Lossy transmission line

This component is a two-port network that represents a lossy wire, or cable, through which an electrical signal propagates. 

Multisim uses the distributed model to represent a lossy transmission line. In the distributed model all of the transmission line parameters (resistance, conductance, capacitance, and inductance) are continuously distributed throughout the length of the wire/cable. 

The distributed line model is divided into infinitesimally small line segments (Δx). Each transmission line parameter is infinitesimally small, and the connections between the elements are not treated as perfect conductors (that is, they have impedance). 

The distributed model is in contrast to the simpler lumped element model, which assumes that the transmission line parameters (resistance, conductance, capacitance, and inductance) are lumped into ideal electrical components that are connected by perfectly conducting wires. This approach requires a finite number (usually large) of line segments in order to accurately model the behavior of the transmission line. The more line segments that are used the better the lumped element model will match the actual behavior of the transmission line. This usually leads to very large netlists and long simulation times. Unlike the lumped model, the distributed model does not require the determination of the correct number of line segments. 

During transient analysis, the simulator uses the impulse response convolution method to solve the distributed line model. The simulator will limit the simulation step size to accurately model the frequency response of the transmission line. For this reason, short transmission lines will have long simulation run times. 

The uniform constant-parameter distributed transmission line model can be used to model the following types of lines:

  • RLC (uniform transmissin lines with series loss only)
  • RC (uniform RC lines)
  • LC (lossless transmission lines)
  • RG (distributed series and parallel conductance).