PAPER Wide-Band Coaxial-to-Coplanar Transition

2030 PAPER Wide-Band Coaxial-to-Coplanar Transition Toshihisa KAMEI a),yozoutsumi, Members, NguyenQUOCDINH, and Nguyen THANH, Student Members SUMMARY Targeting the transition from a coaxial wave guide to a coplanar wave guide (CPW), a microwave and millimeter-wave wide-band coaxial-to-coplanar transition is proposed. This design connects the coaxial inner conductor to the CPW center conductor in a perpendicular manner to directly couple, on the same plane, the radial high-frequency electric field in the coax to the gap between the CPW s center conductor and ground plane. The performance of the proposed transition was compared with those of the conventional transition by experiments and electromagnetic field simulations, and it was found that the proposed method is independent of CPW shape and that it exhibits good matching performance in comparison to the conventional method especially in high-frequency bands above 15 GHz. key words: coaxial wave guide, coplanar wave guide, transition 1. Introduction In a coplanar wave guide (CPW) for a fixed dielectricsubstrate thickness, there are an infinite number of combinations of center-conductor width (W) and centerconductor/ground-plane gap (S) to achieve matching impedance (50 Ω). In microwave and millimeter wave devices, specific W and S combinations for satisfying this 50 Ω requirement are determined according to the application in question. It is therefore essential in a coaxial-to-coplanar transition that conversion over a wide band be possible with minimal mismatch with respect to these various combinations of W and S. However, in the conventional method that connects the coaxial inner conductor and CPW center conductor on the same plane, higher-order modes arise at the connection section when W is extremely smaller or larger or S is extremely smaller than the diameter of the coaxial inner conductor. This results in the insertion of unnecessary reactance in the line at discontinuous sections in a serial or parallel manner thereby generating a large mismatch. In this paper, we propose a new type of coaxial-tocoplanar transition capable of wide-band conversion for the various combinations of W and S that achieve 50 Ω while being independent of the values of W and S. This is accomplished by connecting the coaxial inner conductor perpendicular to the CPW center conductor so that the radial highfrequency electric field of the coax directly couples on the same plane with the high-frequency electric field in gap S of Manuscript received March 16, 2007. Manuscript revised June 6, 2007. The authors are with The National Defence Academy, Yokosuka-shi, 239-8686 Japan. a) E-mail: kamei@nda.ac.jp DOI: 10.1093/ietele/e90 c.10.2030 the CPW. 2. Structure of Coaxial-to-Coplanar Transition Figures 1 and 2 show the structures of conventional coaxialto-coplanar transition and proposed coaxial-to-coplanar transition, respectively. In these figures, the transistions of both sides are connected by a 50 Ω CPW (length l) inorder to measure the transmission performances. In Fig. 1, the inner and the center conductors connect on the same plane, but here, the two-dimensional radial high-frequency electric field of the coax couples with the high-frequency electric field in the CPW s gap S having a one-dimensional-type of cross section. This coupling, as a result, has a large dependence on the values of W and S in the CPW. Here, if coax diameter and W are significantly different, or if S is extremely smaller, incorporating a tapered (c) Fig. 1 Structure of conventional Coaxial-to-Coplanar transition. Top plan view. Sectional view. (c) Enlarged view. Copyright c 2007 The Institute of Electronics, Information and Communication Engineers

