SWR Measures Match—Not Gain, F/B or Efficiency
SWR Measures Match—Not Gain, F/B or Efficiency
A calibrated SWR sweep can validate mismatch magnitude at a stated reference plane. It cannot show where accepted power goes, how radiation varies with angle or whether a rear null survives the installed environment.
A low-SWR reading may accompany an excellent antenna, a lossy antenna or a dummy load. The reading is real; the distinction lies in what it observes. Reliable antenna work assigns each performance claim to a measurement that can actually see that quantity.
Measurement boundary: SWR describes mismatch magnitude at one electrical port. Gain and front-to-back ratio require calibrated field data with direction. Radiation efficiency requires radiated power relative to accepted power.
1. What a Calibrated One-Port Measurement Establishes
A vector network analyser measures complex reflection coefficient S11 at its calibration plane. For a real reference impedance Z0 under the usual power-wave convention:
Γ = S11 = (Zin − Z0)/(Zin + Z0)
SWR = (1 + |Γ|)/(1 − |Γ|)
Preflected/Pincident = |Γ|2
Paccepted/Pincident = 1 − |Γ|2
The Keysight Network Analyzer Basics note treats reflection and transmission as different measurements. An SWR display retains only |Γ|; complex S11 also retains phase and can be converted to input impedance when its assumptions are met.
A useful report includes frequency span, reference impedance, calibration method and plane, fixture or feedline, instrument uncertainty, antenna configuration and nearby environment. If complex measured and modelled S11 agree after those boundaries are aligned, the model has passed an input-impedance test.
Nothing in SWR identifies azimuth, elevation, polarization, beamwidth, sidelobes, a forward direction or a rear direction. It also does not divide accepted power between radiation and loss.
2. Accepted Power Is Not Radiated Power
The accepted fraction is determined by mismatch. What happens after power crosses the reference plane requires another power balance:
Paccepted = Pradiated + Ploss
ηrad = Pradiated/Paccepted
ηtotal = (1 − |Γ|2)ηrad
The last relation includes mismatch and is commonly called total efficiency; the terminology and system boundary should be stated. A feedline, transformer, matching network or choke belongs inside the efficiency boundary if it lies between the declared input plane and the radiator.
| SWR | |Γ| | Reflected incident power | Accepted incident power |
|---|---|---|---|
| 1.10:1 | 0.0476 | 0.23% | 99.77% |
| 1.20:1 | 0.0909 | 0.83% | 99.17% |
| 1.50:1 | 0.2000 | 4.00% | 96.00% |
| 2.00:1 | 0.3333 | 11.11% | 88.89% |
The NIST method for determining a lower bound on antenna efficiency explicitly separates mismatch and loss and uses reverberation-chamber observations plus stated assumptions. That is a much richer evidence set than an ordinary S11 sweep.
3. Two Counterexamples Keep the Terms Straight
A 50 Ω dummy load can show almost 1:1 SWR while converting nearly all accepted RF power to heat. An efficient antenna can show the same port match while radiating most of its accepted power. Identical SWR does not imply identical radiation efficiency.
Conversely, an ideal lossless antenna at 2:1 SWR accepts 88.89% of the incident travelling-wave power and radiates all of that accepted power. Its radiation efficiency is 100%, while its total efficiency and realized gain include the 0.51 dB mismatch penalty.
Keep the boundary explicit. Multiple reflections, transmitter behaviour and feedline loss can change delivered system power. They still do not let a single port-reflection result separate radiation from dissipation.
4. Gain, Directivity and Realized Gain
IEEE 145-2025 is the active IEEE standard for antenna terminology. For radiation intensity U(θ,φ):
Prad = ∫4π U(θ,φ)dΩ
D(θ,φ) = 4πU(θ,φ)/Prad
G(θ,φ) = ηradD(θ,φ)
Grealized(θ,φ) = (1 − |Γ|2)G(θ,φ)
Directivity describes how radiated power is distributed with angle. Gain also includes radiation efficiency. Realized gain additionally includes mismatch at the stated port. SWR supplies only that last mismatch factor; it does not supply radiation intensity, total radiated power or loss.
NIST Technical Note 1551 provides useful antenna definitions in a radiometry context. For general antenna terms, IEEE 145 remains the direct definitions reference.
5. Front-to-Back Ratio Requires Directional Data
Antenna orientation is not an S11 coordinate. Rotate a directional antenna by 180° in a uniform environment without changing its feed geometry and its input match may remain essentially unchanged even though its main lobe now points the other way.
F/B = 10 log10(Ufront/Uback)
F/B = 20 log10|Efront/Eback| for the same field component, distance and wave impedance
A valid F/B result defines the forward direction and whether “back” means exactly 180° from it, the strongest rear lobe or a rear-sector value. It also states frequency, elevation, polarization, angular resolution, range geometry, multipath control and uncertainty. A deep rear null is especially sensitive to element-current error, feedline current, ground reflection and nearby objects.
