| Summary: | 碩士 === 國立雲林科技大學 === 電機工程系 === 103 === This thesis proposes novel designs of branch-line and rat-race couplers. As compared with conventional couplers, whose structures are symmetric with respect to both the horizontal and vertical plane of symmetry (POS), the couplers proposed here are symmetric with respect to only the horizontal POS. This leads to three, instead of two, possibly different characteristic impedances (Z1, Z2, and Z3) and electrical lengths (θ1, θ2, and θ3) required for the new designs. The additional impedance Z3 can be used to adjust the circuit bandwidth. In the analysis, even and odd circuit analyses were first applied to find the corresponding ABCD matrices, which were subsequently transformed into scattering matrices. From such analyses, scattering parameters of the entire coupler and the required characteristic impedances can be determined. For validation, we have designed and fabricated six single-band couplers, all with the same center frequency of 2.45 GHz. These circuits include branch-line and rat-race couplers with the power ratios of 1:1, 2:1, and 3:1, summing up to six of them.
For applications where two communication bands are required in the same circuit, dual-band brand-line and rat-races couplers were also designed and fabricated in this study. For the former, the center frequencies are 0.9/2.45 GHz, and for the latter, 0.915/2.45 GHz. To achieve dual-band operations, each transmission-line sections in the previous designs were replaced with three sections, the middle of which was further substituted with an all-pass coupled-line section. Formulas available in the literature were applied to find the appropriate lengths and characteristic impedances for the three sections in order to obtain the prescribed ratio of the dual operating bands.
The circuit simulator Microwave office and full-wave simulator IE3D were both employed to carry the simulations. The circuits were designed and fabricated using either R04003 or FR4 substrates, and measured using an Agilent E5071C vector analyzer. Results showed that the simulation agree reasonably well with the measurement.
Keywords：All-pass coupled line、Ring coupler、Branch line coupler
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