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 Some even say VSWR is “outdated” or “inaccurate.”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|)

Where:

Γ = (ZL - Z0) / (ZL + Z0)
  • ZL is the load impedance.
  • Z0 is the system impedance (typically 50 ohms).

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.

Adding Return Losses in a System: The Real Advantage Here's a powerful reason to consider return loss over VSWR in multi-component systems: you can add return losses (in dB) to estimate total system loss when moving from one component to another.

Why? Because decibels are logarithmic and represent ratios. If Component A has a return loss of 20 dB and Component B has a return loss of 20 dB, the cumulative mismatch loss can be roughly approximated by adding the two dB values—as long as the assumptions about isolation and directionality hold.

With VSWR, this becomes much more complicated. VSWR is not additive. To analyze multiple components using VSWR, you have to:

  • Convert each VSWR to |Γ|
  • Convert |Γ| to return loss
  • Add dB values (if appropriate)
  • Convert back to VSWR if needed

It’s messy and error-prone. Return loss or S11 values in dB give you a more modular and analytical way to understand what’s happening at each stage in your RF chain.

Also, VSWR is inherently a source-to-load concept. It tells you what happens when power moves forward. But return loss can be viewed in either direction:

  • Source to load (typical usage)
  • Load to source (when analyzing reflections from the far end, useful for analyzing subsystems)

This bidirectional utility is extremely helpful when designing or troubleshooting systems with multiple interfaces.

Debunking the Nonsense

“VSWR is not accurate, use dB instead.”

⚠️ Misleading.

  • VSWR and return loss are mathematically interconvertible.
  • Saying one is “more accurate” than the other is like saying inches are more accurate than centimeters.

“VSWR hides mismatch loss.”

⚠️ Not quite.

  • VSWR doesn't directly show you how much power is lost due to mismatch—it shows reflection.
  • But neither does return loss—unless you convert it to power.

“VSWR is old-school.”

⚠️ Irrelevant.

  • It's still used because it’s easy to interpret.
  • A rig manual showing “SWR must be below 2:1” is not bad advice—it’s practical.

Practical Use in Ham Radio

When to use VSWR:

    • Quick tuning.
    • Checking if your antenna tuner or matchbox is doing its job.
    • Quick on-air performance checks.

When S-parameters (S11/S21) make more sense:

    • Designing filters, baluns, transmission lines, and broadband antennas.
    • Lab measurements with a VNA.
    • Analyzing complex systems or debugging.

A Balanced Take

Both VSWR and S11 (or return loss) describe the same reality, but from different angles. In ham radio:

  • VSWR is intuitive and fast.
  • Return loss in dB is more granular, especially when making plots or comparing systems.
  • S11 in complex form (magnitude and phase) is powerful for design.

Conversions (Cheat Sheet)

Given VSWR, you can get Return Loss:

|Γ| = (VSWR - 1) / (VSWR + 1)
Return Loss = -20 * log10(|Γ|)

And vice versa:

|Γ| = 10^(-Return Loss / 20)
VSWR = (1 + |Γ|) / (1 - |Γ|)

Bottom Line

  • Saying “use dB instead of VSWR” is like saying “use kilograms instead of pounds.” It doesn’t make your measurement better—just different.
  • If you're working with antennas, use what makes sense for your application.
  • Don’t throw away VSWR—it’s still one of the most practical indicators in ham radio.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.