Design of Compact Directional Couplers for UWB Applications

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 2, FEBRUARY 2007

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Design of Compact Directional Couplers for UWB Applications Amin M. Abbosh and Marek E. Bialkowski, Fellow, IEEE

Abstract—This paper presents a simple design method for a class of compact couplers, which offer coupling in the range of 3–10 dB over an ultra-wide frequency band from 3.1 to 10.6 GHz. The proposed couplers are formed by two elliptically shaped microstrip lines, which are broadside coupled through an elliptically shaped slot. Their design is demonstrated for a 3-, 6-, and 10-dB coupling assuming a 0.508-mm-thick Rogers RO4003C substrate. Results of simulation and measurements show that the designed devices exhibit a coupling of 3 1 dB, 6 1.4 dB and 10 1.5 dB across the 3.1–10.6-GHz band. This ultra-wideband coupling is accompanied by isolation and return loss in the order of 20 dB or better. The manufactured devices including microstrip ports occupy an area of 25 mm 15 mm. Index Terms—Compact ultra-wideband (UWB) couplers, coupled circuits, directional couplers, planar coupler design.

I. INTRODUCTION

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ROADBAND microwave directional couplers are a very important category of passive microwave circuits. They are used to combine or divide signals with appropriate phase of 90 , and are commonly used in microwave subsystems such as balanced mixers, modulators, and antenna beam-forming networks [1]. In addition, they are essential for developing the cost-effective measurement equipment [2]–[4]. Our particular interest in these devices is with respect to developing an ultra-wideband (UWB) microwave imaging system for breast cancer detection [5], [6]. In these and many other applications, the required couplers are often required to be accomplished in planar (stripline or microstrip) technology. In order to achieve their broadband operation, the approach of coupled transmission lines can be employed. The inherent feature of this approach is that matching and directivity is perfect, and independent of frequency, at least under ideal conditions. However, the challenge is to obtain a tight coupling in the range of 3–6 dB. Using coupled microstrip lines, the tight coupling can be accomplished using the Lange [7] or tandem coupler configurations [8]–[10]. However, they require wire crossovers, which

Manuscript received March 29, 2006; revised June 14, 2006. This work was supported by the Australian Research Council under Grant DP0449996 and Grant DP0450118. The authors are with the School of Information Technology and Electrical Engineering, The University of Queensland, St. Lucia, Qld. 4072, Australia (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2006.889150

is inconvenient from the manufacturing point-of-view. In addition, the Lange coupler features narrow strips, which create additional manufacturing problems due to the requirement for strict etching tolerances. In turn, the broadband tandem coupler may require wiggles or serpentines to equalize even- and odd-mode phase velocities [9], [11] when realized in microstrip technology. In order to avoid these problems, the slot-coupling approach involving a double-sided substrate, which was first proposed by Tanaka et al. [12], can be applied to realizing a tight coupling. The structure is formed by two microstrip lines separated by a rectangular slot in the common ground plane. Its design formulas were given in [13]. When one aims only at the design of a 3-dB coupler, an alternative is the microstrip-slotline approach, which was described by de Ronde [11]. In contrast to Tanaka et al., the de Ronde’s approach preserves the one-layer microstrip format of the coupler at an expense of etching both sides of a ceramic substrate. One side of this coupler is formed by two parallel connected microstrip lines, while the other one includes a straight slotline with two circular terminating slots. In addition, de Ronde suggested the use of a capacitive disc below the slotline to enhance broadband performance. A very important feature of this coupler is a multioctave operation and a very compact size. By introducing modifications to the original de Ronde’s design, Garcia [14] demonstrated an alternative configuration of a compact planar 3-dB coupler operating, similarly as de Ronde’s device, over the 4 : 1 bandwidth. In his design, Garcia avoided the circular terminating slots and the capacitive disc. Instead, he enlarged the size of a slot below the microstrip layer. This could be the key to achieving UWB performance. By neglecting the capacitive disc beneath the slot, which appeared in the original de Ronde’s configuration, Schiek [15], and then Hoffmann and Siegl [16], produced the design rules for the microstrip-slot 3-dB coupler. However, for the simplified configurations, their designs were not as broadband as offered by de Ronde and Garcia. In this paper, we describe a class of compact planar couplers, which are capable of providing coupling between 3–10 dB over an ultra-wide frequency band. In order to find initial dimensions of these devices, simple design equations similar to the ones described in [13] are applied. Final dimensions are obtained with the use of full-wave electromagnetic analysis software package such as Ansoft’s High Frequency Structure Simulator (HFSS). The validity of the presented designs is confirmed experimentally.

