✍ Scribed by Takao Fujii; Tadashi Kawai; Isao Ohta; Yoshihiro Kokubo
No coin nor oath required. For personal study only.
In a coupled-line quarter-wave directional coupler consisting of inhomogeneous waveguides such as microstrip lines, directivity is degraded due to the difference in the phase velocity of the even and odd modes. In order to rectify this problem, various methods have been proposed. In the present paper, it is shown that the characteristics can be improved with an extremely simple matching circuit based on a design method using the equivalent admittance. Specifically, by providing some freedom in the setting of even and odd mode impedances in the coupled line section, more effective improvement of characteristics is achieved. First, a design chart is presented for the transmission line width and spacing, with the coupling coefficient at the time of perfect matching as a parameter. Next, it is demonstrated that the directivity is substantially improved if a 60-Ω distributed coupled line is matched to 50-Ω input and output transmission lines via a simple impedance step matching circuit. Further, by including a short-circuited quarter-wave stub, a directivity of more than 20 dB over a relative bandwidth of more than 47% is obtained. The validity of the present design method is confirmed by means of an electromagnetic field simulator (HFSS) and a test experiment.
📜 SIMILAR VOLUMES