Also known as: RL
Return loss is the ratio, expressed in decibels, of the power incident on an impedance boundary to the power reflected back from it: RL = −20·log₁₀|Γ|, where Γ is the reflection coefficient.[^wiki] A large return loss (say 20 dB or more) means very little power comes back and the match is good; a small return loss (a few dB) means a large fraction reflects and the match is poor. It measures exactly the same mismatch as the standing-wave ratio, just on a logarithmic scale.
How it works
When a wave meets a mismatched load, a fraction of its power reflects, set by the reflection coefficient’s magnitude: reflected power is |Γ|² of incident power. Return loss simply expresses that fraction in decibels and flips its sign so the number is positive and larger-is-better:
-
RL = 0 dB → Γ = 1 → all power reflected (open or short circuit). -
RL = 10 dB → Γ ≈ 0.316 → 10 % of power reflected. -
RL = 20 dB → Γ = 0.1 → 1 % of power reflected. -
RL = ∞ dB → Γ = 0 → perfect match, nothing reflected.
A rough rule of thumb across RF engineering is that a return loss above 10 dB (VSWR below about 2:1) is acceptable for most receive and many transmit uses, while sensitive or high-power systems aim for 15–20 dB or better. Note the sign convention: return loss is conventionally quoted as a positive number of decibels, so “improving” the match means the number goes up. Some instruments instead report the negative S11 value; the two describe the same thing and only the sign differs.
In practice
Return loss is a frequency-dependent curve, not a single number. An antenna presents its lowest reflection near resonance and worsens toward the band edges, so a sweep reveals the usable bandwidth as the span where the curve stays above the chosen threshold. Connectors, adaptors, and cable faults each add their own small reflections, and their contributions can add or partly cancel depending on phase and spacing — which is why a chain of individually decent parts can still show a disappointing aggregate return loss.
Return loss should not be confused with insertion loss, which is power lost passing through a component (heat, radiation) rather than power reflected back. A good filter has high return loss in its passband and low insertion loss; a bad match shows the opposite.
Relevance to SDR
For a receive-only SDR, return loss quantifies how much of the antenna-captured signal is turned away at the input connector instead of reaching the low-noise amplifier and ADC. It is measured with a vector network analyzer or a simpler antenna analyzer as the magnitude of the input S-parameter S11. Keeping return loss high across the band of interest maximises the delivered signal and therefore the signal-to-noise ratio the decoder receives.
GopherTrunk itself measures nothing in the analog domain — it consumes IQ samples after the front end — so return loss is an antenna-and-cabling property that shapes the quality of the samples GopherTrunk is handed, not something the software reports. For operators chasing a marginal control channel, checking the antenna’s return loss on the target frequency is a practical first step before blaming the decoder.
Sources
| [^wiki]: Return loss — Wikipedia, the RL = −20·log10 | Γ | definition, sign conventions, and relation to reflected power. |