Engineering Reference
RF Impedance, VSWR and Transmission-Line Terms Reference
RF lookup for characteristic impedance, reflection coefficient, VSWR, return loss, mismatch loss, velocity factor, electrical length, coax loss, and matching terms.
- Reading Time
- 12 min
- Format
- Terms and relationship lookup
- Updated
- September 29, 2026
Core RF Impedance Terms
| Term | Symbol | Unit | Meaning | Boundary |
|---|---|---|---|---|
| Characteristic impedance | Z0 | Ω | Traveling-wave voltage/current ratio of a uniform transmission line | Not the DC resistance of the cable |
| Load impedance | ZL | Ω | Impedance terminating the line | May be complex and frequency-dependent |
| Reflection coefficient | Γ | ratio or % | Complex reflected-to-incident voltage-wave ratio | Magnitude alone omits phase |
| VSWR | S | ratio | Standing-wave voltage maximum divided by minimum | Ranges from 1 to infinity for passive one-port magnitude cases |
| Return loss | RL | dB | Positive mismatch metric defined as -20 log10 |Γ| | Higher positive return loss indicates a smaller reflection |
| Reflected power fraction | Pr/Pf | ratio or % | Power-wave fraction associated with mismatch | Equals |Γ|² under the adopted reference conditions |
| Mismatch loss | ML | dB | Available-power reduction from mismatch | Does not include cable dissipation |
| Velocity factor | VF | ratio | Propagation speed divided by vacuum light speed | Can vary with construction and frequency |
| Electrical length | θ | degrees or rad | Phase accumulated over a physical line length | Changes with frequency |
| Insertion / cable loss | L | dB | Power loss through a component or line under stated conditions | Keep sign convention explicit |
| Quarter-wave transformer impedance | Zt | Ω | sqrt(Zsource × Zload) for the ideal real-resistance case | Narrowband and sensitive to electrical length |
| Loaded Q for simple L match | Q | ratio | sqrt(Rhigh/Rlow - 1) for the ideal resistive case | Not component Q or a universal bandwidth guarantee |
Mismatch Relationships
| Quantity | Relationship | Ideal match | Complete reflection |
|---|---|---|---|
| Reflection coefficient magnitude | |Γ| | 0 | 1 |
| VSWR | (1 + |Γ|) / (1 - |Γ|) | 1:1 | ∞ |
| Return loss | -20 log10 |Γ| | ∞ dB | 0 dB |
| Reflected power | |Γ|² × 100% | 0% | 100% |
| Mismatch loss | -10 log10(1 - |Γ|²) | 0 dB | ∞ dB |
Canonical calculation check: for VSWR 2:1, the project utility returns 33.333% voltage-wave reflection and 9.542 dB return loss.
Transmission-Line Lookup
| Need | Relationship | Use | Caveat |
|---|---|---|---|
| Propagation velocity | v = c × VF | Cable or dielectric wave speed | Use datasheet VF at relevant frequency |
| Wavelength | λ = v / f | Physical distance per cycle | Not free-space wavelength when VF < 1 |
| Electrical length | θ = 360° × l / λ | Phase length of a physical line | Frequency-dependent |
| Propagation delay | t = l / v | One-way travel time | Connector and device delays are separate |
| Loss scaling | Ltotal = attenuation × length | First-order specified-frequency estimate | Cable data and interpolation conditions govern |
Matching and Loss Anchors
| Example | Canonical result | Interpretation |
|---|---|---|
| 50 Ω to 100 Ω quarter-wave transformer | 70.710678 Ω | Ideal real-resistance design at one frequency |
| 10 dB cable loss | 10.000% output/input power | Dissipative loss, separate from mismatch |
| 90° line | l = λ / 4 | Quarter-wave only in the selected medium |
| Matched 50 Ω system | ZL = Z0 = 50 Ω | Γ = 0 in the ideal reference-plane model |
Reference-Plane and Measurement Boundaries
Impedance and reflection are frequency-dependent complex quantities. A measured value belongs to a calibration plane, bandwidth, fixture and instrument setup. Cable loss can make a remote mismatch appear smaller at the instrument because the reflected wave is attenuated twice. De-embedding, connector repeatability and calibration quality matter.
Common Errors
- Treating characteristic impedance as DC resistance.
- Using signed Γ where a magnitude formula is required.
- Calling 0 dB return loss a good match.
- Using 20 log for a power ratio without converting the quantity.
- Equating low VSWR with low cable loss.
- Ignoring the measurement reference plane.
- Using free-space wavelength for a cable section.
- Assuming velocity factor is constant at every frequency.
- Applying sqrt(ZS ZL) to arbitrary complex impedances.
- Treating a quarter-wave transformer as broadband.
- Adding linear loss ratios as though they were dB.
- Ignoring connector, launch and fixture discontinuities.
Support reference
FAQ
What is characteristic impedance?
It is the traveling-wave voltage-to-current ratio of a uniform transmission line. It is not the same as the cable's DC resistance.
How are VSWR and reflection coefficient related?
For reflection-coefficient magnitude below one, VSWR = (1 + |Γ|) / (1 - |Γ|), and |Γ| = (VSWR - 1) / (VSWR + 1).
What is return loss?
Return loss is -20 log10 |Γ| dB. Under this positive convention, a larger return-loss value means a smaller reflected wave.
Is 0 dB return loss a good match?
No. Under the positive return-loss convention, 0 dB corresponds to complete reflection. An ideal match has infinite return loss.
What reflected power corresponds to Γ?
Under the adopted real-reference power-wave conditions, reflected power fraction is |Γ| squared.
Does a low VSWR mean the cable has low loss?
No. VSWR describes mismatch. A well-matched cable can still have significant dissipative attenuation.
What does velocity factor change?
It changes propagation velocity, wavelength, delay and physical length for a specified electrical length. It does not directly set characteristic impedance.
When does a quarter-wave transformer work?
The ideal sqrt(ZS ZL) relationship applies to real resistances at the design frequency with a lossless 90-degree line. It is inherently frequency-sensitive.
Can an L network match any two impedances?
The simple calculator relationship assumes positive real source and load resistances. Complex impedances require reactance cancellation and a fuller matching design.
Can I add RF losses in dB?
Independent cascaded gains and losses may be added algebraically in dB when reference planes and sign conventions are consistent.
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