KAMEI et al.: WIDE-BAND COAXIAL-TO-COPLANAR TRANSITION 2031 part in the CPW in the discontinuous part as shown in Fig. 1 can reduce mismatch to some extent. In Fig. 2, in contrast, the inner and the center conductors of the two lines connect at a right angle suggesting that the radial high-frequency electric field of the coax can directly couple with the high-frequency electric field in gap S of the CPW in a uniform manner and on the same plane without being dependent on the values of W and S. SMA connectors comprise general-use connectors, which have the structure shown in Fig. 3, and connectors, which have the structure shown in Fig. 3. Fig. 3 is used with microstrip lines and coplanar waveguides (CPWs) when the center conductor is thin. In this paper, the transmission performances of the conventional transtion shown in Fig. 1 are compared to those of the proposed transition shown in Fig. 2 when they were used with the two different types of connectors (Figs. 3 and ), in which the inner conductors diameters are different. For the 50 Ω coaxial wave guide, an external conductor diameter (D o )is4.1mmφ and an inner conductor diameter (D i )is1.2mmφ in Fig. 3 and an external conductor diameter (D o )is2.1mmφ and an inner conductor diameter (D i )is0.65mmφ in Fig. 3. 3. Transmission Performances of Coaxial-to-Coplanar Transition Using center-conductor width W as a parameter, Fig. 4 shows the relationship between CPW characteristic impedance Z c [Ω] and distance S between CPW center conductor and ground plane for a CPW of conductor thickness t=9 µm formed on a dielectric substrate with relative dielectric constant ε r =3 and substrate thickness h=350 µm. For each of the W and S combinations (W=100 µm, S=16 µm), (W=200 µm, S=24 µm), (W=300 µm, S=30 µm), (W=600 µm, S=48 µm), and (W=900 µm, S=65 µm) for which Z c =50 Ω can be nearly achieved, we determined S 21 transmission performances by experiment and electromagnetic field simulations using the transition structures of Figs. 1 and 2 with Figs. 3 and connectors. In the experiment, we connected each of the transition structures of Figs. 1 and 2 to both ends of the CPW with line length l=25.4mm to determine transmission performances S 21. For the electromagnetic field simulations, we employed 3D electromagnetic simulation software called MW Studio (from CST) and used the finite integration technique (an algorithm for solving Maxwell equations in integral form) and the perfect boundary approximation technique (an expanded algorithm using a cubic mesh that can perform analysis while preserving complex shape information including surfaces) as analysis techniques. As for simulation conditions, we established a minimum 9 µm-square mesh to analyze the smallest gap section of the CPW and a maximum 900 µm-square mesh to analyze space away from gap sections and perform calculations including those for conductor loss and dielectric loss. (c) Fig. 2 Structure of proposed Coaxial-to-Coplanar transition. Top plan view. Sectional view. (c) Enlarged view. Fig. 3 Structure of Coaxial connector (sectional view). D o =4.1 mm Φ, D i =1.2 mm Φ. D o =2.1 mm Φ, D i =0.65 mm Φ. Fig. 4 Relation between characteristic impedance Z c and W, S of coplanar waveguide.

2032 Fig. 5 Frequency performances of S 21 of conventional Coaxial-to- Coplanar Transition. W=300 µm, S=30 µm (experiment). W=600 µm, S=48 µm (experiment). (c) W=900 µm, S=65 µm (experiment). (d) W=300 µm, S=30 µm (simulation). (corresponding to Fig. 1 with Fig. 3 connector) Fig. 7 Frequency performances of S 21 of proposed Coaxial-to-Coplanar Transition. W=300 µm, S=30 µm (experiment). W=600 µm, S=48 µm (experiment). (c) W=900 µm, S=65 µm (experiment). (d) W=300 µm, S=30 µm (simulation). (corresponding to Fig. 2 with Fig. 3 connector) Fig. 6 Frequency performances of S 11 of conventional Coaxial-to- Coplanar Transition. W=300 µm, S=30 µm (experiment). W=600 µm, S=48 µm (experiment). (c) W=900 µm, S=65 µm (experiment). (corresponding to Fig. 1 with Fig. 3 connector) First, for the Fig. 1 transition with incorporating a tapered part in CPW by using Fig. 3 connector, Fig. 5 shows experimental results (S 21 ) for the above (W=300 µm, S=30 µm), (W=600 µm, S=48 µm), and (W=900 µm, S=65 µm) combinations and simulation results (S 21 )forthe (W=300 µm, S=30 µm) combination. It can be seen that experimental and simulation values agree well and that a standing wave forms due to mismatch in the two transitions connected to both ends of the CPW. Also, for high-frequency bands in excess of 15 GHz when using the conventional transition shown in Fig. 1, S 21 values differ greatly depending on the values of W and S even if Z c of 50 Ω in the CPW is maintained. That is to say, S 21 values deteriorate the smaller the values of W and S become. This is because it is difficult for the coaxial radial high-frequency electric field having a two-dimensional element in the cross