Two-Port S-Matrix
[b₁] [S₁₁ S₁₂] [a₁] [b₂] = [S₂₁ S₂₂] [a₂] S₁₁ = b₁/a₁|a₂=0 : Input reflection (port 2 terminated in Z₀) S₂₁ = b₂/a₁|a₂=0 : Forward transmission S₁₂ = b₁/a₂|a₁=0 : Reverse transmission (port 1 terminated) S₂₂ = b₂/a₂|a₁=0 : Output reflection Stored in .s2p file: # GHz S DB R 50 freq S11_dB S11_ang S21_dB S21_ang S12_dB S12_ang S22_dB S22_ang
Two-Port Network Classifications
| Network Type | Condition | Example |
|---|---|---|
| Reciprocal | S12 = S21 | Passive: filter, cable |
| Non-reciprocal | S12 ≠ S21 | Active: amplifier, isolator |
| Lossless | |S11|²+|S21|² = 1 | Ideal transmission line |
| Matched | S11 = S22 = 0 | Attenuator with matched ports |
| Symmetric | S11 = S22 | Symmetric ladder filter |
Converting Two-Port S to ABCD
ABCD matrix = transmission (chain) matrix for cascade analysis: A = (1+S11)(1−S22)/(2S21) + S12/(2S21) × ... [Full formula depends on normalization — see ABCD Matrix glossary page] Key use: cascade of N two-ports: [ABCD_total] = [T₁][T₂]...[Tₙ] RF View Circuit Simulator uses this internally for all cascade simulations.
RF View Two-Port Analysis: RF View displays all four S-parameters of any .s2p file. Select S11, S21, S12, or S22 individually. Smith chart shows S11 or S22 impedance. Free on Android.