Crosstalk or coupling phenomenon between two parallel conductors with a ground or reference conductor is well known in electromagnetic compatibility and microwave circuit fields. In this paper we consider the characteristics of a long two-conductor line embedded in an inhomogeneous medium while taking account of the difference between two phase constants in independent propagation modes of two parallel lines. The characteristics of near-end and far-end crosstalk and of through-end transmission are discussed for frequencies up to more than the frequency where the line length is 10 times the wavelength. Interesting properties, such as very strong and null crosstalk phenomena that occur at the far end and non-transmission at the through end, appear in the envelope characteristics. The reason for these properties is clarified by making theoretical and experimental investigations.

For common-mode noise current reduction, a CMC (Common Mode Choke) is widely used in signal transmission line circuits consisting of a ground and two conductors (a balanced transmission line). However, a common-mode noise current reduction characteristic is not clearly analyzed yet in the case that a CMC is inserted in a balanced transmission line. In this paper we propose the calculation method of CMC insertion losses and derive an equation to analyze the common-mode current characteristics of a balanced transmission line with a CMC inserted. The analyzed frequency range is from 100 kHz to 100 MHz. We conclude that in the frequency range up to 30 MHz: (1) the proposed insertion loss calculation method is useful for analyzing CMC insertion losses in differential-mode and in common-mode; (2) the derived circuit equation can be applied for analyzing the common-mode current characteristics of a balanced transmission line locally unbalanced with conditions of a CMC inserted; (3) the proposed calculation method may give the expected results that a CMC should be placed in a signal source side of an unbalanced point of a pair-cable for reduction of common-mode currents; and (4) if it is placed in a terminal (or load) side of an unbalanced point, there is no effect, or rather common-mode currents are increased by the insertion of a CMC.