When Antenna Claims Outrun Antenna Physics
When Antenna Claims Outrun Antenna Physics
A useful antenna can still carry an overstated claim. The problem is not experimentation; it is turning a good match, an interesting pattern or a successful contact into proof of something else.
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.
Readers have sent me questions about antennas they had seen online, in videos, in articles and in printed magazines. The names and designs were different, but the questions were remarkably similar.
- Can this antenna really have that much gain?
- Does this SWR curve really prove high efficiency?
- Can two short radials really make a directional antenna?
- Is this still a no-radial antenna if it uses a counterpoise?
- Can Reverse Beacon spots prove that the antenna performs as claimed?
Those are good questions. Many of these antennas work: they tune, make contacts and may be clever, compact and useful. The trouble starts when the explanation goes further than the observation supports.
A low SWR becomes “efficiency.” A normalised polar plot becomes an absolute gain figure. A counterpoise becomes invisible in the description but essential during setup. A spot on another continent becomes proof of a radiation pattern.
I am deliberately leaving the people out of this discussion. Builders, experimenters and activators do not need a personality contest; the recurring claim patterns need an engineering answer. The physics does not care who made the claim.
Where the Interpretation Goes Too Far
The distinction I keep coming back to in these reader questions is simple: an antenna can be useful and overmarketed at the same time. Saying that a claim is overstated is not saying that the antenna is useless.
The words have to identify what the result actually describes:
- “efficient” becomes radiation or total efficiency at stated frequencies;
- “4 dB gain” becomes realised gain relative to a named reference, direction, polarisation and elevation;
- “directional” becomes a pattern or front-to-back ratio under stated geometry;
- “no radials” becomes a documented return-current path;
- “broadband” becomes an impedance or realised-gain bandwidth at stated limits; and
- “field proven” becomes a repeatable test protocol with uncertainty.
Evidence rule: every number needs a metric, reference plane, frequency, configuration and uncertainty. Without those, a precise-looking result can still be ambiguous.
SWR and Mismatch Efficiency: The Exact 96%
Start with the reader's SWR question. The familiar “1.5:1 means 96% efficient” statement contains a correct calculation and an incorrect leap. For a real reference impedance, the magnitude of the reflection coefficient is:
|Γ| = (SWR − 1) / (SWR + 1)
The mismatch efficiency at that reference plane is:
ηm = 1 − |Γ|²
At 1.5:1 SWR, |Γ| = 0.2 and ηm = 0.96. So “96%” is not invented: 96% of the incident power is accepted at that plane, and the corresponding mismatch loss is about 0.18 dB. This is the matched-source convention; transmitter behaviour and source re-reflections are separate system questions.
The error is renaming mismatch efficiency as radiation efficiency. A matched resistor can accept nearly all incident power and convert nearly all of it to heat.
That answers the question: the SWR curve does not prove high radiation efficiency. It proves something useful about the port match. SWR is not a power meter for radiation.
Reference plane matters. If the analyser is at the shack end of a lossy feedline, line attenuation can make the measured SWR look better while consuming power before it reaches the antenna.
Radiation Efficiency, Total Efficiency and Gain
A NIST antenna-efficiency paper distinguishes radiation efficiency from total efficiency: radiation efficiency is radiated power divided by net power accepted at the antenna port; total efficiency also includes mismatch.
The useful relationships are:
| Quantity | Meaning | What it does not prove |
|---|---|---|
| Mismatch efficiency ηm | Fraction not reflected at a stated plane. | Radiation, ground or loading-coil efficiency. |
| Radiation efficiency ηrad | Prad / Paccepted at the antenna port. | Input match or directional concentration. |
| Directivity D | Pattern concentration relative to an isotropic radiator with the same radiated power. | Ohmic or ground loss. |
| Gain G | ηradD, referenced to accepted input power. | Mismatch unless “realised gain” is specified. |
| Realised gain | Gain including mismatch at the defined port. | Feedline loss upstream unless included explicitly. |
ITU-R BS.705-1 makes the central distinction: directivity is a property of pattern shape, while gain also accounts for antenna efficiency.
A Component Efficiency Is One Factor in the System
The same leap appears when a transformer result is made to stand in for the whole antenna. If a transformer has a measured loss of 0.3 dB, that corresponds to a power ratio of about 93.3% under the stated fixture, impedance and calibration. That can be good component performance. It is not automatically 93.3% antenna-system efficiency.
A simplified cascade is:
ηsystem = ηtransformer × ηfeedline × ηmismatch × ηradiation
Each factor must refer to compatible ports and conditions, without counting a loss or mismatch twice. Multiplying independently measured matched-component figures is only an approximation when the installed components interact through mismatch or unintended current paths. Transformer loss can also change with impedance, common-mode current, voltage, frequency, temperature and drive level, so a single matched two-port result is not a universal power rating.