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slot by their rectangular equivalents, as shown in Fig. 1(b). In this case, the rectangular microstrip width and length are and , respectively. The rectangular slot width and length are denoted by and . For the equivalent rectangular shaped microstrips and slot, the analysis and design procedure is similar to the one described in cou[13]. Assuming that the coupler is required to have pling, the even and odd mode characteristic impedances are calculated using (1) and (2) as follows: (1) (2)

Fig. 1. (a) Layout of the proposed wideband coupler including microstrip ports. (b) Equivalent configuration used to work out initial dimensions. (c) Electric field lines for odd- and even-mode excitation.

II. DESIGN The configuration of a compact coupler, which is capable of providing a tight coupling over an ultra-wide frequency band, is shown in Fig. 1(a). In concept, it is similar to the one of Tanaka et al. [12]. The differences concern the shaping of the broadside coupled strips and the slot. They seem to be the key factors behind the UWB performance. The coupler consists of three conductor layers interleaved by two dielectrics. The top conductor layer includes ports 1 and 2. The bottom conductor layer is similar to the top layer, but the ports here are ports 3 and 4. Note that ports 3 and 4 are on opposite sides of the substrate compared to ports 1 and 2, but that this is not a limitation in many applications. The two layers are coupled via a slot, which is made in the conductor supporting the top and bottom dielectrics. As observed in Fig. 1(a), the two microstrip conductors and the slot are of an elliptical shape. The curved microstrip lines are included to make connections to subminiature A (SMA) ports. By assuming that the curved microstrip lines are shortened to zero length, the structure features , double symmetry with respect to the horizontal plane . For in which the slot is located, and the vertical plane the purpose of analysis and design of this coupler, it is sufficient to utilize only the horizontal symmetry plane. In this case, an even-odd mode approach with respect to ports 1 and 3 can be applied to analyze this circuit [17]. The initial analysis and design procedure can be simplified by approximating the two elliptical conducting patches and the

where is the characteristic impedance of the microstrip ports of the coupler. and the coupling factor is Assuming that and can be calculated 3, 6, or 10 dB, the values of from (1) and (2) and are given as follows: 120.5 and 20.7 for dB, 86.7 and 28.8 for dB, and 69.4 dB. and 36.0 for Before commencing the design, we consider the operation of this coupler for the odd and even modes. When the odd mode is excited, the slot can be replaced by a perfect electric conductor. The resulting upper part of the equivalent coupler shown in Fig. 1(b) becomes a microstrip line whose charac. The width realizing can be teristic impedance is determined using standard design equations for a microstrip transmission line [17]. Alternatively, the static formulas described in this paper can be used. From Fig. 1(c), one can see that, in the odd mode, the electric field concentrates mostly in the parallel-plate region formed by the patch and ground plane. A fringe effect, also observed in Fig. 1(c), is less pronounced as becomes large in comparison with the for small substrate thickness . A different wave propagation condition occurs under the even-mode wave excitation. For this mode, the magnetic conductor replaces the slot in the ground plane. Its presence pushes an electric field (launched from the microstrip port) outside the parallel-plate region. This is because the magnetic conductor forming the lower plate does not allow the electric field to be perpendicular to its surface. As a result, the even-mode wave travels in two antipodal slot regions outside the parallel-plate region, as shown in Fig. 1(c). In order to enable a smooth launch of the even-mode wave from the microstrip port to the two antipodal slotlines, the transition formed by the elliptically shaped patches and the ground slot, as shown in Fig. 1(a), is required. and of the equivalent rectangular The dimensions shaped coupler [see Fig. 1(b)], providing the required evenand odd-mode characteristic impedances, are determined using a static approach similar to the one presented in [13]. By using and are given by (3) and (4) as follows: this approach, (3) (4)

ABBOSH AND BIALKOWSKI: DESIGN OF COMPACT DIRECTIONAL COUPLERS FOR UWB APPLICATIONS

where

is the first kind elliptical integral and . Following [13], the parameters and culated using (5) and (6) as follows:

are cal-

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TABLE I VALUES OF DESIGN PARAMETERS IN MILLIMETERS

(5) (6) is the width of the where is the thickness of the substrate, is the width of the top and bottom microstrip patches, and slot of Fig. 1(b). Using the analysis in [18], the ratio of elliptical functions appearing in (3) and (4) can be approximated by the following:

