The 96% SWR Myth: What 1.5:1 Actually Proves
The 96% SWR Myth: What 1.5:1 Actually Proves
The arithmetic is sound: 1.5:1 SWR corresponds to 96% mismatch acceptance at one declared RF port. The myth begins when that number is relabelled as antenna or system efficiency.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
The paper under discussion: Greg Mihran, KJ6ER, Challenger Halfwave Antenna, revision February 2025. The design is a portable, one-band-at-a-time off-centre-fed half-wave vertical using a telescoping whip, linked counterpoise, 4:1 unun and feed-line choke.
The paper presents a practical build that can make contacts and be useful in the field. My disagreement is narrower and technical: the paper moves from measured or modelled quantities to antenna-efficiency conclusions that those quantities do not establish.
On page 11, the paper says that at or below 1.5:1 SWR, “your antenna is 96% efficient with that SWR.” The 96% result is exact for mismatch efficiency under the stated port assumptions. It is not radiation efficiency, transformer efficiency, choke effectiveness, gain or complete-system efficiency.
Short version: keep every percentage attached to its denominator and reference plane. SWR tells you how incident power divides into accepted and reflected power at one port. It does not tell you where accepted power goes next.
The 96% Arithmetic Is Real
For a load impedance ZL connected to a line with real positive characteristic impedance Z0, the load reflection coefficient is:
Γ = (ZL − Z0)/(ZL + Z0)
SWR = (1 + |Γ|)/(1 − |Γ|)
ηm = 1 − |Γ|2 = 4·SWR/(SWR + 1)2
At SWR = 1.5:1:
|Γ| = (1.5 − 1)/(1.5 + 1) = 0.2
ηm = 1 − 0.22 = 0.96 = 96%
Mismatch loss = −10 log10(0.96) ≈ 0.177 dB
At the same calibrated plane, for the stated traveling-wave boundary, net accepted power is incident power minus reflected power:
Paccepted = P+ − P− = P+(1 − |Γ|2)
That is the whole 96% result. Of the incident power at that plane, 96% is accepted by everything beyond the plane and 4% is reflected there. Keysight’s RF power-transfer and mismatch guidance treats complex reflection coefficient, mismatch and measurement uncertainty as port quantities. It does not rename accepted power as radiated power.
Accepted Power Still Has Several Destinations
Once power crosses the named plane, it can be radiated by the intended structure, radiated by unintended feed-line current, dissipated in conductors, soil, transformer, choke, connectors or loading, or returned through another interaction. The SWR number does not separate those destinations.
| Quantity | Definition at a declared boundary | What it cannot establish alone |
|---|---|---|
| Reflection coefficient Γ | Complex reflected-to-incident wave ratio at a calibrated port | Radiation, loss distribution or pattern |
| Mismatch efficiency ηm | 1 − |Γ|² for the stated traveling-wave case | Where accepted power goes |
| Component transmission efficiency | Power transmitted through one defined device under its test terminations | Losses and radiation elsewhere in the installation |
| Radiation efficiency ηrad | Power radiated by the defined antenna divided by power accepted at its terminal boundary | Mismatch unless the definition explicitly includes it |
| Directivity D | Angular concentration of radiated power | Conductor, ground, matching or mismatch loss |
| Gain G | Radiation efficiency multiplied by directivity under the IEEE antenna boundary | Mismatch unless reported as realised gain |
| Realised gain Gr | Gain including mismatch for the declared port and polarisation | Unmodelled installation changes or an unspecified measurement uncertainty |
IEEE 145-2025 establishes the antenna terminology that keeps gain, directivity and efficiency distinct. The names are not interchangeable even when every result is expressed as a percentage or in decibels.
A Dummy Load Exposes the Category Error
A well-matched dummy load can present nearly 1:1 SWR and accept almost all incident power while converting nearly all of it to heat. That does not make it an efficient radiator. A lossy antenna can likewise present an excellent match because useful radiation resistance and unwanted loss resistance both contribute to its terminal impedance.
Conversely, an efficient radiator can be mismatched to a 50 Ω feed line. Match and radiation efficiency are different axes. Improving one may improve realised performance, but one does not prove the other.
The 4:1 Argument Is Useful—but Bounded
The Challenger paper’s use of a 4:1 unun is technically interesting. An ideal transformer with turns ratio n transforms impedance by n2; a 2:1 voltage ratio therefore gives a 4:1 impedance ratio. If the installed off-centre feedpoint presents a suitable impedance, that ratio can be practical.
Zsecondary/Zprimary = n2
ηcomponent = 10−LdB/10 when LdB is a positive, correctly measured insertion loss
The paper cites 0.34 dB and 0.24 dB insertion-loss magnitudes for two named 4:1 devices, corresponding arithmetically to about 92.5% and 94.6% transmitted power. Those percentages can describe the devices under their measurement terminations, frequency, level and fixture. They cannot be carried across the antenna terminal and relabelled as whole-antenna radiation efficiency.
