Chapter: Transmission Lines and Waveguides - Transmission Line Theory

Reflection Coefficient

The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered.

REFLECTION COEFFICIENT

 

Reflection coefficient

 

The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave. The reflection coefficient is closely related to the transmission coefficient.

 

 

Reflection occurs because of the following cases:

 

1)when the load end is open circuited

 

2)when the load end is short-circuited

 

3)when the line is not terminated in its characteristic impedance.

 

When the line is either open or short circuited, then there is not resistance at the receiving end to absorb all the power transmitted from the source end. Hence all the power incident on the load gets completely reflected back to the source causing reflections in the line. When the line is terminated in its characteristic impedance, the load will absorb some power and some will be reflected back thus producing reflections.

 

Reflection Coefficient can be defined as the ratio of the reflected voltage to the incident voltage at the receiving end of the line Reflection Coefficient K=Reflected Voltage at load /Incident voltage at the load.

K=Vr/Vi

 

Telecommunications

 

In telecommunications, the reflection coefficient is the ratio of the amplitude of the reflected wave to the amplitude of the incident wave. In particular, at a discontinuity in a transmission line, it is the complex ratio of the electric field strength of the reflected wave (E − ) to that of the incident wave (E + ). This is typically represented with a Γ (capital gamma) and can be written as

 


 

The reflection coefficient may also be established using other field or circuit quantities.

 

The reflection coefficient can be given by the equations below, where ZS is the impedance toward the source, ZL is the impedance toward the load:


Simple circuit configuration showing measurement location of reflection coefficient.


Notice that a negative reflection coefficient means that the reflected wave receives a 180°, or π, phase shift.


The absolute magnitude (designated by vertical bars) of the reflection coefficient can be calculated from the standing wave ratio,

 

SWR:


 

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