for for

(7) and for given values The synthesis task of determining and is accomplished by solving (3)–(7) using the of Gauss–Newton iteration method. The last step of the design procedure concerns the determiis chosen to be nation of the coupler’s length. Here, , where is the effective wavelength for the microstrip line and can be calculated using standard formulas such as those presented in [17]. Formulas (3)–(7) enable calculations of the equivalent parameters of the rectangular shaped coupler of Fig. 1(b). The next step is to work out the dimensions of the elliptically shaped counter part. Due to compact size, where the dimension is equal or less than a quarter of the effective wavelength, one can expect a similar performance when the rectangular and elliptically shaped couplers occupy an approximately equal area. Using this equivalence principle and assuming that the mean algebraic length of the elliptically shaped coupler is equal to its rectan, then the gular counterpart such that width of the microstrip and the width of the slot for the elliptically shaped coupler can be obtained using (8) and (9) as follows: (8) (9) are adjusted by iteratively The final dimensions , , and running the finite-element method design and analysis package Ansoft HFSSv9.2. In order to test the coupler experimentally, its ports need to be connected to SMA coaxial connectors. To minimize possible reflections, curved microstrip lines, as shown in Fig. 1(a), can be used. Our simulations have revealed that for high-quality impedance match, the radius of these curved lines should not be less than twice the width of the microstrip line.

Fig. 2. Simulated performance of the designed 3-dB directional coupler.

III. RESULTS The validity of the presented design method is tested in examples of 3-, 6-, and 10-dB directional couplers aimed for operation in the 3–10-GHz frequency band. For this band, the center frequency of operation is 6.5 GHz. A Rogers RO4003C substrate featuring a dielectric constant of 3.38 and a loss tangent of 0.0027, 0.508-mm thickness, plus 17- m-thick conductive coating is selected for the couplers development. , , and Using the proposed method, the dimensions are determined and are shown in Table I. One can find that the obtained values are not too far off from the ones calculated using mm using [13] or mm using (3)–(9). First, mm, mm, and mm. (4), mm (for mm), mm, Therefore, and mm. The return loss, coupling, and isolation of the designed couplers are first verified using HFSS. Fig. 2 shows the simulated amplitudes of the scattering parameters for the designed 3-dB coupler. These are followed by results of the phase difference between the two output ports, as shown in Fig. 3. It is clear that the designed coupler features UWB characteristics. The coupling is 3 0.8 dB for the 3.1–10.6 GHz band. The isolation and return loss are better than 28 and 22 dB, respectively, for the band. In Fig. 3, it is observed that the phase difference between ports 2 and 3 is 90 1 over the band. This

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Fig. 3. Simulated phase characteristic.

Fig. 5. Simulated performance of the designed 10-dB directional coupler.

Fig. 6. Manufactured 3-dB coupler. Fig. 4. Simulated performance of the designed 6-dB directional coupler.

result together with the magnitude results shown in Fig. 2 indicates that the coupler operates as a backward wave quadrature coupler [17]. Figs. 4 and 5 show the simulated amplitudes of the scattering parameters for the designed 6- and 10-dB couplers. It can be seen that, for the 6- and 10-dB couplers, the best result for the coupling is obtained for frequencies around the center frequency. The gradual deviation from the specified value of coupling then occurs. In general, the three couplers feature quite a good UWB performance despite only being formed by a one-quarter-wave section of (nonuniformed) coupled lines. The directional couplers are then manufactured and tested using a vector network analyzer. The photograph of the one of the manufactured 3-dB couplers is shown in Fig. 6. The overall dimensions of the coupler including bent microstrip lines are 25 mm 15 mm, indicating that the device is of a very compact size. The manufactured 6- and 10-dB couplers have the same size.

The measured results are presented in Figs. 7–9. As observed in Figs. 7–9, all of the manufactured couplers show UWB behavior with coupling 3 0.8, 6 1.4, and 10 1.5 dB for the 3-, 6-, and 10-dB couplers, respectively, across the 3.1–10.6-GHz band. The isolation is better than 23, 20, and 19 dB, while the return loss is better than 21, 18, and 19 dB for the 3-, 6-, and 10-dB couplers, respectively. As observed from the presented data in Figs. 7–9, the operation of the 3-dB coupler seems to be best and is superior over the one of Garcia [14], which showed the 3 dB 1-dB bandwidth from 4.5 to 8 GHz and the isolation of around 20 dB. The manufactured 6- and 10-dB couplers exhibit some insertion losses, which are not observed in the simulated results. These can be due to conduction and dielectric losses, the difficulty of manual aligning the two microstrip layers forming this type of coupler, and coaxial connectors. The 6- and 10-dB couplers have a smaller width than the 3-dB coupler and as such they are more sensitive to aligning errors. However, in general, the agreement between the simulated and measured results can be considered as very good.

