band operation is still of much interest. While the con- ventional microstrip branch-line coupler suffers from the defi- ciencies of large size, narrow bandwidth, radiation loss, and cross-talk issues, several design techniques were proposed to reduce the coupler size or realize dual-band characteristic [1–5]. One method to reduce the size of conventional couplers is to use purely distributed elements in conjunction with high- impedance transmission lines to approximate quarter-wave trans- mission-line behavior [1,2]. Conversely, many circuit models were proposed to realize the dual-band characteristic [3–5]. In recent years, substrate integrated waveguide (SIW) has attached much interest. Because of its advantages of low loss and insensitivity to outside interference, many SIW directional couplers were proposed [6,7]. However, at L/S-band, when com- pared with microstrip or coplanar waveguide couplers, SIW cou- plers are still large. Recently, a novel transmission line named substrate integrated coaxial line (SICL), a.k.a fenced stripline, was presented and experimentally verified [8]. The SICL is a planar coaxial line, comprising a conductive signal strip sand- wiched between two grounded dielectric layers and side-limited by two rows of metallic via-holes. It has a broadband single- mode operation thanks to the TEM mode. The two rows of metallic via-holes can avoid the propagation of the unwanted parallel-plate mode. Additionally, SICLs can be implemented using the conventional multilayer PCB process and therefore easily integrated with other planar circuits. A 3-dB branch-line coupler based on the SICL technology was designed and achieved good performance [9], which showed great size reduc- tion compared with SIW couplers. In this work, dual-band SICL branch-line couplers with con- ventional and compact structures are designed, fabricated, and measured. Both of the proposed couplers achieve good perform- ance at required bands. 2. DESIGN OF DUAL-BAND SUBSTRATE INTEGRATED COAXIAL LINE COUPLER The SICL is implemented in a two-layer dielectric substrate with three metallic layers, as shown in Figure 1. The inner layer conductor is sandwiched by two grounded dielectric layers and side-limited by two rows of metallic via-holes. In this work, both the top and bottom dielectric layers use FR-4 material with a relative dielectric constant of 4.4 and a loss tangent of 0.027. Each substrate has a thickness of 0.8 mm. A FR-4 layer of 0.15 mm with the same dielectric constant is used to bond the top and bottom dielectric layers. 2.1. Dual-Band Substrate Integrated Coaxial Line Coupler The traditional design of the 3-dB SICL branch-line coupler requires two pairs of quarter-wavelength SICLs with characteris- tic impedances of 35 and 50 X, which operates in a single band. According to Ref. 4, a coupler with proper choice of circuit top- ologies can operate at two arbitrary frequencies (f 1 and f 2 ). Four half-wavelength short-circuit stubs are introduced to achieve the dual-band characteristic. Figure 2(a) shows the circuit structure of the dual-band branch-line coupler. Let Z 0 be the characteristic impedance of input and output ports, Z 0 5 50 X. Then, the val- ues of Z 1 , Z 2 , and Z 3 are given by Ref. 4 Z 1 5 Z 0 ffiffi 2 p  1 cos dp 2   (1) Z 2 5Z 0  1 cos dp 2   (2) Z 3 5 Z 0 11 ffiffi 2 p  1 sin dp 2   tan dp 2   (3) where Figure 1 Top and side views of the SICL structure. 1246 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 57, No. 5, May 2015 DOI 10.1002/mop