Reflection Coefficient
Understanding the Reflection Coefficient
The reflection coefficient is perhaps the most fundamental parameter in RF engineering. It quantifies how much of the incident signal is reflected back at any impedance discontinuity. Every port match specification (return loss, VSWR, S11) is derived from the reflection coefficient.
Reflection Coefficient Relations
- From impedance: Gamma = (Z_L - Z_0) / (Z_L + Z_0)
- Return loss: RL = -20 log10(|Gamma|) dB
- VSWR: VSWR = (1 + |Gamma|) / (1 - |Gamma|)
- Mismatch loss: ML = -10 log10(1 - |Gamma|^2) dB
|Gamma| = 0 (perfect match): RL = inf, VSWR = 1.0
|Gamma| = 0.05: RL = 26 dB, VSWR = 1.11
|Gamma| = 0.1: RL = 20 dB, VSWR = 1.22
|Gamma| = 0.2: RL = 14 dB, VSWR = 1.50
|Gamma| = 0.33: RL = 9.5 dB, VSWR = 2.0
|Gamma| = 0.5: RL = 6 dB, VSWR = 3.0
|Gamma| = 1.0 (total reflect): RL = 0 dB, VSWR = inf
Frequently Asked Questions
What is the reflection coefficient?
The reflection coefficient Gamma is the ratio of reflected to incident wave at a discontinuity. |Gamma| = 0 means perfect match (no reflection). |Gamma| = 1 means total reflection. It is related to return loss, VSWR, and S11.
What causes reflections?
Any impedance discontinuity causes reflections. When a wave traveling on a Z0 line encounters a load Z_L != Z0, part of the wave is reflected. The bigger the impedance difference, the larger the reflection coefficient.
What is an acceptable reflection coefficient?
|Gamma| < 0.1 (RL > 20 dB, VSWR < 1.22) is good for most applications. |Gamma| < 0.05 (RL > 26 dB, VSWR < 1.11) is excellent. |Gamma| > 0.33 (RL < 10 dB, VSWR > 2.0) indicates significant mismatch requiring attention.