A class of nonuniform coupled transmission lines and their equivalent circuits

Abstract

AbstractY‐type and Z‐type parabolic‐taper single‐mode transmission lines and their equivalent circuits are first discussed. Then, by superposition of two of the above single modes, the TEM mode along a parabolic‐taper coupled transmission line (PTCTL) is analyzed. According to the choice of Y‐type or Z‐type for tapered single modes and the choice of combinations of balanced‐unbalanced modes or open‐short line modes as two independent superposed modes, there can be six types of PTCTL. Their equivalent circuits are also easily shown. Through the superposition of the voltages and currents of two Y‐type single‐mode parabolic‐taper lines, a Y‐type parabolic‐taper coupled two‐wire line is obtained. Similarly, from two Z‐type single‐mode parabolic‐taper lines, a Z‐type parabolic‐taper coupled two‐wire line is obtained. Furthermore, from a Y‐type parabolic‐taper line and a Z‐type parabolic‐taper line, we derive a mixed type of parabolic‐taper coupled two‐wire line. In any case there are two forms of combination, balanced‐unbalanced modes and short‐open line modes as superposition of two independent modes. An obtained nonuniform coupled two‐wire line does not always have the same taper. A special case of interest is that in which one of the two wires of a coupled line has decreasing taper and the other has increasing taper. Finally, a low‐pass filter is designed by the use of nonuniform coupled transmission lines and it is shown that the line length along the axis can be reduced.