Transmission Line Effect

Created:5/29/2020Last Updated:07/06/2023


When the frequency or trace length of an electrical signal surpasses a certain threshold, the signal on a conductor begins to exhibit transmission line effects. At this point, voltage and current are treated as traveling waves, and factors such as wave speed, signal integrity, and trace impedance become dependent on the characteristics of the transmission medium.


 Transmission lines are conductors used to carry electrical signals. At higher frequencies, the signal travels as a transverse electromagnetic wave (TEM) guided by the structure of the transmission line, such as impedance-controlled microstrip or stripline on a PCB. Waveguides, on the other hand, are specialized transmission lines typically constructed as hollow metal tubes, designed for very high-frequency signals. In PCB routing, a stripline trace acts as a parallel plate waveguide.

Practical Application

In the context of PCB microstrip or stripline signal transmission interfaces, the presence of transmission line effects is determined by examining the interface length (L) relative to the electrical wavelength (λ) of the highest spectral content of the signal. If the trace length is greater than a tenth of the electrical wavelength of the highest spectral frequency of the signal (L > 0.1λ), we consider the trace to exhibit transmission line effects.

Another way to determine if a trace exhibits transmission line effects based on propagation delay, the following timing-based rule can be applied: Propagation Delay (τ) > 0.25T_rise If the propagation delay on the transmission line from transimter to receiver is greater than 0.25 times the signal's rise time (T_rise), it suggests that the trace exhibits transmission line effects. 



Further Reading

For more detailed information on the relationship between a signal's rise time and bandwidth, you can refer to the provided reference and further reading link: "Bandwidth of a signal from its rise time" available at