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Reflection Coefficients of Transmission Line

Just as a plane wave is partially reflected at a media interface when the intrinsic impedances on either side of the interface are dissimilar (η1 ≠ η2), the guided wave on a transmission line is partially reflected at the load when the load impedance is different from the characteristic impedance of the transmission line (ZR ≠ Z0).

In a mismatched line, only a part of the incident power is absorbed by the load. The remainder goes back along the line towards the source and is known as the reflected power. It implies that in a mismatched line the power is transferred in both directions along the line. However, in this case, the voltage-to-current ratio must be the same for the reflected power and for the incident one, because this ratio is determined by the characteristic impedance of the line.

The actual voltage at any point along the line is the vector sum of the incident voltage and of the reflected voltage, taking into account the phases of each component. The same is true for the current. The effect of the incident and reflected components on the behaviour of the line can be understood by considering two limiting cases:
  1. Short-circuited line
  2. Open-circuited line

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