A lower impedance ratio can be easier to realise with low loss than a much higher ratio in some designs because it may need fewer turns and less extreme voltage/current transformation. It is not a universal ranking. Topology, winding, core, complex load, bandwidth, power, temperature, common mode and the measurement method still decide the result.
Mini-Circuits’ transformer guidance defines insertion loss relative to an ideal transformer of the same ratio in a correctly terminated test system. It also documents the effects of magnetising inductance, interwinding capacitance, leakage, core/conductor loss, frequency, temperature and applied level.
Matching and Choking Need Different Evidence
The 4:1 device addresses differential impedance transformation. The feed-line choke is meant to add complex impedance to the exterior-shield common-mode path. These are related parts of one installation, but their measurements are not interchangeable.
- Differential transformer loss: measure transmission with the appropriate impedance transformation, terminations, fixture removal and power regime.
- Differential choke insertion loss: verify that the wanted coaxial mode passes within the required loss and match limits.
- Common-mode choke performance: measure complex common-mode impedance and installed exterior current across frequency.
- Thermal margin: verify transformer, choke, coax and connectors at the intended waveform, duty cycle, mismatch and ambient temperature.
A choke can show very low differential insertion loss yet provide insufficient common-mode impedance at one band. It can also provide useful resistive common-mode impedance and dissipate unwanted common-mode power as heat. Calling either percentage “choke efficiency” without naming the mode and boundary creates another denominator problem.
The Linked Counterpoise Is Part of the Antenna System
The February 2025 paper says the Challenger needs no radials while also specifying a short linked counterpoise. That is reasonable terminology if radial is reserved for a particular geometry. It does not mean the system has no return conductor.
The linked counterpoise carries current and couples to soil, tripod, feed line and surrounding objects. Its length, routing, height, conductor loss and ground coupling can change terminal impedance, current distribution, pattern and efficiency. It should be included inside the antenna-system boundary rather than dismissed as a negligible accessory.
Calling the conductor a radial, counterpoise or short leg does not determine its physics. Measure or model its current and loss, then perturb its placement to test sensitivity.
A Choke Does Not Make Coax Participation Binary
The paper correctly makes a feed-line choke part of the design. The remaining question is how much exterior current exists after the choke on each band. Choking impedance is finite, complex and frequency dependent; the installed common-mode circuit also depends on feed-line length, route, mast, equipment, operator proximity and distributed coupling.
Exterior-shield current can radiate. That radiation is not necessarily heat loss, but it changes the antenna boundary, current distribution and pattern. If the coax is omitted from the model while carrying measurable current in the field, modelled radiator gain and installed-system gain no longer describe the same object.
The ARRL installed common-mode procedure uses a calibrated current probe along the cable before and after adding a choke. Sampling several positions matters because the exterior path can support standing-wave maxima and minima.
Midpoint Current Does Not Prove Efficiency
A half-wave-like conductor commonly has a current maximum away from its ends. That current distribution helps explain where radiation originates and how the structure is fed. It does not quantify conductor heat, ground loss, transformer loss, common-mode split or total radiated power.
Two antennas can show similar normalised current shapes while carrying different absolute current for the same incident power or losing different amounts in their conductors and surroundings. A current maximum is a topology result; efficiency needs a power balance.
NEC Results Are Conditional Engineering Evidence
The paper uses 4NEC2 to select dimensions and compare patterns. That is a useful design method when geometry, conductor loss, ground model, feed arrangement, networks and feed-line paths correspond to the intended installation.
NEC calculates the model it is given. It cannot account for omitted coax, choke behaviour, tripod currents, lossy joints, layered or non-uniform soil, nearby objects or operator coupling. A far-field gain number therefore inherits every material, boundary and geometry assumption in the model.
The Lawrence Livermore NEC record explicitly identifies wires, surfaces, homogeneous ground, loads, networks, transmission lines, currents and patterns as model inputs and outputs. That is the right reading: NEC is a quantitative solver, while correspondence between model and deployment is a separate validation task.
Gain Is Not Efficiency Wearing dBi
Peak gain and takeoff angle can compare installed communication behaviour, but they do not isolate radiation efficiency. Directivity can increase in one direction by redistributing the same radiated power. Loss can decrease gain without changing the normalised pattern shape much.
G(θ,φ) = ηradD(θ,φ)
Grealised(θ,φ) = ηmG(θ,φ) for the declared port and polarisation boundary
A model or measurement can support a radiation-efficiency conclusion only when gain and directivity share the same antenna boundary and the mismatch treatment is explicit. IEEE 149-2021 describes controlled antenna-pattern and property measurements, test-facility requirements and instrumentation boundaries.
