| Summary: | 碩士 === 國立交通大學 === 電信工程研究所 === 102 === In this thesis, two types of couplers are presented. Both of them have features of arbitrary power division ratios. One of them has the large size reduction, which is compared with conventional rat-race coupler. The other one has the dual-band operation, and ability of arbitrary phase difference. The designs of these two couplers which improve performances of conventional couplers.
The first two chapters introduce the applications, operating principal and theory of branch-line coupler and rat-race coupler. In the third chapter, a novel transmission-line element that is able to exhibit a high-impedance passband is proposed and applied for coupler realization with a large power division ratio. The analysis of the transmission-line element was conducted by its equivalent circuit model and the closed-form equations for the pole and zero frequencies of the effective impedance were derived to help control the passband impedance and phase responses by the lumped parameters. The high-impedance feature of the presented artificial line was exploited to experimentally develop a 12 dB ring coupler, which occupies only 29% footprint of the conventional coupler at 3.5 GHz. Experimental results are in good agreement with the simulation data.
In the fourth chapter, a directional coupler that allows for arbitrary power division ratios as well as arbitrary phase differences at dual frequencies of interest. Explicit design equations in terms of dual-band power-dividing ratios and phase
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differences will be given here and were derived based on the even- and odd-mode decomposition analysis. To illustrate the design procedure, examples will be provided and the relevant studies on the coupler’s electrical parameters for a wide range of dual-band specifications and frequency ratio were conducted, by which the graphical solutions can be readily used as the further design tool. In addition, the resulting operational bandwidths are included and discussed for completeness. To validate our idea, four coupler prototypes were carried out according to the given guidelines. Excellent agreement is obtained between the measured and full-wave calculated results. The functional versatility of the proposed simple structure is well suited to applications in dual-band integrated modules, such as the beam-forming networks, for both loss and size reduction.
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