6. Reference Plane, Feed Network and Common Mode
An analyser at the station end of a feedline does not directly observe the radiator terminals. A matching network can transform impedance, while loss in the network and line attenuates both outward and returning waves. For a uniform line:
Γin = ΓLe−2(α+jβ)l
|Γin| = |ΓL|e−2αl
A good instrument-end SWR may therefore coexist with feed-network loss. Compare at the same plane by including the real network in the model, moving the calibration plane or characterizing and de-embedding the intervening network.
Common-mode current adds another path. Current on the coax exterior, mast or nearby conductors can change loss and radiation pattern while the one-port match still looks good. NIST’s discussion of antenna-measurement challenges treats unwanted common-mode current as a measurement concern. Document choke impedance and cable routing, measure exterior current where practical and repeat after a controlled routing change.
7. Impedance Agreement Is Partial Model Validation
Terminal resistance can contain radiation resistance, conductor loss, ground loss, matching loss and unintended feedline participation. Different current distributions can produce similar terminal impedance yet different gain and pattern. That non-uniqueness matters for parasitic and phased arrays because their pattern depends on complex element currents.
Report impedance agreement narrowly: measured complex input impedance agrees with the model within the declared frequency, reference-plane, configuration and uncertainty limits. Validate current distribution, gain, efficiency and pattern with independent observables.
8. Measurements That Address the Actual Claim
IEEE 149-2021 is the active recommended practice for measuring transmitting and receiving antenna properties. It covers radiation-pattern measurement and antenna test facilities.
- Gain: use calibrated substitution, two-antenna or three-antenna methods, or a suitable near-field measurement transformed to the far field. NIST’s Technical Note 1311 describes an extrapolation-range gain and polarization method.
- Pattern and F/B: measure calibrated field over the required azimuth and elevation angles with polarization, distance, reflections and uncertainty controlled.
- Radiation efficiency: measure radiated versus accepted power, derive ηrad from gain and directivity, integrate a calibrated full-sphere pattern, or use an applicable calibrated chamber or Wheeler-cap method.
- Array operation: measure complex element currents or port waves where accessible, characterize the feed network and mutual coupling, and verify the installed pattern.
NIST’s reverberation-chamber work on radiation and total efficiency measures the missing power relationship. A one-direction field comparison can demonstrate a directional level difference, but without sufficient pattern data it cannot distinguish higher total radiation from redistribution toward that direction.
9. Impedance Bandwidth Is Only One Bandwidth
A low-SWR span is impedance bandwidth under the tested conditions. Gain, efficiency, beam direction, F/B, null depth, matching-network loss and common-mode current can change across the same span.
The NTIA Antenna System Guide distinguishes terminal-impedance bandwidth from bandwidth defined by acceptable gain, pattern or another performance criterion. Loss can also broaden a match, so a wide, smooth SWR curve is not automatically a wide radiation-performance result.
10. Match Every Claim to an Observable
| Claim | Suitable evidence | Report at least |
|---|---|---|
| Input match and impedance bandwidth | Calibrated complex S11 | Touchstone data, reference plane, fixture, feedline and environment |
| Relative or absolute gain | Calibrated substitution, antenna method or controlled field comparison | Power convention, reference antenna, direction, polarization and uncertainty |
| F/B, beamwidth and lobe direction | Azimuth/elevation pattern or suitable near-field scan | Angular convention, resolution, multipath control and field levels |
| Radiation efficiency | Radiated-versus-accepted power, G/D, chamber or applicable Wheeler method | System boundary, mismatch treatment, feed-network loss and uncertainty |
| Array pattern | Complex element data plus measured pattern | Amplitude, phase, coupling, network loss and common-mode current |
| Model validation | Agreement across independent observables | Geometry, material losses, ground, convergence, currents, power budget and measurements |
Bottom line: SWR validates mismatch magnitude at a declared port. Gain needs calibrated field transfer, F/B needs angular data, and radiation efficiency needs radiated power relative to accepted power. Good engineering keeps those conclusions separate.
Primary technical references
- IEEE 145-2025 — Standard for Definitions of Terms for Antennas
- IEEE 149-2021 — Recommended Practice for Antenna Measurements
- NIST — Antenna gain and polarization calibration services
- NIST TN 1311 — Extrapolation-range gain and polarization measurements
- NIST — Radiation and total efficiency in a reverberation chamber
Mini-FAQ
- What does SWR prove about an antenna? SWR quantifies mismatch magnitude at a stated reference plane. Calibrated complex S11 can also support input-impedance validation, but neither measurement establishes gain, pattern or radiation efficiency.
- Can an inefficient load have a one-to-one SWR? Yes. A dummy load can accept almost all incident power and convert it to heat while presenting an excellent match.
- Can a high-SWR antenna still be radiation-efficient? Yes. Radiation efficiency is radiated power divided by accepted power. Mismatch is a separate factor in total efficiency and realized gain.
- Can SWR validate an antenna model? It can validate modelled port impedance at an aligned reference plane. It cannot independently validate current distribution, gain, pattern or efficiency.
- What measurement establishes front-to-back ratio? A controlled angular field or pattern measurement that defines the forward and rear directions, elevation, polarization, resolution and uncertainty.
- What measurement establishes radiation efficiency? A method that relates radiated power to accepted power, directly or through a recognized method such as gain-to-directivity comparison, full-sphere integration, a calibrated chamber or an applicable Wheeler cap.