Transients in lossy transmission lines

TL;DR

This paper uses numerical inverse Laplace transforms to analyze how conductor and dielectric losses affect the transient response of non-ideal transmission lines, and explores methods to approximate and improve modeling accuracy.

Contribution

It demonstrates that low-loss approximations are sufficient for modeling transients and investigates various factors influencing the response, including parasitic effects and cooling to reduce conductor losses.

Findings

Transient shape is sensitive to conductor losses but not dielectric losses.

Low-loss approximations accurately model the transient response.

Cooling the cable reduces conductor losses, approximating ideal line behavior.

Abstract

Numerical inverse Laplace transforms are used to analyze the transient response of non-ideal transmission lines to a voltage step. We find that the detailed shape of the transient response is sensitive conductor losses, but insensitive to dielectric losses. Furthermore, we find that the low-loss approximations for the transmission line propagation constant and characteristic impedance in the complex-frequency domain are sufficient to accurately model the observed transient response. We also investigate the effects of: (1) a parasitic capacitance terminating the open end of the coaxial transmission line, (2) the input impedance of the oscilloscope used to make the measurements, and (3) the finite rise time of the voltage step. Finally, we cool a semi-rigid coaxial cable in liquid nitrogen so as to reduced conductor lossless and observe a transient response that is closer to that expected from an ideal lossless line.