Four-Port Coupler S-Parameter Definitions
Port 1 = Input, Port 2 = Through, Port 3 = Coupled, Port 4 = Isolated
S21: Through path insertion loss (main line, should be ~0 dB for loose coupling)
S31: Coupling factor (power sampled to monitor port, e.g., −10 dB = 10%)
S41: Isolation (leakage to isolated port, should be << S31)
S11: Input return loss (should be <−20 dB for good coupler)
S22: Through port return loss (should be <−20 dB)
S33: Coupled port return loss (should be <−15 dB)
Directivity = Isolation − Coupling = −20·log₁₀|S41| − (−20·log₁₀|S31|)
= 20·log₁₀(|S31|/|S41|) [dB, higher = better]
Performance Specifications by Coupling Value
| Coupling (dB) | Main Path Loss | Directivity (typical) | Use Case |
|---|---|---|---|
| −3 dB (50%) | −3 dB | 20–30 dB | Power splitter function |
| −10 dB (10%) | −0.46 dB | 25–35 dB | Power monitoring, ALC |
| −20 dB (1%) | −0.04 dB | 25–40 dB | Low-perturbation sampling |
| −30 dB (0.1%) | −0.004 dB | 25–40 dB | Precision VSWR bridge |
Analysis Steps in RF View
- Load coupler .s4p file → extract port pairs using SNP Converter
- Main path: ports 1,2 → s21_through.s2p → verify S21 ≈ expected through loss
- Coupled path: ports 1,3 → s31_coupled.s2p → verify S21 = coupling factor ±0.5 dB flat across bandwidth
- Isolated path: ports 1,4 → s41_isolated.s2p → S21 should be < (coupling − directivity)
- Directivity = S31 − S41 → should be >25 dB for good measurement coupler
Why Directivity Matters for VSWR Measurement
When measuring VSWR with a coupler: Minimum detectable |Γ| = 10^(−Directivity/20) Directivity = 25 dB → min |Γ| = 0.056 → min VSWR = 1.12:1 → 26 dB RL Below this, reflected and direct leakage signals mix, giving incorrect VSWR reading. For VSWR measurements >40 dB RL: need Directivity > 40 dB → use calibrated VNA.
RF View Coupler Analysis: Load 4-port coupler .s4p, extract coupling (S31) and isolation (S41) paths, measure directivity = S31 - S41 in dB with delta marker. Free on Android.