The remaining power can still be lost in the loading coil, conductors, soil, contacts or nearby materials. The return system and the coax can also participate in radiation, changing the pattern; their current is not automatically a dissipative loss. A component can be efficient while the complete installation is mediocre. Those remain different claims.
“No Radials” Still Needs a Return-Current Map
An unbalanced radiator does not operate without a return path. The return may use explicit radial wires, a counterpoise, a vehicle body, a metal roof, mast, coax exterior, capacitive coupling to earth, station wiring or some combination.
“Radial” and “counterpoise” can describe different geometries and design intentions, so it is too broad to say that every current-carrying return conductor is literally a radial. The honest requirement is simpler: identify it, include it in the model and disclose any installation dependence.
If changing a return conductor changes impedance, external feedline current, loss or pattern, it is an active part of the antenna system. Calling it a tuning tail does not remove that role.
So my answer to “no radials, but a counterpoise?” is not a vocabulary trick. Ask where the return current flows. If the counterpoise completes the current path and sets the tuning, it is part of the antenna—not an optional accessory you can leave out of the performance explanation. Changing the name does not change the current.
If a folded counterpoise makes an 80 m installation practical, explain that useful compromise. If a controlled section of coax provides the return, explain where it ends and why the choke belongs there. You cannot use the return system for the good parts of the claim and ignore it for the inconvenient parts.
Two Radials Can Create Directivity—But Gain Also Depends on Loss
An asymmetric return system can redistribute radiation and therefore create real directivity. This is not merely a visual artefact. If the total radiated power were unchanged, more radiation in one solid angle would require less elsewhere.
The unproven leap is from an asymmetric normalised pattern to an absolute forward-gain claim. Resistive loss in soil and conductors can reduce total radiated power; unintended feedline current can also change the pattern and loss. A normalised polar plot hides that scale.
That is the important answer to the two-radial question: yes, a return layout can make the antenna directional; no, its lopsided plot alone does not establish a claimed 4 dB forward advantage. Directivity is real. The missing information is the absolute scale, losses and comparison conditions.
Correct test: compare realised gain or field strength in the claimed direction for equal transmitter power at a stated reference plane. Also report efficiency and the complete three-dimensional pattern.
Front-to-Back Ratio Is Not Forward Gain
Front-to-back ratio compares radiation in two directions. It can improve because the rear field fell, because the forward field rose, or both. It contains no absolute scale.
A 20 dB front-to-back ratio could describe a strong forward lobe and ordinary rear field, or a mediocre forward field and a very deep rear null. Report forward gain and front-to-back ratio separately, with the angles and polarisation used.
If the forward field stays the same while the rear field falls, the front-to-back figure improves without a stronger forward signal. That may still be useful rejection. It is not the same claim as extra forward gain.
NEC Produces the Answer to the Model You Built
NEC is an electromagnetic solver, not an automatic representation of a park. Lawrence Livermore National Laboratory's NEC-5 can calculate currents, impedance, patterns and near fields for wires and conducting surfaces above ground.
Its validation manual separates two error classes:
- numerical-model error from segmentation, mesh, junction treatment and solver limitations; and
- physical-model error from simplifying or omitting parts of the real structure and environment.
For a portable antenna, publish the model file and state wire diameter, segmentation, source model, loads, real-ground parameters, height, radial layout, feedline exterior, choke impedance and nearby conductors. Run convergence and sensitivity checks instead of quoting a one-decimal gain from one geometry.
From Polar Plot to Picnic Table
A model can validly predict what a defined geometry does. The question is whether the field deployment reproduces that geometry.
Coax routing, radio and battery leads, operator distance, wet soil, metal furniture, fences and vehicles can change current distribution. Do not demand that every small object be modelled; identify the variables likely to carry significant current or scatter a significant field, then test sensitivity.
A good engineering statement is: “Within these assumptions, the model predicts this behaviour.” A stronger field claim needs field evidence.
This is why the picnic-table questions matter. A pattern predicted for one return layout does not automatically follow the antenna into every park. Keep the useful prediction, but do not sell its assumed surroundings as a property of the whip.
RBN and WSPR Can Support Comparisons—With Experimental Design
The Reverse Beacon Network is a network of receiving stations that reports what it hears and offers tools for quantitative comparisons as heard by a chosen reverse beacon on a chosen band and time.
That makes RBN more than anecdote, but not an antenna range. Propagation, transmit power, timing, frequency, receiver noise, AGC/calibration, polarisation and geometry remain variables.
