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Can S11 or S21 be less than 0?
Posted Apr 15, 2015, 9:45 p.m. EDT RF & Microwave Engineering Version 4.4 2 Replies
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Hi,
All
In the module 'Frequency Selective Surface, Periodic Complementary Split Ring Resonators' on the website of COMSOL, we can get analysis of S-Parameter by 1D plot group. The module has already got results of S11dB and S21dB, what I want to analysis are S11 and S21, so I add emw.S11 and emw.S21 in a new 1D plot group without any other operation. After plot, what I got makes me confused: I saw part values of S11 and S21 are LESS THAN 0.
I wornder if I understand this physical significance in a correct way, because when I repeat paper 'oe-21-5-6153,' I got negative value, too.
Please help me out!
THANK YOU!
BEST,
Y,Zou
All
In the module 'Frequency Selective Surface, Periodic Complementary Split Ring Resonators' on the website of COMSOL, we can get analysis of S-Parameter by 1D plot group. The module has already got results of S11dB and S21dB, what I want to analysis are S11 and S21, so I add emw.S11 and emw.S21 in a new 1D plot group without any other operation. After plot, what I got makes me confused: I saw part values of S11 and S21 are LESS THAN 0.
I wornder if I understand this physical significance in a correct way, because when I repeat paper 'oe-21-5-6153,' I got negative value, too.
Please help me out!
THANK YOU!
BEST,
Y,Zou
2 Replies Last Post Apr 22, 2015, 11:55 p.m. EDT
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Posted:
10 years ago
Actually, S11dB, S21dB, etc. should by definition always (I think) be negative. Sometimes the negative of actual S parameters in dB are reported in practice (when the negative is understood), but that is not technically correct.
We have seen COMSOL report positive values for S parameters in dB on occasion when there were problems in the simulations. We think we have seen occasions (at least in 4.2) where COMSOL reported incorrect absolute values of S parameters but had the correct value in dB. Haven't checked that in a recent version, but suggest you look at S dB parameters, and suggest you see if 4.3b or 5.1 show similar results (we had many problems with 4.4).
David
We have seen COMSOL report positive values for S parameters in dB on occasion when there were problems in the simulations. We think we have seen occasions (at least in 4.2) where COMSOL reported incorrect absolute values of S parameters but had the correct value in dB. Haven't checked that in a recent version, but suggest you look at S dB parameters, and suggest you see if 4.3b or 5.1 show similar results (we had many problems with 4.4).
David
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Posted:
10 years ago
S11 and S21 are complex numbers. The real and/or imaginary parts of these numbers can easily be positive or negative. In typical real-world engineering problems, these numbers have magnitudes less than 1, since they represent signal couplings from either a port to itself (S11, a reflection) or to another port (S21, a transmission). The phases of these numbers generally depend on the port geometry, mode(s), the media, the terminating impedances involved, etc. Do a search on "S-matrix" on the internet to learn more.