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An effective method for the time-domain characterization of lossless multiconductor transmission lines with cross-sectionally inhomogeneous dielectrics is presented. Lines of this type are characterized by multiple propagation modes having different velocities. Time-domain reflectometry is used to obtain the characteristics impedance and the modal velocities of the line. A pulse or step-function response of the line is used to obtain the modal amplitudes which, in turn, determine the velocity matrix. The appropriate multiconductor transmission-line equations are solved to obtain the per-unit-length inductance and capacitance matrices in terms of the measured characteristic-impedance and velocity matrices. The method is concise and complete and identifies the propagation modes in a way that permits direct physical interpretation of the results. The time-domain experimental results for a four-conductor transmission line are presented and are found to be in good agreement with independent frequency-domain measurements.