second-passband for dual-band applications, IEEE Microwave Wire- less Compon Lett 16 (2006), 360–362. 3. J.-W. Baik, S. Pyo, W.-S. Yoon, and Y.-S. Kim, Dual-mode dual- band bandpass filter for single substrate configuration, Electron Lett 45 (2009), 982–984. 4. A. G€ or€ ur, Description of coupling between degenerate modes of a dual-mode microstrip loop resonator using a novel perturbation arrangement and its dual-mode bandpass filter application, IEEE Trans Microwave Theory Tech 52 (2004), 671–677. 5. S. Amari, Comments on “Description of coupling between degener- ate modes of a dual-mode microstrip loop resonator using a novel perturbation arrangement and its dual-mode bandpass filter applica- tions, IEEE Trans Microwave Theory Tech 52 (2004), 2190–2192. 6. ADS User Manual, Agilent Technologies, Inc., Fremont, CA, (2007). V C 2014 Wiley Periodicals, Inc. NOVEL COMPACT INVERTED U-SHAPED DIRECTIONAL COUPLER USING PARALLEL DUAL TRANSMISSION LINES TECHNIQUE Siti Fatimah Ausordin, Sharul Kamal Abdul Rahim, Norhudah Seman, Raimi Dewan, Dayang Norkhairunnisa Abang Zaidel, BashirMuhammad Sa’ad, and Suhanya Jayaprakasam Wireless Communication Centre (WCC), Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia; Corresponding author: sharulkamal@fke.utm.my Received 19 May 2013 ABSTRACT: A novel compact design of a 3-dB inverted U-shaped directional coupler using dual parallel transmission lines on double layer substrates is presented in this article. Dual parallel transmission lines technique is used as a replacement of the conventional single transmission line. Hence, a smaller size and wider bandwidth of direc- tional coupler is successfully achieved. The proposed coupler is highly adaptable to various microwave applications for microwave circuitry such as the Butler Matrix (BM) beamforming network. The reduction in size gives better flexibility for the integrated structure of BM system. The proposed 2.45-GHz coupler successfully obtaining size reduction up to 40% compared to the rectangular multilayer directional coupler. In addition, 25% bandwidth performance was achieved. Dimension and S- parameter performance comparison has been made between the pro- posed coupler which use dual parallel transmission lines technique in multilayer structure and rectangular multilayer coupler. The measure- ment results are in good agreement with simulated ones in terms return loss, coupling, through and isolation. V C 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:251–256, 2014; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28041 Key words: multilayer technology; dual transmission lines technique; directional coupler; compact design; improved bandwidth 1. INTRODUCTION The directional coupler is an indispensable microwave passive component that is used in varieties of microwave system designs such as balance amplifiers [1], power dividers [2], phase shifters [3], mixers [4], and Butler matrix beam forming networks [5–8]. The directional coupler is the most basic and popular type of hybrid couplers, in which the phase difference between the out- put ports is 90 o . Traditionally, there are two common problems associated with the 90  hybrid coupler’s design: (i) large size, especially at low frequencies and (ii) narrow operating band- width. Compact couplers are always desired for seamless inte- gration with existing microwave devices, which consequently reduces the devices’ overall cost and bulkiness. Conversely, bet- ter bandwidth enables versatility of the couplers’ usage in vari- ety of applications [9]. Several techniques have been proposed by past researches to reduce the size of the directional couplers. Techniques such as defected ground structure [10], varying dielectric constant near the side wall [11], left-handed transmission lines [12], lumped element [13], and dual transmission lines in single layered sub- strate [14] are some of the common techniques that have been proposed. The techniques successfully achieved size reduction of the directional coupler up to 64%. However, the trade off in the size reduction is the narrow bandwidth of 15%. In contrast, other proposed technique in [15] has been effec- tively used in overcoming bandwidth limitations, achieving improvement of up to 30%. Their methods, however, have resulted in increased fabrication complexities due the use of waveguide in their design, incurring significant additional costs. Another method has been proposed in [16] that incorporate a coupler by using multisection coplanar waveguide technology. Although, this method resulted in bandwidth improvement of 40%, this design coupler has more intricate implementation due to the need of bond wire connecting series and shunt arm of coupler. An alternative method to overcome bandwidth limitations is by implementing couplers on multilayered structures [17,18]. Multilayer technology is chosen in previous researches by con- sidering their capability of providing tight coupling, high power capability, and also size reduction of structure. The reported rectangular-shaped [17] and elliptically-shaped [18] couplers achieve good bandwidth performance over their targeting fre- quencies, 1.8 GHz and ultrawide band (UWB) frequencies. However, these design structures use a more costly Rogers RO4003C substrate that enables a low-loss structure for UWB operation. In order to overcome the size reduction and bandwidth issues simultaneously, a hybrid of parallel dual transmission lines [14] and multilayer technology [17] techniques have been chosen and implemented for the proposed coupler in this article. Based on literature reviews, the implementation of dual transmission lines technique in a multilayered structure has never been investi- gated. This structure targets non-UWB operations; hence, a low- cost FR4 substrate offers an extremely competitive compromise between bandwidth improvement and size reduction compared to other non-UWB techniques [10–14]. The proposed compact 3-dB inverted U-shaped directional coupler is finally achieved by converting the single transmission line patch of both top and bottom layers in the [17] to parallel dual transmission line each using techniques reported in [14]. 2. COUPLER DESIGN In this section, design dimensions of the proposed compact 3-dB inverted U-shaped directional coupler, which implemented dual transmission lines in multilayer structure operating at 2.45 GHz, is computed and presented. The proposed coupler design is simulated using computer simulation technology (CST) Micro- wave Studio 2010. An FR-4 substrate is used with the thickness of 0.8 mm, copper thickness of 0.035 mm, dielectric constant of 4.5, and loss tangent of 0.019. The design of the proposed compact coupler structure con- sists of a rectangular ground plane in the middle layer of the two 45 3 15 mm 2 substrates is presented here. Top conductor layer consists of port 1 and port 3 while the bottom conductor layer consists of port 2 and port 4, as shown in terms of layout DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 1, January 2014 251