[常见问题]Why S21 at dc can be smaller than 1, although electrically connected 2-port structure?原因

[sell=1]Why S21 at dc can be smaller than 1, although electrically connected 2-port structure? Wp0L!X=0  
(Last modified: 24 Apr. 2007) R}VEq gq  
If S21 at dc is smaller than 1, this might happen due to the following reasons: wjnQK  
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Reason 1: Port1 and Port2 have different line impedances Z1<>Z2, resulting in an amplitude of S11 at dc > 0 ( S11ampli=abs(Z1-Z2)/(Z1+Z2) ). Therefore also S21 at dc has to be physically smaller than 1. An identical case occurs, when Port1 and Port2 are explictily renormed to different line impedances. 9Vh>ty1|_  
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Solution: When renorming both ports to the same impedance, S21 at dc should be 1. ^ua8Ya  
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Reason 2: The open boundary condition (PML) acts at dc as a small resistor. syR +;  
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Solution: use magnetic boundaries, where metallic materials are touching the boundary. ZB@Bj>,bp  
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Reason 3: Mismatch of mode pattern at dc (only possible for inhomogenous port) [>y0Xf9^  
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Solution: change port evaluation frequency to 0 (or use full deembedding, which typically runs longer) \yNe5  
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Reason 4: tan-delta fit of lossy dielectrics does not result in zero conductivity at dc (a leakage current flows through the substrate) 6}xFE]Df-Y  
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Solution: try to modify the material model in a way, so that the curves eps'' and tan delta start at 0 for dc frequency. (robust check: simulate lossless dielectrics) c[RkiV3  
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了解一下楼主所说的问题

好像以前遇到过算出来的S21总是为0的情况