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If you want more information about S parameters, you can read "Microwave Engineering" by David M. Pozar. This book explain about S parameters with clear enough.
For antenna example, S11 is ok when the value is usually below -10 dB. While S21, S12, etc. is important for element that has more than one port. For example, directional coupler has 4 port, so it has S11, S12, S13, S14, etc.
Sxj , is the ration between the voltage wave in the port x , and the voltage wave in the port j
S11 , represent the reflection coefficient , S21 represnt the output wave to the input wave which is the gain ,
Sij=Vi-/Vj+, where I is the output port and J is the input port, - is incident onto the port from the network and + is incident from the port to the network. The port has a reference plane from which we relate everything to.
But how is this valid in non-TEM waveguides?
Problem 1: We can't measure V or I in RF. (Remember we redefined voltage and current).
Solution: Measure it using power. (Sij=sqrt(Pi-)/sqrt(Pj+), where P+=0.5|V+|2/Z, P-=0.5|V-|2/Z, so we defined the a=V+/sqrt(Z), b=V-/sqrt(Z). and the powers become P+=0.5|a|2, P-=0.5|b|2. Therefore Sij=b/a.
Problem 2: This wll cause the S parameters to be dependant on Zc.
Solution: We'll use Zc=50 Ohms as our standard, relate everything to it.
Problem 3: We need a matched load over the RF spectrum to be able to measure the S parameters.
Solution: Use the standard 50 Ohms load.
Solution 3: Futher discussions
Some people will say the standard load termination will still cause reflections into the port, thats correct, but it'll cause no input wave from the terminated port. i.e. The reflected wave is reflected from the inside of the port, not incident from outside the refrence plane (remember V+ is defined as incident from outside the refrence plane), so it won't contradict out definition of the S-parameters.
You can check Wikipedia's link on S parameters for more info
I found your first answer to be appropriate for the spirit of the original question. What I liked was the simple, straightforward response that provided insight into what s-parameters describe.
I have answered questions in a similar way and often felt the need to respond with more detail as you have here. I understand your dilemma.
The other answers provided were also valid and I have no argument with any of them. Again, I liked your brief response.
to understand whether s-parameters values are good/bad, you need to really understand what each s-parameter stands for (SAJ25 post is good and succinct on this).
Then you also need to know what the device you are measuring is supposed to be doing:
- are you expecting power to move between ports x and y (for Sxy) and in which direction?
etc
Probably not needed now, but if anyone comes across this. When reading up on s-parameters, just think, S_ab is the wave out port a from an input at port b. This is only for matched loads at all ports though.
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