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J. of Electromagn. Waves and Appl., Vol. 17, No. 5, 707–718, 2003 A SMALL SIZE 3 DB 0◦ /180◦ MICROSTRIP RING COUPLERS A. Mohra Microstrip Department Electronics Research Institute Cairo, Egypt A. F. Sheta Electronic Engineering Department College of Engineering, King Saude University P.O. Box 800, Riyadh 11421, Saudi Arabia S. F. Mahmoud Electronic Engineering Department Kuwait University P.O. Box 5969, Safat 13060, Kuwait Abstract—New small size 3 dB microstrip ring couplers suitable for microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs) are presented. Area saving more than 85% of that of the conventional ring coupler can be achieved using the proposed configurations. Small size couplers based on the proposed configurations and designed at 900 MHz have been implemented on RT/duroid 5880 dielectric material. The theoretical and experimental results show favorable comparison. 1 Introduction 2 Ring Coupler Using T-Shape Section Equivalent to Quarter Wave Line 3 Ultra-Miniaturized Hybrid Ring Coupler Configura- tions 3.1 Using  Circuit Equivalent 708 Mohra, Sheta, and Mahmoud 3.2 Coupled Line Short Circuited at Its Diagonal 3.3 Implementation of the Three Quarter Line in Serpentine Form 4 Design Cases and Experimental Results 4.1 Coupler Design Based on T-Shape Equivalent to λ/4 Line (Fig. 4) 4.2 Coupler Design Based on T-Shape Equivalent to λ/4 Line and the Implementation of 3λ/4 Line in Form of Serpentine Shape (Fig. 8) 4.3 Coupler Design Based on T-Shape Equivalent to λ/4 Line and -Shape Equivalent to 3λ/4 Line (Fig. 6) 5 Conclusion References 1. INTRODUCTION Hybrid couplers are used as components, in almost every RF system, such as power combiners and dividers, de(modulators), balanced mixers, image rejection mixers, balanced amplifiers and feed network in antenna arrays. The 0◦ /180◦ hybrid coupler is preferred in some applications, namely, mixers, modulators, and isolated power splitters since the isolation between its input ports may be independent of the value of the two balanced impedance loads [1]. The circumference of the conventional 0◦ /180◦ hybrid ring coupler is 1.5 λg (see Fig. 1). At the lower frequency of the microwave band such as mobile frequencies such size is too large for system integration and MMICs applications, since a large circuit area results in high chip cost. Several design techniques have been proposed to reduce the coupler size. A quarter wavelength pair of coupled lines short-circuited at their diagonal ends has been used to replace the three quarter wavelength line [2]. The circumference of such coupler has been reduced to 1 λg . However this technique requires a very tightly coupled line section that is difficult to fabricate with simple microstrip technology. The design of 3 dB reduced size hybrid ring based on λg /6 or λg /8 sections has been studied [3]. The circumference reported is 1.25 λg . Another approach to reduce the ring coupler size requires; (1) using a small section of transmission line with a specified characteristic impedance instead of the λg /4 line; and (2) replacing the three quarter wavelength line by a one-quarter-wavelength line with phase inverter [1, 4]. Based on this approach the 1.5 λg circumference has been reduced to 0.67 λg A small size 3 dB 0◦ /180◦ microstrip ring couplers 709 3 1 λ g/4 λ g/4 λ g/4 2 Zo =70.7 Ω 4 3 λ g/4 Figure 1. Layout of the conventional ring coupler based on λg /4 line. [4]. Significant increase in bandwidth to exceed one octave has been obtained. The circuit is composed of a coplanar strips (CPS) ring and coplanar wave-guides (CPW) feed lines. In this case air bridges, which are potentially expensive, are needed. A crossover of the two strips on the ring is also required to achieve 180◦ phase shift (phase inverter). Design theory of a reduced size ring coupler using phase inverter in different topologies has been studied [1]. In this paper, new miniaturized 0◦ /180◦ ring coupler configura- tions are introduced. The replacement of each quarter wavelength line by its T -shape equivalence described earlier by the authors [5] for hybrid quadrature couplers will be introduced in Section 2. This ap- proach leads to more than 70% area saving, ending up with 0.75 λg circumference. Further size reduction can be achieved by replacing the three quarter wavelength line either by: (1) its  circuit equivalent, (2) a pair of coupled lines short circuited at their diagonals, or (3) the same line length in serpentine shape. These configurations will be described in section 3. The proposed structures are suitable for hybrid MICs and MMICs applications. With this approach more t

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