Learning Forum

[CB]

How does a transmission line signal reflection or transmission happen?

[NKC]

Hi, 
A transmission line carries a signal. As it propagates down a transmission line the signal will travel along the line until it hits some discontinuity in its path. A discontinuity can be any impedance mismatch or change a change in boundary condition. If the transmission line is a trace on a printed circuit board the trace can be split from one line to two lines hence a change in the physical model of the line as it is split. Depending how the split section is designed the some of the signal will travel through to the two new lines and some will be reflected back to the single line. This all depends on the impedance of the line sections. The signal will go along with little reflection if the two split lines has an impedance less than the single transmission line. Depending upon the magnitude and phase of the reflected signal back onto the single transmission line the incoming signal can be distorted. Besides a physical split in the PCB transmission line a physical component ( active or passive) placed in line with the trace will also cause some reflections as will a via. For wireless transmission, if the signal is transmitted in air and the signal sees a wall, building some different material the signal sees a change in its boundary condition and like above some of the signal will be transmitted or reflected depending on the material properties of the objects on either side of the boundary. If the signal sees a higher impedance when it propagates into the material at the other side of the boundary the signal will be reflected as in the case with the transmission line. So both impedance mismatch or boundary conditions matter when determining if a signal is transmitted or reflected.