Scattering Parameters: Basic questions

[dalarev]

Scattering parameter say, S11, can be defined as the ratio of the reflected wave from port 1 to the incident wave on port 1. If it is a 2-port network, we imply the condition that the load at port 2 is matched to the characteristic impedance of the line.

Take this excerpt from another website:

"If the output port 2 is terminated, that is, the transmission line is connected to a matched load impedance giving rise to no reflections, then there is no input wave on port 2."

My confusion it the "no input wave on port 2" part. A matched load impedance at port 2 will give rise to no reflections. Fine, I believe that. That can be seen from the definition of the reflection coefficient, Gamma.

However, there is no input wave on port 2? If there is no incident wave on port 2 to begin with, then why does the load impedance need to be matched? There is no danger of reflecting anything anyway, so why match the load (at port-2) to the line?

 

[Valkarie]

A:A load that is not matched to the line will reflect some of the incident wave. This is why it is important to match the load to the line, so that no reflections occur. If the load is matched, then there will be no input wave on port 2, as all the power will be absorbed by the load.

 

[Mene]

I can provide some clarification on this topic. Scattering parameters, also known as S-parameters, are commonly used in the field of radio frequency engineering to describe the behavior of a two-port network. They are used to characterize the transmission and reflection of electromagnetic waves at the input and output ports of a network.

In the excerpt provided, the author is discussing the concept of a matched load impedance at port 2. This means that the impedance of the load connected to port 2 is equal to the characteristic impedance of the transmission line. When this condition is met, there will be no reflections at port 2, as all of the incident energy will be absorbed by the load.

The statement "there is no input wave on port 2" is referring to the fact that in this scenario, there is no wave traveling from the load back towards the network. This is because the matched load impedance ensures that all of the incident energy is absorbed and there is no reflected wave. Therefore, there is no need for an input wave on port 2, as it would not contribute to the overall behavior of the network.

The reason for matching the load impedance at port 2 to the characteristic impedance of the line is to ensure maximum power transfer and minimize reflections. Even though there may not be any incident waves on port 2, it is still important to match the load impedance to prevent any potential reflections that could occur if the load impedance was not properly matched.

In summary, the concept of a matched load impedance at port 2 is to ensure that there are no reflections and maximum power transfer, even though there may not be any incident waves on that port. This is a common practice in radio frequency engineering to optimize the performance of a two-port network.