SWR, Feedline Loss and Radiated Power
SWR, Feedline Loss and Radiated Power
SWR describes reflection at a declared transmission-line plane. It does not, by itself, determine feedline loss, tuner loss, antenna efficiency, radiation pattern or radiated power.
A useful RF power statement always names the quantity and its plane: incident, reflected or net power at the transmitter connector; power accepted at the tuner input; power delivered to the feedline; power accepted by the antenna; or power actually radiated. Those values are related, but they are not interchangeable.
Video context: this article is a technical commentary on the embedded discussion, which invokes the ARRL Antenna Book while combining SWR, impedance, delivered power, tuner behaviour and feedline loss. The analysis checks those claims quantity by quantity and reference plane by reference plane; it is not a criticism of citing the book itself.
Use the framework below to compare each statement in the video with the corresponding reflection, accepted-power, network-loss and radiation quantity.
Engineering principle: draw the system, mark every reference plane, and keep reflection, component dissipation, antenna efficiency and radiation pattern as separate quantities.
1. Reflection Coefficient Comes First
For a load impedance ZL connected to a uniform line with a real positive characteristic impedance Z0, the load-plane voltage reflection coefficient is:
ΓL = (ZL − Z0) / (ZL + Z0)
SWR = (1 + |Γ|) / (1 − |Γ|)
SWR retains only the magnitude of the complex reflection coefficient. It discards phase, so one SWR value does not identify one impedance. On a 50 Ω line, 25 Ω and 100 Ω resistive loads both give 2:1 SWR, as do many reactive loads.
A vector network analyser can measure complex S11 at a calibrated plane. SWR or return loss calculated from that measurement describes the impedance seen at that plane and frequency. Moving the plane through a feedline rotates the reflection phase; line attenuation also changes its magnitude.
Scope of the familiar equations: the simple |Γ|² power relation assumes the usual real reference impedance and compatible power-wave definition. General waveguide theory requires more care for lossy structures or complex reference impedances.
2. Incident, Reflected and Accepted Power Share a Plane
At one plane in a real-Z0 system, compatible incident and reflected wave powers give:
Pref / Pinc = |Γ|²
Pnet = Pinc − Pref
Paccepted / Pinc = 1 − |Γ|²
At the antenna terminals, net power crossing into the antenna is its accepted power. At the input of a feedline, the same difference is net power entering the entire downstream line-and-load network. It is not automatically the power accepted at the far-end antenna.
The often-tabulated quantity −10 log10(1 − |Γ|²) is mismatch loss relative to incident power at that interface. It is not heat produced by reflection. Actual heat requires a lossy line, tuner, conductor, dielectric, loading network, ground system or other dissipative element.
| SWR | |Γ| | Reflected fraction | Accepted fraction | Interface mismatch loss |
|---|---|---|---|---|
| 1.5:1 | 0.200 | 4.0% | 96.0% | 0.18 dB |
| 2.0:1 | 0.333 | 11.1% | 88.9% | 0.51 dB |
| 3.0:1 | 0.500 | 25.0% | 75.0% | 1.25 dB |
| 5.0:1 | 0.667 | 44.4% | 55.6% | 2.55 dB |
These values describe one interface under the stated normalization. They do not include feedline attenuation, tuner loss, transmitter power reduction, antenna loss or directionality.
3. Standing Waves Describe a Field Pattern, Not a Loss Mechanism
Incident and reflected waves superpose along a line, creating position-dependent voltage and current maxima and minima. The envelope can be stationary while each travelling-wave component has a propagation direction. Net time-average power flow is the difference between their powers at a given plane.
A lossless line with a mismatched load does not dissipate power merely because it has standing waves. A real line does dissipate power, and the nonuniform voltage and current can increase dielectric and conductor loss relative to matched-line operation. The same maxima can raise stress in connectors, tuners and insulation.
At a fully reflecting lossless termination, incident and reflected powers are equal and net average power into the load is zero. At a partly absorbing antenna port, the difference is accepted by the antenna and is then divided between radiation and internal loss.
4. Feedline Loss Changes Both Power and Apparent SWR
A datasheet's matched-line attenuation is a necessary starting value, not the complete loss of a mismatched installation. Frequency, line length, conductor and dielectric properties, temperature, connectors and the load reflection all matter.
For a uniform line with one-way power transmission factor L, the load reflection appears at the input approximately as:
|Γin| = L |Γload|
L = 10−A/10, where A is one-way matched-line attenuation in dB
The reflection travels toward the load and back relative to the input incident wave, so input return loss improves by about twice the one-way attenuation. A lossy cable can therefore make shack-end SWR look lower while delivering less power to the antenna.
Do not estimate antenna power by taking transmitter power and subtracting only a catalogue cable-loss number. For a defensible budget, characterise the line as a two-port over frequency, include connectors and adapters, use the actual complex load, and account for multiple reflections with the measured source and load conditions.
5. A Tuner Changes the Match at Its Input Plane
A tuner transforms impedance between its output and input. When adjusted near the transmitter, it can present a low-reflection input to the radio while the line between tuner and antenna still carries a high-SWR wave pattern. A tuner located at the antenna feedpoint can instead establish a low-reflection boundary for the main feedline, subject to its own output connection and antenna load.
Every real tuner has insertion loss, finite component Q, voltage and current limits, thermal limits and a restricted load range. A 1:1 indication before the tuner establishes only the input match at that operating point; it does not establish tuner efficiency or antenna efficiency.
The transmitter also belongs in the model. Its protection system may reduce generated power as mismatch rises. Its output network, isolator or tuner can absorb or re-reflect a returning wave. The steady-state incident wave on a mismatched line can therefore include the effect of repeated interactions; it is not always identical to the power the amplifier would deliver to a nonreflecting load.
