It doesnвЂ™t matter whether we are dealing with digital pulses or harmonic AC waves, an incoming wavefront of an electromagnetic wave can reflect off of the interface between two materials. S-parameters and Input ImpedanceДЄll transmission lines are media used to direct propagation of an electromagnetic pulse or wave. Transmission Line Reflection Coefficient vs. Instead, we need S-parameters and input impedance to properly describe signal behavior at an impedance discontinuity along a transmission line. In a channel with finite size and definite geometry, signals will not propagate as plane waves and their reflection cannot be described using the transmission line reflection coefficient. Unfortunately, most designers who are not versed in signal integrity analysis may not know that the reflection coefficient is not a complete metric for describing reflection from the load on a transmission line. New designers often refer to the reflection coefficient to describe reflections off the load end of a transmission line. Plane waves reflecting off of water are described with a reflection coefficient. The reflection of a plane wave can be perfectly described using a reflection coefficient, but this is not the whole story in a complex structure like a printed circuit board.Дesigners need to use input impedance and S-parameters to describe reflections in transmission lines. All electromagnetic waves experience some reflection when they reach the interface between two media that have refractive index contrast.

How to design transmission lines