Help needed on defniition of S12 vs S21
My belief is that if the system is perfectly matched (S11=S22=0), then the ratio of the signals at port 1 to port 2 are given by S21. So if the input to port 1 is 1 V, the output from port 2 is 10 V, then S21 is 10.
My colleage believes the opposite, in other words S12 would be 10. Reading
http://www.antenna-theory.com/defini...parameters.php
where it says:
S-parameters describe the input-output relationship between ports (or terminals) in an electrical system. For instance, if we have 2 ports (intelligently called Port 1 and Port 2), then S12 represents the power transferred from Port 1 to Port 2. S21 represents the power transferred from Port 2 to Port 1. In general, SNM represents the power transferred from Port N to Port M in a multi-port network.
My belief is the information at http://www.antenna-theory.com/defini...parameters.php is wrong, so I'm intersted in the views of others.
I must admit, I tend to trust http://www.antenna-theory.com/ far more than Wikipedia, but in this case, I think Wikipedia
http://en.wikipedia.org/wiki/Scatter...t_S-Parameters
is right.
Dave
Hi,
The S21 is the S transmit parameter from the port 1 to the port 2 and the ratio of a2/a1.
a1 is the complex electrical field 'measured on port 1' and a2 is the complex electrical field 'measured on port 2' .
The Sij parameters are the complex value type with two components : amplitude and phase or, real and imaginary.
If the modulus of S21=0 (-inf in dB) => not energy transmitted.
If the modulus of S21=1 (0 dB) => all energy transmitted, zero loss ! from port 1 to port 2 : a1=a2 and log(1)=0dB
I hope to be clear...
BR
The first paragraph on the antenna theory page has multiple errors. Interestingly though the paragraph right below figure 1 does get the relationship right.
In the above Figure, S21 represents the power received at antenna 2 relative to the power input to antenna 1. For instance, S21=0 dB implies that all the power delivered to antenna 1 ends up at the terminals of antenna 2 (which isn't physically possible). If S21=-10 dB, this implies that 1 Watt delivered to antenna 1 (or 0 dB), ends up as -10 dB at antenna 2, or 0.1 Watts.
I agree, there are multiple errors. Most obviously a confusion between power and voltage.
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I agree, there are multiple errors. Most obviously a confusion between power and voltage.
Had higher hopes for that website when first encountering it sometime back, but, sad to say it looks almost like a simple 'industry front' website ... this (these errors) only go to underscore or reinforce that thought.
Compare that website with:
. . http://www.microwaves101.com/
run by somebody with I would judge to have actually worked in the field with his subject.
_Jim