Build a Power Budget Without Double Counting
A useful system statement names successive planes and non-overlapping terms. One possible bookkeeping form is:
Pradiated,intended = Pincident,port · ηm,port · ηfeed/match · ηrad,structure
This product is valid only when each factor is defined between compatible planes and losses are counted once. If the SWR plane is before a lossy transformer, accepted power already includes every downstream destination. If radiation efficiency is computed for a model that already includes conductor and ground loss, do not multiply those losses again. If coax exterior radiation is present, decide whether the defined antenna includes it and state that boundary.
| Proposition in the February 2025 paper | What the evidence establishes | What completes the claim |
|---|---|---|
| 1.5:1 SWR corresponds to 96% | 96% mismatch acceptance at the declared port | Separate transformer, choke, conductor, ground and radiation accounting |
| A low-loss 4:1 unun improves the feed | Component transmission under defined terminations can be high | Complex installed load, power/thermal verification and complete-system budget |
| The choke keeps coax out of the antenna | A suitable choke can reduce exterior current | Installed current sweep versus frequency, position and feed-line routing |
| Midpoint current is high | The model has a half-wave-like current distribution | Absolute currents and accepted-versus-radiated power |
| NEC predicts gain and angle | The declared model produces those outputs | Model assumptions, loss inputs and controlled field validation |
| The linked counterpoise is short | The chosen geometry can be tuned to resonance | Counterpoise/coax currents, soil coupling, pattern and sensitivity tests |
A Measurement Plan That Can Support the Percentages
- Freeze the boundary. Draw the whip, counterpoise, transformer, choke, coax, tripod, feed-line route and measurement planes.
- Measure Γ at the antenna-system input. Calibrate or de-embed to the declared plane and report complex impedance, not only minimum SWR.
- Characterise the 4:1 device. Use the relevant complex load, frequency and fixture; check insertion loss, return loss and temperature at operating power.
- Characterise the choke in both modes. Record wanted-mode transmission, complex common-mode impedance, installed exterior current and thermal behaviour.
- Map the return-current sensitivity. Reposition the linked counterpoise and feed line in controlled steps and repeat impedance and current measurements.
- Validate gain comparatively. Use a known reference antenna, fixed range geometry, stable instrumentation, identical feed planes, polarisation and time window.
- Reserve absolute efficiency for an appropriate method. Combine calibrated gain/directivity or another valid efficiency technique with a complete uncertainty and boundary statement.
A field-strength A/B test can provide valuable relative realised-gain evidence when site, geometry, propagation, power and instrumentation are controlled. It does not by itself separate directivity from radiation efficiency or yield a universal absolute percentage.
Keysight’s field cable-and-antenna guidance keeps return loss, insertion loss and calibration plane separate. That discipline is exactly what the 96% discussion needs.
The Constructive Conclusion
The Challenger concept does not become invalid because one percentage is labelled too broadly. A one-band-at-a-time off-centre-fed half-wave with a linked counterpoise, suitable 4:1 transformer and measured choke can be a practical portable antenna.
The engineering disagreement is about the claim boundary:
- 1.5:1 SWR gives 96% mismatch acceptance at a declared port.
- A measured transformer insertion loss describes that transformer under the test conditions.
- A choke needs common-mode impedance and installed-current evidence.
- Radiation efficiency needs accepted and radiated power for one defined antenna boundary.
- Gain and pattern need their directivity, efficiency, mismatch and installation assumptions stated.
Those statements strengthen the portable design rather than dismiss it. A percentage becomes useful when readers can see exactly what was measured, what was assumed and which part of the system it describes.
Engineering References
- Greg Mihran, KJ6ER: Challenger Halfwave Antenna, February 2025 mirror
- IEEE 145-2025: Standard for Definitions of Terms for Antennas
- IEEE 149-2021: Recommended Practice for Antenna Measurements
- Keysight: RF Power Transfer, Reflection and Mismatch Uncertainty
- Keysight: Precise Cable and Antenna Measurements in the Field
- Mini-Circuits: How RF Transformers Work and How They Are Measured
- ARRL: Common-Mode Current and Common-Mode Chokes
- Lawrence Livermore National Laboratory: NEC-5.0
Final rule: 96% is not false; “antenna efficiency” is the wrong label for that SWR calculation. Keep the number, keep the reference plane, and measure every remaining path before extending the conclusion.
Mini-FAQ
- What does 1.5:1 SWR actually mean? It gives |Γ| = 0.2 and 96% mismatch acceptance at the declared port under the stated traveling-wave assumptions. It does not give radiation efficiency.
- Can a 4:1 unun be a low-loss choice? Yes. A suitable 4:1 device can have low insertion loss for the relevant complex load, frequency and power, but that component result is not whole-system efficiency.
- Does low choke insertion loss prove the coax is isolated? No. Wanted-mode insertion loss and common-mode suppression are different measurements. Verify complex choking impedance and installed exterior-shield current.
- Does a midpoint current maximum prove good efficiency? No. It supports a half-wave-like current shape. Radiation efficiency still requires accepted and radiated power with conductor, ground and matching losses included.
- Is the linked counterpoise electrically minor because it is short? Not necessarily. It is part of the return-current system, and its coupling, loss, route and current can affect match, efficiency and pattern.
- Can NEC establish the Challenger’s field performance? NEC can calculate the declared geometry and loss model. Installed performance still requires the coax, choke, return path, ground and surroundings to be modelled or measured consistently.