314 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 42, NO. 4, NOVEMBER 2000 The Improvement of Tri-Plate Line Performance By Using Corrugated Transitions Ruey Bing Hwang, Member, IEEE Abstract—A novel structure is investigated for the improvement of tri-plate line (TPL) performance. Based on experience in the design of horn antennas, we introduce corrugations of the tran- sition section of the TPL in order to achieve better field uniformity and to reduce the return loss. A corrugated transition waveguide may be viewed as the cascade of bifurcated metal waveguides. We employed the building-block approach, which breaks the overall structure into cascaded subcells, each of which may be analyzed rigorously by the mode-matching method. Thus, the input–output relation of each subcell is obtained as well as that of the overall structure. This enables us to carry out a parametric study on the field distribution inside the TPL test fixture, so that we can opti- mize its design for any required specifications. Index Terms—Bifurcation, corrugated transitions, method of mode matching, tri-plate line (TPL). I. INTRODUCTION A CCORDING to the recommended practice SAE J1113-25 “electromagnetic compatibility measurement procedure for vehicle components-immunity to radiated electromagnetic fields 10-kHz to 200-MHz tri-plate line (TPL) method,” [1], [2] a TPL is employed to facilitate the testing of automotive electronic components such as transmission control modules, sensors, and the other subsystems associated with wire har- nesses. A TPL looks like a Crawford cell without side walls [10], as shown in Fig. 1(a). It is designed to generate a uniform electric field in the transverse plane for electromagnetic sus- ceptibility (EMS) testing [3], [6], [7]. The fields are transferred through a taper transition from a small parallel-plate waveguide to a large one in which the equipment under test (EUT) is placed [4]. However, the shape of taper transition should be well designed to maintain the field uniformity inside the uniform region as well as to reduce the return loss [5]. In this paper, the taper transition of a TPL is viewed as a horn antenna and the technique of a corrugated horn antenna is em- ployed for the design of a TPL with fins. We observe that a TPL with fins may be considered as the cascade of different subcells, each of which contains a finite length of uniform metal wave- guide and a bifurcated metal waveguide. To simplify the for- mulation of this problem, we adapt the building-block approach such that the input–output relation of each subcell is obtained Manuscript received September 3, 1999; revised May 23, 2000. This work was supported by the National Science Council under Contract NSC 89-2213- E009-074 and by the Ministry of Education and the National Science Council, R.O.C., under Contract 89-E-FA06-2-4. The author is with the Microelectronics and Information Systems Research Center, National Chiao-Tung University, Hsinchu, Taiwan 300, R.O.C. (e-mail: rbhwang@eic.nctu.edu.tw). Publisher Item Identifier S 0018-9375(00)10200-5. (a) (b) Fig. 1. Structural configurations. (a) Conventionally used TPL. (b) TPL with corrugated transitions. first by the rigorous mode-matching method. Then the results are combined to yield the input–output relation of the overall structure. This paper is organized as follows. A detailed description of the problem is given in Section II, including the structure pa- rameters. In Section III, the input–output relations of a typical subcell, which consists of a bifurcated waveguide and a uniform waveguide, is obtained by the rigorous mode-matching method. The building-block approach is then employed to deal with the field analysis of the cascaded subcells. Numerical examples are given in Section IV. Finally, some concluding remarks are given in Section V. II. STATEMENT OF THE PROBLEM As described in the preceding section, the taper transition is used to reduce the return loss and to maintain the field unifor- mity. However, in this paper, stress is placed on the characteris- tics of wave propagation in the transition region. The structure of nonuniform parallel-plate waveguide (PPWG) with fins is considered as a prototype to study the physical mechanism in order to develop criteria of design for the TPL with fins. As shown in Fig. 1(b), the structure is constructed as the cascade of subcells, each of which consists of a bifurcated waveguide and a uniform waveguide of finite length. To demonstrate the 0018–9375/00$10.00 © 2000 IEEE