section normal to the CPW substrate surface (Fig. 1) to couple smoothly with the high-frequency electric field in gap S of the CPW having almost a onedimensional shape much smaller than the coax diameter. As a result, higher-order modes are generated and reactive energy builds up in the discontinuous part. Figure 6 shows S 11 (experimental values) for the transition shown in Fig. 1 for the above three combinations of WandS. Figure 7 shows S 21 for the proposed transition shown in Fig. 2 with Fig. 3 conector for the above three combinations of W and S. These results show that the mismatch at the two transitions connected to both ends of the CPW has been reduced and that the standing wave has become smaller. Also, for high-frequency bands up to 27 GHz, S 21 values are nearly the same for the various combinations of W and S provided that 50 Ω is maintained for Z c of the CPW. The reason for this is thought to be as follows. As shown in Fig. 2, the radial high-frequency electric field of the coaxial wave guide s dominant mode can be made to uniformly couple on the same plane with the 2D-like high-frequency electric field in the CPW s S section, this regardless of the values of W and S. In any case, there is very little generation of higher-order modes. Simulation results for the (W=300 µm, S=30 µm) combination are also shown here together with experimental results, and it can be seen that they agree. Figure 8 also shows S 11 (experimental values) for the transition shown in Fig. 2 for the above three combinations of W and S. As the experiment results in Figs. 5 to 8 demonstrate, the proposed transition s result (W=300 µm, S=30 µm) shows an improvement in S 21 of about 4 db for high frequency bands above 20 GHz and an improvement of about 3dB in S 11 likewise for high frequencies above 20 GHz compared with the conventional one. Next, when the narrow inner conductor coaxial connector for the transition to the narrow center conductor CPW is used, the transmission performances (S 21,S 11 ) are obtained by the computer simulations. For each of the W and S combinations (W=100 µm, S=16 µm), (W=200 µm, S=24 µm), and (W=300 µm, S=30 µm) for which Z c =50 Ω

KAMEI et al.: WIDE-BAND COAXIAL-TO-COPLANAR TRANSITION 2033 can be nearly kept, we examined S 21 and S 11 by electromagnetic field simulations using the transition structures of Figs. 1 and 2 with Fig. 3 connector. Fig. 8 Frequency performances of S 11 of proposed Coaxial-to-Coplanar Transition. W=300 µm, S=30 µm (experiment). W=600 µm, S=48 µm (experiment). (c) W=900 µm, S=65 µm (experiment). (corresponding to Fig. 2 with Fig. 3 connector) Fig. 9 Frequency performances of S 21 and S 11 of the conventional Coaxial-to-Coplanar Transition by computer simulation. W=100 µm, S=16 µm. W=200 µm, S=24 µm. (c) W=300 µm, S=30 µm. (corresponding to Fig. 1 with Fig. 3 connector) Fig. 10 Frequency performances of S 21 and S 11 of the proposed Coaxialto-Coplanar Transition by computer simulation. W=100 µm, S=16 µm. W=200 µm, S=24 µm. (c) W=300 µm, S=30 µm. (corresponding to Fig. 2 with Fig. 3 connector) Figure 9 shows simulation results for the above three W and S combinations. The conventional transition with incorporating a tapered part in the CPW by using a connector of Fig. 3 can obtain a better performance comparing with the one of using Fig. 3 connector. The S 21 is 4 db or less up to 27 GHz. Figure 10 shows S 21 for the proposed transition with Fig. 3 conector for the above three combinations of W and S. These results also show that the mismatch at the two transitions connected to both ends of the CPW has been reduced and that the standing wave has become smaller and improves from S 21 of the conventional transition when the width of the center conductor of connected CPW becomes 300 µm or less. The S 21 is 2 db or less up to 27 GHz. As the simulation results in Figs. 9 and 10 demonstrate, the proposed transition s result (W=100 µm, S=16 µm) shows an improvement in S 21 of about 2 db for the higher frequency range above 20 GHz and an improvement of about 5 db in S 11 likewise for the higher frequency range above 20 GHz compared with the conventional one. It was also found that the proposed method is independent of CPW shape (W and S) and that it exhibits good matching performances in comparison to the conventional method in high-frequency