Crosstalk between coaxial transmission lines

The general theory of coaxial pairs was dealt with in an article on “The Electromagnetic Theory of Coaxial Transmission Lines and Cylindrical Shields” by S. A. Schelkunoff (B. S. T. J., Oct., 1934). The present paper considers a specific aspect of the general theory, namely, crosstalk. Formulae for the crosstalk are developed in terms of the distributed mutual impedance, the constants of the transmission lines and the terminal impedances. Some limiting cases are given special consideration. The theory is then applied to a few special types of coaxial structures studied experimentally and a close agreement is shown between the results of calculations and of laboratory measurements. If the outer members of coaxial pairs are complicated structures rather than solid cylindrical shells, the crosstalk formulae still apply but the mutual impedances and the transmission constants which are involved in these formulae must be determined experimentally since these quantities cannot always be calculated with sufficient accuracy. The crosstalk between coaxial pairs with solid outer conductors rapidly decreases with increasing frequency while the crosstalk between unshielded balanced pairs increases. In the low frequency range there is less crosstalk between such balanced pairs than between coaxial pairs but at high frequencies the reverse is true. The diminution of crosstalk between coaxial pairs with increasing frequency is caused by an ever increasing shielding action furnished by the outer conductors of the pairs. Finally, crosstalk in long lines using coaxial conductors is discussed and the conclusion is reached that, unlike the case of the balanced structure, the far-end crosstalk imposes a more severe condition than the near-end crosstalk in two-way systems which involve more than two coaxial conductors.