Why Do Electrical Waves Reflect in a Transmission...

Why Do Electrical Waves Reflect in a Transmission Line? When a voltage pulse enters a long transmission line, it does not appear everywhere instantly. Instead, it travels through the line as an electromagnetic wave. The line has a property called characteristic impedance, represented by Z-zero. While the wave is travelling, voltage and current maintain a fixed ratio determined by this impedance. Now the wave reaches the load. If the load impedance exactly matches the line impedance, all the energy enters the load. There is no reflection. But if the impedances are different, the load cannot absorb the entire incoming wave. The remaining energy travels backward as a reflected wave. At an open circuit, current cannot flow. So the reflected voltage adds to the incoming voltage, producing nearly double voltage at the end. At a short circuit, voltage must become zero. Therefore, the reflected voltage returns with opposite polarity and cancels the incoming voltage. The amount and direction of reflection are described by the reflection coefficient. This is why impedance matching is essential in communication cables, high-frequency circuits and power-system travelling-wave protection.