bands above 15 GHz. As explained, the transmission performances of the conventional transition were compared to the transmission performances of the proposed transition using two different types of connectors in which the inner conductors diameters are different. Much better transmission performances were obtained from the proposed transition when a connector with a large-radius inner conductor (Fig. 3) was used. In contrast, better performances were obtained from the conventional transition when a connector with a small-radius inner conductor (Fig. 3) was used. However, when a connector with a small-radius inner conductor was used in the proposed transition, the improvement in transmission performances was even more pronounced than in the conventional transition. 4. Electric Field Distribution of Coaxial-to-Coplanar Transition We performed electromagnetic field simulations at 20 GHz to compute the absolute-value intensity distributions of high-frequency electric fields for the transition structures of Figs.1 and 2 using the (W=300 µm, S=30 µm) combination. The electric-field distribution for the substrate surface and that for the cross section that includes the plane of symmetry about the transition center are shown in Figs. 11 and, respectively, for the Fig. 1 transition with Fig. 3 connector and in Figs. 12 and, respectively, for the Fig. 2 transition with Fig. 3 connector. In both figures, the input power into a coaxial guide is assumed to be 1 W. In the conventional transition shown in Fig. 11, the electric field in the transition section extends as far as the outer side of the coaxial wave guide s external conductor. In addition, it is necessary to consider the generation of higher

2034 Fig. 11 Electric field s distribution in conventional Coaxial-to-Coplanar transtion at 20 GHz (W=300 µm, S=30 µm). Topplanview. Cross sectional view. (corresponding to Fig. 1 with Fig. 3 connector) Fig. 12 Electric field s distribution in proposed Coaxial-to-Coplanar transtion at 20 GHz (W=300 µm, S=30 µm). Topplanview. Cross sectional view. (corresponding to Fig. 2 with Fig. 3 connector) order modes on both coaxial side and CPW side to satisfy the boundary condition at the discontinuation of coaxial to CPW transition. Especially, the higher-order modes on the CPW side extend as a surface wave to sites far away from the CPW s center conductor on the ground plane of the CPW s substrate surface. Radiation occurs in this propagation process where the high frequency energy propagates on the substrate surface through the higher-order modes. But in the proposed transition shown in Fig. 12, we see that there is little generation of higher-order modes and that the dominant mode of the coaxial wave guide makes a smooth transition to the dominant mode of the CPW. Next, we also performed electromagnetic field simulations at 20 GHz to compute the absolute-value intensity distributions of high-frequency electric fields for the transition structures of Figs.1 and 2 using the (W=100 µm, S=16 µm) combination. The electric-field distribution for the substrate surface and that for the cross section that includes the plane of symmetry about the transition center are shown in Figs. 13 and, respectively, for the Fig. 1 transition with Fig. 3 connector and in Figs. 14 and, respectively, for the Fig. 2 transition with Fig. 3 connector. The input power into a coaxial guide is also assumed to be 1 W in both figures. Looking at the electromagnetic field distribution (Fig. 13) of the conventional transition when the connec- Fig. 13 Electric field s distribution in conventional Coaxial-to-Coplanar transtion at 20 GHz (W=100 µm, S=16 µm). Topplanview. Cross sectional view. (corresponding to Fig. 1 with Fig. 3 connector)

