Short Answer
S11 and return loss describe the same physical measurement — the reflected power at port 1 — but use opposite sign conventions. S11 is a complex number; when expressed in dB for a passive device, it is always negative. Return loss is defined as the positive version of that same number.
Formula Comparison
S11 (dB) = 20·log₁₀|Γ| [negative for passive: e.g., −10 dB] Return Loss (dB) = −20·log₁₀|Γ| [positive: e.g., +10 dB] Return Loss = −S11(dB) S11 = −Return Loss Examples: S11 = −10 dB → Return Loss = +10 dB → |Γ| = 0.316 → VSWR = 1.92 S11 = −20 dB → Return Loss = +20 dB → |Γ| = 0.100 → VSWR = 1.22
Why Two Conventions Exist
S11 comes from the scattering matrix formalism where negative dB means the reflected wave has smaller amplitude than the incident wave. Return loss (RL) was defined earlier in the context of transmission line theory where engineers wanted a positive number that got larger as matching improved ("more return loss = better"). Both conventions are used in industry:
- VNA displays: Often show S11 in dB (negative) — you see −10 dB on screen
- Datasheets: Often specify "Return Loss > 14 dB" (positive number)
- Antennas: "VSWR < 2:1" corresponds to "RL > 9.5 dB" or "S11 < −9.5 dB"
Specification Interpretation Table
| Datasheet Says | Means | VNA Screen Shows |
|---|---|---|
| RL > 14 dB | S11 < −14 dB | Trace below −14 dB line |
| RL > 9.5 dB | S11 < −9.5 dB, VSWR < 2:1 | Trace below −9.5 dB line |
| S11 < −20 dB | RL > 20 dB | Trace below −20 dB line |
RF View: RF View can display both "S11 in dB" (negative convention) and "Return Loss in dB" (positive convention) — switch with one tap. VSWR view also shows the same measurement in VSWR units.