EIRP vs SWR: Why I Prefer the 4:1 EFOC Trade-Off

This article started with a question many operators ask: “My EFHW has a perfect match. Your EFOC shows 3:1 SWR. Doesn’t that make it inefficient for DX?” My answer is no. A perfect match tells me that the radio sees a convenient load; it does not tell me how much useful signal leaves the complete antenna system.

For a practical multiband wire installation, I favour the EFOC design choice: work with a more moderate feed impedance, use a 4:1 UNUN for transformation, and give the return current an intentional route. I would rather accommodate manageable mismatch in a suitable feed system than make a flat SWR trace the overriding design objective. That is an engineering preference with a reason behind it—not a claim that every EFOC beats every EFHW in every direction.

Why I Prefer the Lower Transformation Burden

A half-wave end feed normally presents a kilohm-class impedance. The EFOC approach instead uses the complete radiator and return-path geometry to work in a more moderate impedance region suited to 4:1 transformation. The advantage I am pursuing begins there: the matching network does not have to make the same extreme impedance and voltage transformation.

For an ideal transformer, the impedance ratio is the square of the turns ratio. A 4:1 impedance transformation corresponds to a 2:1 voltage/turns ratio; 49:1 corresponds to 7:1. As a circuit illustration, a resistive 200 Ω load needs less terminal voltage for the same accepted power than a 2,450 Ω load: VRMS = √(PR) gives a voltage ratio of √(2450/200) = 3.5. These are nominal comparison loads, not measured EFOC feed impedances or product voltages.

That reduced voltage-transformation demand gives the designer a less demanding starting point for insulation, winding layout and broadband matching. A high-ratio end-feed transformer has to reconcile enough low-frequency magnetizing inductance with high-frequency leakage, capacitance and conductor effects across its intended bands. Avoiding some of that transformation burden is a positive reason to choose the 4:1 architecture, even if its installed SWR is not 1:1.

Fewer turns are not a free efficiency certificate. A lower ratio is not a winding recipe, and reducing turns indiscriminately can raise volts per turn and flux density. Core loss still depends on material, frequency, flux, geometry and temperature. Winding topology also matters. The useful claim is that I have chosen a less extreme matching task; the actual loss and power rating belong to the completed transformer under its intended load.

The Return Path Is Part of the Design Advantage

The EFOC29 combines its radiator with an integrated 4:1 UNUN and a specified return arrangement. In the coax-counterpoise version, a deliberate section of the coax exterior participates in the antenna; the choke establishes the intended transition to the station-side feedline. The separate M6-terminal version uses a wire counterpoise instead. Follow the installation guide for the configuration in use; they are not interchangeable layouts.

I want that return conductor and its boundary to be designed in, not discovered later as RF on the shack wiring. This makes installation choices explicit: where the current is intended to flow, what geometry sets the impedance, and where common-mode impedance is needed. A real choke has finite impedance, so the boundary is controlled rather than mathematically perfect.

A well-engineered EFHW also needs a return path and can provide one deliberately. The distinction is not that the EFOC has found an exception to current continuity. It is the combination I prefer for this application: moderate-impedance transformation and an intentional, installable return arrangement, with the remaining mismatch handled as a system trade-off.

A Modest Mismatch Can Be the Better Trade

A tuner can present the radio with its required load while leaving standing waves on the antenna-side cable. The extra cable and tuner loss must be paid, but there is no rule saying that bill must be larger than the loss avoided elsewhere. If a less demanding matching arrangement saves more power than the additional tuner and feedline losses consume, more power reaches the radiator. That is why “3:1” alone does not settle the EFOC-versus-EFHW question.

Nor is 3:1 automatically acceptable. A long, lossy cable run, a difficult complex load, transmitter foldback or excessive tuner voltage can erase the advantage. Use a feedline and tuner suited to the installed impedance, stay inside every component's ratings, and distinguish SWR before the tuner from what the transmitter actually sees. Matching the radio safely is necessary; making the entire antenna-side system display 1:1 is not the same objective.

Use EIRP, ERP and Radiated Power Correctly

The 2024 ITU Radio Regulations define equivalent isotropically radiated power, or e.i.r.p., in a given direction as power supplied to the antenna multiplied by antenna gain relative to an isotropic radiator in that direction. Effective radiated power, or e.r.p., uses the gain of a half-wave dipole as the reference. For the same direction and polarization:

EIRP is directional. It can be greater than total radiated power because directivity concentrates radiation into some directions at the expense of others. Total radiated power instead integrates radiation over the full sphere. “Power remaining after loss” is neither quantity until the loss boundary and radiation pattern are established.

