VSWR vs dB Loss (S11 and S21): Cutting through the common confusion
When it comes to measuring how well an antenna system is performing, especially in amateur radio, two schools of thought often emerge: those who swear by VSWR (Voltage Standing Wave Ratio) and those who claim that return loss in dB or S-parameters like S11 and S21 are more accurate or modern. Some even say VSWR is “outdated” or “inaccurate.” Let’s break it all down with both clarity and precision—without oversimplifying the physics.
What Is VSWR, Really?
VSWR is a scalar expression of how much RF power is reflected back from a load (like an antenna). It’s derived from the reflection coefficient (Γ):
VSWR = (1 + |Γ|) / (1 - |Γ|)
Γ = (ZL - Z0) / (ZL + Z0)
- ZL is the load impedance.
- Z0 is the system impedance (typically 50 Ω).
Important Notes:
- VSWR is unitless.
- A VSWR of 1:1 means perfect match (0% reflected power).
- A high VSWR (say, 5:1) means a poor match (significant power is reflected).
What About Return Loss?
Return Loss expresses the reflected power in dB:
Return Loss (dB) = -20 * log10(|Γ|)
Higher values = better match, just like lower VSWR = better match.
| VSWR | Return Loss (dB) | Reflected Power (%) |
|---|---|---|
| 1.0 | ∞ | 0% |
| 1.5 | 14.0 | ~4% |
| 2.0 | 9.54 | ~11% |
| 3.0 | 6.0 | ~25% |
| 10.0 | 1.74 | ~67% |
They’re just different lenses on the same phenomenon.
S-Parameters (S11 and S21): The RF Engineer's View
In RF engineering, S-parameters are the standard for analyzing networks:
- S11: Reflection coefficient at port 1 (how much is reflected). This is your VSWR and Return Loss.
- S21: Forward transmission from port 1 to 2 (e.g., antenna input to output of a filter). It tells you how much power gets through.
So when someone says “use S11 instead of VSWR,” they’re actually talking about the same measurement, just in vector (complex) form.
Return Loss in System Analysis
Return loss in dB is especially useful in multi-component systems because dB values are additive. If one component has 20 dB return loss and the next also has 20 dB, you can estimate the cumulative mismatch impact simply by addition — something you cannot do with VSWR directly.
Debunking the Nonsense
“VSWR is not accurate, use dB instead.”
Misleading. VSWR and return loss are mathematically interconvertible. Saying one is “more accurate” is like saying inches are more accurate than centimeters.
“VSWR hides mismatch loss.”
Not quite. VSWR shows reflection, not absolute power loss — but neither does return loss unless converted. Both are just different forms of the same data.
“VSWR is old-school.”
Irrelevant. VSWR remains practical and intuitive, which is why most transceivers still display it.
Practical Use in Ham Radio
When to use VSWR:
- Quick tuning checks
- Validating antenna tuner performance
- On-air safety for rigs (below 2:1)
When S-parameters (S11/S21) make more sense:
- Designing filters, baluns, transmission lines, and broadband antennas
- Lab measurements with a VNA
- Analyzing complex RF systems
Conversions (Cheat Sheet)
|Γ| = (VSWR - 1) / (VSWR + 1)
Return Loss = -20 * log10(|Γ|)
|Γ| = 10^(-Return Loss / 20)
VSWR = (1 + |Γ|) / (1 - |Γ|)
Bottom Line
- VSWR and return loss are two ways of describing the same reflection phenomenon.
- Neither is “better” — each has contexts where it is more practical.
- Don’t discard VSWR — it remains the fastest indicator for antenna health in amateur radio.
Mini-FAQ
- Is return loss always better than VSWR? — No, they’re interchangeable; return loss is handier in dB math, VSWR is handier for quick tuning.
- Why do rigs still show SWR? — Because it’s intuitive: below 2:1 means safe operation, above 3:1 often means retune or fix the antenna.
- Is S11 just another name for VSWR? — Yes, S11 in magnitude form equals the reflection coefficient, from which VSWR and return loss are derived.
Interested in more technical content like this? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Feel free to contact RF.Guru.