A useful on-air A/B test should:
- switch antennas rapidly enough that propagation is approximately common;
- use the same transmitter power and feed reference plane;
- compare reports from the same receiving stations;
- repeat many cycles, bands, times and path directions;
- avoid cherry-picking only successful spots; and
- report the distribution and uncertainty, not only the best result.
Such a test can estimate relative on-air performance on sampled paths when transmitter and receiving conditions are adequately controlled. It still does not directly deliver absolute efficiency or a complete three-dimensional pattern.
So a reader's overseas spot is valuable: the signal reached that receiver strongly enough to decode at that time. It is not, by itself, the answer to “how much gain?” or “how efficient?” Controlled comparisons can tell us more; a map of successful spots cannot fill in everything they did not measure.
Contacts Prove a Link Closed
A completed QSO proves that, under those conditions, the link budget was sufficient. It sets a lower bound on usable performance for that path and mode. It does not isolate antenna gain, efficiency or pattern from propagation, receiver sensitivity, noise, coding and operator technique.
This is not an insult to practical operating. “Easy to deploy and made the required contacts” can be the most important result for a portable antenna. It is simply a different claim from “4 dB gain” or “90% radiation efficiency.”
“But I made contacts with it” is not a rebuttal to that distinction. The contacts are not in dispute. The unsupported explanation is.
Shape Does Not Supply an Unstated Reference
Diamond, rhombic, folded, spiral and fractal describe geometry. Geometry controls current distribution, impedance, bandwidth and pattern, but the name does not import the performance of a larger antenna that looks similar.
A true gain claim still requires a reference and loss accounting. For a reciprocal antenna receiving a polarisation-matched far-field wave with conjugate matching, effective aperture and gain are linked by Ae = λ²G/(4π), with G in linear units in the arrival direction. A compact structure may be useful, but visual resemblance is not a gain measurement.
A small diamond-shaped wire does not inherit a large travelling-wave rhombic's performance simply because the outline looks familiar. A folded path may solve a packaging or matching problem; that is a useful reason to build it without attaching an unmeasured gain claim.
Useful Antennas Deserve Honest Claims
None of these answers means a portable antenna must be perfect. A compact system with moderate loss can be the right choice for a balcony or a trip. Two radials can be a practical return system. A transformer-fed wire can be convenient. An unusual geometry can be worth experimenting with.
The engineering advantage may be faster deployment, a more repeatable return path, less component loss or useful directivity in a particular direction. Say which advantage the design provides and explain the mechanism. Do not make every benefit mean “gain.”
A useful antenna can simply be useful. It does not need a fictional efficiency number to justify the contacts someone enjoyed making with it.
Better Claims Are More Useful
| Overclaim | Defensible wording |
|---|---|
| “1.5:1 SWR means 96% efficient.” | “Mismatch efficiency is 96% at this plane; radiation efficiency was not measured.” |
| “No radials.” | “No separate radial wires; the documented RF return is the counterpoise and controlled coax section.” |
| “Two radials add 4 dB.” | “This model predicts directional redistribution; absolute realised gain awaits loss and field validation.” |
| “NEC proves it.” | “The converged model predicts this result for the stated geometry and ground.” |
| “RBN proves gain.” | “Repeated switched comparisons show this relative result on the sampled paths.” |
| “It made DX, so it is efficient.” | “It closed the required links under these conditions.” |
The Practical Verdict
The reader questions lead back to the same distinction: the danger is not experimentation; it is certainty without the conditions that make the claim true.
The 96% from a 1.5:1 SWR belongs to mismatch, not automatically to radiation. A low-loss transformer does not account for the rest of the installation. A counterpoise is not nothing. Two radials may produce useful directivity, but a front-to-back ratio or normalised plot is not an absolute forward-gain result. A model predicts its model; a spot records a received signal; a contact establishes a working link.
Those are worthwhile results. Give each one credit for what it establishes. If the antenna is quick to deploy and makes the required contacts, say so. If a complete comparison demonstrates lower loss or stronger signal, explain why and under which conditions. That is a stronger argument than borrowing certainty from the wrong measurement.
The opposite of antenna hype is not negativity. The opposite of antenna hype is context. And in RF, context is usually where the physics was hiding.
Mini-FAQ
- Is 1.5:1 SWR really 96% efficient? It is 96% mismatch-efficient at the stated plane, not necessarily 96% radiation-efficient.
- Can two radials create gain? They can create directivity; absolute gain also depends on radiation efficiency and the reference.
- Does front-to-back ratio reveal forward gain? No. It is a ratio between directions and has no absolute scale.
- Can NEC prove an antenna? It can solve and validate a stated model; field claims require the physical installation to match or be measured.
- Can the Reverse Beacon Network compare antennas? Yes, with rapid switching, common receivers, repeated samples and honest uncertainty; it is not an absolute antenna range.