IEEE 145-2025 keeps three antenna quantities distinct:

Those equivalent forms require consistent reference planes. If transmitter power is specified at the shack, account for the tuner, matching transformer and feedline before using antenna-port gain. With each η defined as the net output-to-input power ratio of that stage in the actual connected system, a useful end-to-end form is:

Do not substitute matched-load catalogue efficiencies for those installed-system ratios, or multiply by a mismatch factor twice. Use either accepted power with gain or incident power with realized gain at the same antenna port. For the coax-return EFOC, the radiating shield section belongs to the antenna model, not to an assumed perfectly non-radiating cable-loss block.

What SWR Actually Establishes

For a single-mode line with a real reference impedance:

The “33% reflection” statement at 2:1 confuses voltage-wave amplitude with power. The reflection coefficient magnitude is one third, but the reflected power fraction is its square: one ninth, or 11.11%.

Mismatch loss is not automatically heat. In a lossless line the reflected wave returns to the source. A tuner can transform the input impedance and re-reflect energy toward the load, but the real steady-state result depends on tuner loss, source impedance, line attenuation, load reflection magnitude and phase, and every intervening network.

Extra coax loss cannot be read from SWR alone

Additional line loss under mismatch depends on cable type, length, frequency, matched attenuation and the complex load. The fraction of net line-input power reaching the load is distinct from the complete source-to-load delivery problem, which also includes the tuner and source behaviour. Twenty metres of low-loss cable on 80 m and thirty metres of small cable on 10 m can have the same load SWR but very different loss.

The ARRL transmission-line treatment explicitly distinguishes matched attenuation, total line loss, insertion loss and transducer loss. Keysight's mismatch guidance likewise requires the complex source and load reflection coefficients when re-reflections and measurement uncertainty matter.

A Perfect Match Can Hide Loss—but Does Not Prove It

A lossy network can indeed produce a good input match. A dummy load is the limiting example: nearly all accepted power becomes heat. Long lossy coax also makes the SWR observed at the shack look better because the reverse wave is attenuated on its return trip. Therefore a low shack SWR is not proof of high antenna efficiency.

Transformation ratio alone does not establish insertion loss or power handling. A 49:1, 64:1 or 4:1 network must be evaluated as its implemented circuit under the intended load. The result depends on:

• ferrite material, core volume, number and stacking of cores;
• winding topology, turns, conductor size, insulation and stray capacitance;
• actual complex source and load impedances across frequency;
• common-mode current and the defined return path;
• PEP, average power, waveform, duty cycle and thermal state; and
• the measurement fixture, calibration plane and uncertainty.

Fair-Rite's transformer guidance models leakage inductance, winding capacitance, core loss and conductor loss as frequency-dependent elements. A turns ratio alone cannot supply an insertion-loss or temperature-rise figure.

The compensation capacitor is not magic in either direction

The ARRL EFHW kit places an optional compensation capacitor across the primary—a shunt connection—to compensate the transformer's high-frequency response. It can improve matching; it does not remove the high-impedance transformation task. An ideal capacitor stores and returns energy, whereas a real capacitor and its surrounding circuit carry RF current and have finite loss and voltage limits. It is neither a magic efficiency booster nor proof of inevitable failure. My objection is to using the improved SWR trace as an efficiency measurement. Check the complete network under its intended high-impedance load.

EFHW and 4:1 Off-Centre-Fed Systems Need Their Full Return Paths

An EFHW is a half-wave wire fed near a current minimum and voltage maximum. The ARRL kit describes approximately 2,500 ohms and a 49:1 impedance transformation for one four-band implementation. The exact feedpoint impedance changes with wire diameter, end effects, height, ground, nearby objects and the return path.

A conventional off-centre-fed dipole is a two-arm dipole fed away from its centre. Its unequal arms and surroundings make balance a practical design issue, not something guaranteed by the word “dipole.” A feedpoint near 200 ohms can make 4:1 transformation convenient, but impedance varies with feed fraction and band. The ARRL's tested OCF example uses a 4:1 current balun; that is one valid implementation, not a guarantee that every OCF band presents 200 ohms.

In the EFOC coax-counterpoise configuration, the wire and intended exterior-coax section form the antenna current system. Transformer behaviour, choke placement, station bonding and nearby objects determine how closely the installation follows that intention. Moving the choke or changing the return geometry can change impedance and pattern; it is not an innocuous cable adjustment. EFOC variants with separate return wires must instead be evaluated with those wires included. IEEE work on OCF feedline-current suppression likewise treats feedline radiation as a design variable, not an invisible ideal conductor.

Where the Extra Signal Would Come From

Here is a deliberately hypothetical comparison to make the trade-off concrete. Suppose reducing matching-network loss saves 1.0 dB, while the extra tuner and feedline loss associated with the alternative installation costs 0.4 dB. The net improvement is 1.0 − 0.4 = 0.6 dB, or 100.6/10 ≈ 1.15: about 15% more power reaches the antenna. The installation with the less attractive antenna-side SWR would then be delivering more power, not less.

