IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 46, NO. 12, DECEMBER 1998 2507 Simplified Design Technique for High-Performance Microstrip Multisection Couplers Duncan K. Y. Lau, Student Member, IEEE, Steve P. Marsh, Senior Member, IEEE, Lionel E. Davis, Fellow, IEEE, and Robin Sloan, Member, IEEE Abstract—A new design technique has been developed which improves the design of a microstrip multisection coupler by a simplified version of the nonuniform coupler method. The technique achieves the performance of a smooth, discontinuity free, nonuniform coupler with a sectional breakdown of the coupling along the structure that can be simply and quickly optimized in a commercial simulator such as Libra. The design is further improved by the use of the “saw-tooth” odd- and even-mode equalization technique, and the effectiveness of this is verified experimentally. The measured results demonstrate an improved performance over the multisection coupler in terms of coupling performance and bandwidth, while the size and the fabrication tolerance are better than the nonuniform line coupler. Index Terms—Broad-band, coplanar, microstrip, mode equal- ization, multisecion coupler, nonuniform coupler, saw-tooth. I. INTRODUCTION B ROAD-BAND couplers are commonly employed for phase shifting, power combining and dividing, and power sampling in microwave circuits. In certain specific applica- tions such as image rejection mixers or signal monitoring in electronic warfare (EW) systems, they demand very broad- band couplers with low amplitude and phase errors. The two natural choices for an extended bandwidth coupler will either be the uniform multisection line coupler or the nonuniform line coupler. The multisection line coupler is simple in its design, but the sharp discontinuities between various sections, together with the even- and odd-mode velocity difference, degrade the accuracy of the coupling response and isolation at high frequencies, as shown by a number of examples in the literature [1]–[4]. The nonuniform line coupler [5]–[7] yields better performance because of the smooth tapering along the structure. This is realized by employing a pair of wavy-shaped coupled microstrip lines to achieve a specified overall coupling response. The design begins with Fourier transform analysis, and the synthesized structure is optimized using iterative techniques [8], [9]. These steps cannot be implemented in standard simulators, and the whole process Manuscript received March 27, 1998; revised August 27, 1998. This work was supported by GEC-Marconi Materials Technology. D. K. Y. Lau, L. E. Davis, and R. Sloan are with the Department of Electrical Engineering and Electronics, University of Manchester Institute of Science and Technology (UMIST), Manchester M60 1QD, U.K. S. P. Marsh is with GEC-Marconi Materials Technology Limited, Caswell, Towcester, Northamptonshire NN12 8EQ, U.K. Publisher Item Identifier S 0018-9480(98)09236-9. is rather time consuming. Further, all the examples in the literature [5]–[7] show undesirable ripples and drop-off in coupling as frequency increases. The aim of this work was to develop a design technique which could achieve the better performance of the nonuniform line coupler, by a simplified approximation method, with an optimization procedure which can be carried out quickly on a standard simulator such as Libra. II. DESIGN METHOD A. Nonuniform Line Coupler The first step for the nonuniform line coupler design re- quires the evaluation of the so-called “reflection coefficient distribution function,” (1) where is the phase velocity of the propagating wave on the line, is the center frequency, is the nominal coupling value, and is the distance along the coupler length. The coupling response is obtained by taking the Fourier transform of (1) (2) where is the physical coupler length. The normalized even- mode impedance function is (3) and the continuous coupling distribution function along the coupler is (4) The physical structure of the nonuniform line coupler can then be constructed according to (4). A typical for a three-section nonuniform line coupler is shown in Fig. 1(a). The major problem of the nonuniform line coupler is that it does not allow one to optimize its coupling response easily even with the aid of existing microwave simulation software. 0018–9480/98$10.00  1998 IEEE