Return Loss & VSWR
Impedance match quality — VSWR, return loss, and reflection coefficient.
Reference
Formulas
- Reflection coef. Γ
- = (Z_L − Z_0) / (Z_L + Z_0)
- Return loss
- = −20 · log₁₀(|Γ|) (dB, positive number)
- VSWR
- = (1 + |Γ|) / (1 − |Γ|)
- |Γ|
- = (VSWR − 1) / (VSWR + 1)
- Power reflected
- = |Γ|² · P_incident
Quick table
| VSWR | Return loss | |Γ| | Power reflected |
|---|---|---|---|
| 1.0 : 1 | ∞ dB | 0 | 0% |
| 1.1 : 1 | 26.4 dB | 0.048 | 0.23% |
| 1.2 : 1 | 20.8 dB | 0.091 | 0.83% |
| 1.5 : 1 | 13.9 dB | 0.200 | 4.0% |
| 2.0 : 1 | 9.5 dB | 0.333 | 11.1% |
| 3.0 : 1 | 6.0 dB | 0.500 | 25% |
| 5.0 : 1 | 3.5 dB | 0.667 | 44.4% |
| 10 : 1 | 1.7 dB | 0.818 | 66.9% |
| ∞ : 1 | 0 dB | 1.0 | 100% (short or open) |
Guidelines
- Excellent match
- VSWR ≤ 1.2 (RL ≥ 20 dB)
- Good match
- VSWR ≤ 1.5 (RL ≥ 14 dB)
- Acceptable
- VSWR ≤ 2 (RL ≥ 10 dB)
- Bad — transmitter at risk
- VSWR > 3
Notes
- Most transmitters fold-back or shut down above VSWR ~2:1 to protect output stages.
- Conjugate match maximizes power delivered; equal impedance minimizes reflections. For 50 Ω / 75 Ω real systems they coincide.
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