KAMEI et al.: WIDE-BAND COAXIAL-TO-COPLANAR TRANSITION 2035 and S of the CPW are important parameters that are determined by the requirements and conditions set forth by the constituent technologies of the liquid-crystal layer used as a transmission medium. We therefore feel that the features of our proposed coaxial-to-coplanar transition can be sufficiently demonstrated in this new device. Acknowledgments This research was supported in part by a 2006 Telecommunications Advancement Foundation Grant and a 2005 Murata Science Foundation Grant. The authors extend their appreciation for this gracious support. Fig. 14 Electric field s distribution in proposed Coaxial-to-Coplanar transtion at 20 GHz (W=100 µm, S=16 µm). Topplanview. Cross sectional view. (corresponding to Fig. 2 with Fig. 3 connector) tor with the small-radius inner conductor was used, we can clearly see that the spreading out of the electromagnetic field from the outer conductor of the coaxial connector was less than that in the electromagnetic field distribution shown in Fig. 11. Furthermore, surface waves spreading from the center conductor on the substrate surface of the CPW were also reduced. From Fig. 14, it is clear that conversion from the coaxial dominant mode to the CPW dominant mode was performed more smoothly when the connector with the smallradius inner conductor was used in the proposed transition. 5. Conclusion There are an infinite number of W and S combinations that can achieve a CPW with a characteristic impedance of 50 Ω, and of these, the specific W and S combination as needed by a particular device is determined by the microwave or millimeter-wave application in question. This paper proposed a new coaxial-to-coplanar transition independent of the values of W and S. The proposed transition directly couples on the same plane the radial high-frequency electric field of the coaxial wave guide with the high-frequency electric field in gap S of the CPW. This scheme generates little higher-order modes and exhibits significant improvement in transmission performances compared to the conventional transition particularly at high-frequency bands above 15 GHz. In the future, we plan to apply the proposed transition to a CPW-type liquid-crystal microwave phase shifter. In this phase shifter, which is now under development, W Toshihisa Kamei was born in Tokyo on January 22, 1965. He received the B.S. degree in electrical engineering from The Tamagawa University in 1988, and The M.S. and Dr. Eng. degrees from The Tamagawa University in 1990 and 1996. Since 1993 he has been with The National Defense Academy and is currently research associate in The Department of Communications Engineering. From 1998 to 1999, he was a visiting researcher at The California Institute of Technology, USA. He has been engaged in research on design of waveguide filters, delay equalizer and small antennas, and the microwave adaptive devices using liquid crystal. He is a member of IEEE. Yozo Utsumi was born in Osaka, Japan, on August 3, 1943. He received the B.Eng., the M.Eng. and the Dr. Eng. Degrees from The Osaka Univ. in 1966, 1968 and 1984 respectively. He joined NHK (Japan Broadcasting Corporation), Tokyo, in 1968. Begining in 1971, he worked at NHK s Technical Research Laboratories, where he had been involved in development of the low noise receiver for satellite broadcasting, and also engaged in research and development of VHF, UHF, microwave, millimeter wave circuits and optical fiber transmission system for Hi-Vision. He was installed as the director of The Radio Wave Research Division in 1990, and of The Optical and Radio Wave Research Division in 1993. Since 1994 he worked at their Technical Administration Department, where he had been engaged in development of BS digital broadcasting system. He was installed as the executive director of The Development Center. Also he moved to The National Defense Academy, where he has been involved in development of microwave adaptive devices using liquid crystal, and also engaged in research of microwave and millimeter wave circuits, and education as professor of The Department of Communications Engineering. He is a fellow of IEEE and a fellow of IIITE.

2036 Nguyen Quoc Dinh was born in Phu Tho, Vietnam on April 25, 1981. He received the B.S. degree from The National Defense Academy in 2006. He has been engaged in research on the microwave adaptive devices using liquid crystal. Nguyen Thanh was born in Hanoi, Vietnam on September 20, 1979. He received the B.Eng. degree in electrical engineering from The Military Technical Academy in Vietnam in 2003. He worked at Radar Institute in Vietnam. Since 2006 he has been M.S. level student in The Department of Electrical Engineering of The National Defense Academy in Japan. He has been engaged in research on design of microwave circuits, waveguide filters, and the microwave adaptive devices using liquid crystal.