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In some applications, one may wish to house the designed couplers in enclosures. In this case, it is important to assess the effect of shielding. Here, this problem was investigated only via computer simulations. Only brief comments concerning the results of these simulations are reported. The produced simulation results revealed that a metal cover with a height of 0.5 cm below and above the three investigated couplers did not adversely affect their performance, as the electrical characteristics were very similar to those shown in Figs. 2–5. Only small adverse effects of the enclosure were observed when the shielding height above and below the coupler structure was reduced to 0.25 cm. IV. CONCLUSION

Fig. 7. Measured performance of the manufactured 3-dB coupler.

A simple method has been proposed for the design of compact directional couplers for UWB applications. The proposed devices are formed by a multilayer microstrip structure with broadside slot coupling. The coupling is controlled by elliptical shapes of microstrip conductors and a coupling slot. The design method has been demonstrated for the case of 3-, 6-, and 10-dB coupling. The couplers have been manufactured and experimentally tested. They have shown UWB behavior across the band from 3.1 to 10.6 GHz. Due to compact size and good electrical performance, they should be of considerable interest to the designers of UWB components. Our particular aim is to use them in a UWB microwave imaging instrumentation [4]–[6]. ACKNOWLEDGMENT The authors acknowledge the assistance of D. Bill, K. Bialkowski, and S. Padhi, all with the University of Queensland, Brisbane, Australia, in the manufacturing of the couplers. REFERENCES



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[11] F. C. de Ronde, “A new class of microstrip directional couplers,” in IEEE MTT-S Int. Microw. Symp. Dig., May 1970, pp. 184–189. [12] T. Tanaka, K. Kusoda, and M. Aikawa, “Slot-coupled directional couplers on a both-sided substrate MIC and their applications,” Electron. Commun. Jpn., vol. 72, no. 3, pt. 2, 1989. [13] M.-F. Wong, V. F. Hanna, O. Picon, and H. Baudrand, “Analysis and design of slot-coupled directional couplers between double-sided substrate microstrip lines,” IEEE Trans. Microw. Theory Tech., vol. 29, no. 12, pp. 2123–2129, Dec. 1991. [14] J. A. Garcia, “A wideband quadrature hybrid coupler,” IEEE Trans. Microw. Theory Tech., vol. MTT-19, no. 7, pp. 660–661, Jul. 1971. [15] B. Schiek, “Hybrid branchline couplers—Useful new class of directional couplers,” IEEE Trans. Microw. Theory Tech., vol. MTT-22, no. 10, pp. 804–869, Oct. 1974. [16] R. K. Hoffmann and J. Siegl, “Microstrip-slot coupler design—Part I and II,” IEEE Trans. Microw. Theory Tech., vol. MTT-30, no. 8, pp. 1205–1216, Aug. 1982. [17] D. Pozar, Microwave Engineering, 3rd ed. New York: Wiley, 2005. [18] W. Hillberg, “From approximation to exact relations for characteristic impedances,” IEEE Trans. Microw. Theory Tech., vol. MTT-17, no. 5, pp. 259–265, May 1969. Amin M. Abbosh was born in Mosul, Iraq. He received the M.Sc. degree in communication systems and Ph.D. degree in microwave engineering from Mosul University, Mosul, Iraq, in 1991 and 1996, respectively. Until 2003, he was Head of the Information Engineering Department, Mosul University. In 2004, he joined the Centre for Wireless Monitoring and Applications, Griffith University, as a Post-Doctoral Research Fellow. He is currently a Research Fellow with the School of Information Technology and Electrical Engineering, The University of Queensland, St. Lucia, Queensland, Australia. His research interests include antennas, radio wave propagation, microwave devices, and design of UWB wireless systems.

Marek E. Bialkowski (SM’88–F’03) was born in Sochaczew, Poland. He received the M.Eng.Sc. degree in applied mathematics and Ph.D. degree in electrical engineering from the Warsaw University of Technology, Warsaw, Poland, in 1974 and 1979, respectively, and the D.Sc. Eng. (Higher Doctorate) degree in computer science and electrical engineering from The University of Queensland, St. Lucia, Queensland, Australia, in 2000. He has held teaching and research appointments with universities in Poland, Ireland, Australia, U.K., Canada, Singapore, Hong Kong, and Switzerland. He is currently a Professor with the School of Information Technology and Electrical Engineering, The University of Queensland. He has authored or coauthored over 450 technical papers, several book chapters, and one book. His research interests include antennas for mobile cellular and satellite communications, signal-processing techniques for smart antennas, low-profile antennas for reception of satellite broadcast TV programs, near-field/far-field antenna measurements, electromagnetic modeling of waveguide feeds and transitions, conventional and spatial power-combining techniques, six-port vector network analyzers, and medical and industrial applications of microwaves.