Characteristic impedance of multiconductor transmission lines
Many technical people are familiar with the notion of characteristic impedance when they think of coaxial cables, which are used frequently when transmitting radio-frequency power between antennas and radion transceivers. However, for transmission lines with several conductors, as seen in the figure, the characteristic impedance is represented as a matrix which represents ratios of all possible inner conductor currents to voltages between the inner conductors and the shield, i.e.
Note that the characteristic impedance gives information on inter-conductor coupling, which is very useful if one wants to study cross-talk between, for example, densely packed traces on a multilayer printed circuit board.
This downloadable paper describes the use of the finite element method for generating information on propagation and characteristic impedance information for generalised transmission lines from the full-wave solution of Maxwell's Equations. This paper appeared in the IEEE Transactions on Microwave Theory and Techniques. See my CV page for the full reference.