Those figures are assumptions, not EFOC or EFHW measurements. If the extra feed-system loss were 1.2 dB instead of 0.4 dB, the same 1.0 dB saving would leave a 0.2 dB deficit. The result turns on the actual losses—not on which system wins the SWR contest.

For DX, one more term matters: where the antenna sends that power. At equal transmitter reference power, with all terms evaluated consistently:

If directional antenna gain is equal, the hypothetical 0.6 dB delivery improvement becomes a 0.6 dB EIRP improvement. A 1 dB gain disadvantage in the wanted direction would instead outweigh it. Equal wire length and height do not establish equal gain: multiband long wires develop lobes and nulls, and slope, ground, return conductors and common-mode current can move them.

This gives my design preference a useful boundary. The EFOC is attractive when its moderate transformation task, controlled return and usable pattern preserve more of the station's power in the direction I need than a high-ratio end-feed arrangement does. The criterion is a better complete system, not a compulsory 1:1 at every point along it.

How to Make a Defensible Comparison

1. Declare one power reference plane. Use the transmitter connector, tuner output, feedline input or antenna port consistently.
2. Record the complete geometry. Include both wire arms, height, slope, conductor, ground parameters, transformer, counterpoise, coax route and choke locations.
3. Measure complex impedance at the feedpoint. Calibrate or de-embed to that plane; a shack SWR includes and can be masked by the feedline.
4. Measure matching-network loss. Use a method valid for the high transformation ratio and complex load, and repeat at operating power after thermal stabilization.
5. Calculate mismatched line loss. Use the measured load impedance and actual cable attenuation at every band, not SWR alone.
6. Measure common-mode current safely. Use an appropriate RF current probe; disable transmission before moving chokes, rerouting conductors or changing connections. Do not touch-test a transmitting antenna.
7. Model or measure the pattern. Include real ground and the feedline exterior. Compare realized gain at the azimuth, elevation and polarization of interest.
8. Validate in the field. Use calibrated substitution or field-strength measurements, stable geometry, rapid A/B switching and enough observations to separate propagation variation from the antenna.
9. Publish uncertainty and raw data. Retain S-parameters, calibration files, power readings, temperatures, model files and pattern data.

Receiver reports, FT8 spots and QSOs are useful operational evidence but do not isolate EIRP. Propagation, remote antenna pattern, polarization, receiver calibration, local noise and time variation remain uncontrolled.

Power Handling Is a Separate Test

Lower transformation ratio may reduce voltage ratio in one design, but it does not by itself prove legal-limit capability, cool FT8 operation or freedom from saturation. A transformer rating needs the exact core, winding, load, frequency, PEP, average power, duty cycle, ambient temperature, enclosure and permitted mismatch. Monitor loss, core temperature, winding temperature, impedance drift and common-mode current through a protected power ramp.

Likewise, an EFHW transformer's higher impedance ratio does not prove failure. A sufficiently large, well-designed transformer can perform efficiently within its documented envelope. The useful engineering question is not 4:1 versus 49:1 in isolation; it is whether the complete measured network meets loss, voltage, current and temperature limits for the actual installation.

Legal Power and RF Exposure

Do not treat “legal limit” as one worldwide transmitter or EIRP number. The current Belgian BIPT amateur table, for example, expresses many HF limits as transmitter power but uses ERP or EIRP in specific allocations. Other administrations and licence classes use different definitions. Check the current licence conditions for the operator, band and emission.

If a limit is stated in ERP or EIRP, higher directional gain can require lower transmitter power. Even when the legal limit is transmitter output, a change in realized gain changes fields around the antenna and therefore the exposure assessment.

ICNIRP's 2020 RF guidelines cover 100 kHz to 300 GHz, but applicable limits and assessment procedures come from national law. At HF, near-field electric and magnetic fields, induced or contact currents, antenna geometry, distance, duty cycle, modulation and time averaging can matter. A far-field EIRP shortcut is not automatically valid close to the antenna. Perform the assessment on the higher credible field configuration; transformer loss is not a safety control.

My Answer to the Perfect-Match Question

No: an EFHW showing 1:1 has not beaten an EFOC showing 3:1 simply by winning on the meter. I prefer the EFOC approach when I can use its lower transformation burden and defined return arrangement to build the more efficient, manageable station system. Accepting some antenna-side mismatch can be a sensible price for that choice; it is not a defect to conceal.

I still want a safe load at the transmitter, tolerable feedline and tuner losses, properly controlled common-mode current and a useful radiation pattern. A well-designed EFHW can meet those goals too. But “mine has a perfect match” is not an answer to the engineering argument. Show where the power goes, and where the current flows. That is what decides whether the DX station receives the benefit.

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