Power-limit rule: qualify the transmitter, tuner, feedline, connectors and antenna at the actual frequency, complex load, waveform, duty cycle, ambient temperature and cooling. SWR alone does not calculate peak voltage, current or temperature at every location.
6. Antenna Efficiency and Pattern Require Other Measurements
IEEE antenna terminology separates the accepted power at the antenna port from the power radiated by the antenna. Radiation efficiency is:
ηrad = Pradiated / Paccepted,antenna
Pradiated = ηradPaccepted,antenna
A perfect feedpoint match can terminate in a dummy load or a lossy antenna, so 1:1 SWR does not establish radiation efficiency. Conversely, a highly efficient antenna can be mismatched to the chosen line.
Directivity describes how radiated power is distributed by direction. Gain combines directivity with radiation efficiency but, under the standard definition, excludes impedance mismatch. Realized gain additionally includes mismatch at the declared antenna reference port. For a passive single-port antenna with a real reference impedance:
G(θ,φ) = ηradD(θ,φ)
Grealized(θ,φ) = (1 − |Γ|²)ηradD(θ,φ)
Those equations stop at the antenna reference port. Feedline and tuner losses ahead of that port require additional factors. Front-to-back ratio, null depth and coverage also require pattern measurements; they cannot be inferred from SWR or total radiated power.
7. Build a Complete Station Power Budget
A useful bookkeeping chain is:
- Transmitter plane: measure generated or net delivered power with a sensor whose bandwidth, directionality, range and waveform response are suitable.
- Tuner input and output: measure the match at both planes and establish tuner dissipation or transmission efficiency under the actual complex load and power.
- Feedline input and output: characterise two-port transmission and reflection at calibrated planes, including jumpers, adapters and connectors.
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Antenna port: measure complex impedance or
S11after moving the calibration plane to the feedpoint or de-embedding the line with a validated model. - Antenna radiation: determine efficiency, gain and pattern with an appropriate range, reverberation, gain-comparison, Wheeler-cap or other complete method and report its uncertainty.
Minimum record: frequency, waveform, power definition, calibration plane, reference impedance, line and tuner configuration, complex load, temperature, instrument range, fixture treatment and uncertainty.
8. What Each Measurement Can Prove
| Measurement | What it establishes | What it does not establish alone |
|---|---|---|
| SWR or return loss | Magnitude of reflection at a declared plane and reference impedance | Complex impedance phase, line loss, tuner loss, efficiency, pattern or radiated power |
| Complex S11 | Complex reflection and impedance at a calibrated plane | Where distributed loss occurs or how much accepted power radiates |
| Two-port S-parameters | Frequency-dependent transmission and reflection between declared ports under measurement conditions | High-power thermal margin unless the test also covers power, waveform and temperature |
| Directional power meter | Incident and reverse indications at its coupler plane within directivity, range and waveform limits | Antenna efficiency, pattern, the location of a discontinuity or local maxima elsewhere |
| Efficiency/gain/pattern test | Specified radiation quantities for the installed or test configuration | Performance in a different site, routing, ground condition or uncertainty boundary |
9. Practical Conclusions
- SWR is derived from reflection magnitude at a declared plane; it is not a complete impedance or power measurement.
- Incident minus reflected power is meaningful only when both quantities use the same plane, reference and compatible time definition.
- Interface mismatch loss is not component heat; dissipation requires a lossy network.
- Feedline attenuation can reduce the SWR seen in the shack while reducing antenna power.
- A transmitter-side tuner can create a good input match without lowering SWR on its output line.
- Antenna accepted power divides into radiation and antenna loss according to radiation efficiency.
- Gain, realized gain, pattern and front-to-back ratio require measurements beyond SWR.
- Component safety depends on local voltage, current, heat, waveform and duty cycle, not on one ratio alone.
Primary Sources and Measurement Anchors
- IEEE Std 145-2025, IEEE Standard for Definitions of Terms for Antennas—current definitions for antenna efficiency, gain, directivity and related antenna quantities.
- IEEE Std 149-2021, IEEE Recommended Practice for Antenna Measurements—current measurement practices for antenna pattern, gain and other transmitting/receiving properties.
- Kurokawa, “Power Waves and the Scattering Matrix”—primary power-wave definitions and source/load interaction framework.
- Marks and Williams, “A General Waveguide Circuit Theory”—NIST treatment of travelling waves, pseudo-waves, characteristic impedance and arbitrary reference impedance.
- Keysight, “Benefits of De-Embedding and Match-Corrected Measurements”—current manufacturer measurement guidance on path loss, source/load mismatch and calibrated reference planes.
- Rohde & Schwarz, “Demystifying OTA Testing”—antenna efficiency, gain, realized gain, directivity and EIRP relationships.
Mini-FAQ
- Does 1:1 SWR prove that an antenna is efficient? No. It establishes a low reflection magnitude at one plane. Accepted power can still be dissipated in a tuner, line, loading network, conductors or ground system.
- Does a tuner remove SWR from the feedline? A tuner changes the match at its own ports. A tuner near the radio can give the transmitter a low-SWR input while its output feedline remains mismatched.
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Is reflected-power percentage equal to SWR? No. For a real reference impedance, reflected fraction is
|Γ|², while SWR is(1 + |Γ|)/(1 − |Γ|). - Why can shack-end SWR look better than feedpoint SWR? Feedline attenuation reduces the returning reflection before it reaches the shack. The better reading can therefore accompany lower delivered power.
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What connects accepted power to radiated power? Antenna radiation efficiency:
P_radiated = η_rad P_accepted,antenna. Pattern and directivity then describe where that radiation goes. - What is the safest way to compare station losses? Measure or model each component between declared planes under the actual frequency, complex load, waveform, duty cycle and temperature, with an uncertainty budget.