EIRP, Realized Gain and SWR: How to Compare EFHW and OCF Systems

SWR describes impedance mismatch at a declared plane. It does not, by itself, reveal transformer heating, feedline loss, radiation efficiency, common-mode current or the field in a DX direction. The right comparison combines all of those quantities without renaming assumed loss as measured EIRP.

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, include tuner, matching-transformer and feedline efficiencies before using antenna-port gain. A useful end-to-end form is:

Do not multiply by a mismatch factor twice. Use either accepted power with gain or incident power with realized gain at the same antenna port.

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; source re-reflection and a tuner add further dependencies. 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 uses an optional primary compensation capacitor to counter unwanted secondary capacitance on the upper bands. Such a capacitor can improve the input response of a particular transformer. It may also create higher circulating current or voltage in a poorly chosen network. Neither outcome is universal. Measure the complete transformer with its intended high-impedance load across frequency and power; do not infer efficiency from the external SWR trace.

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 balanced antenna fed away from its centre. A feedpoint near 200 ohms can make a 4:1 transformation convenient, but the impedance varies sharply with feed fraction and from band to band. The ARRL's tested OCF example uses a 4:1 current balun; “4:1” is not a guarantee that every band presents 200 ohms.

RF.Guru's EFOC17 and EFOC29 use a coax-counterpoise architecture rather than a conventional balanced OCF dipole. The intended exterior section of coax participates in the return path, while the choke defines its boundary. The complete radiating system therefore includes the wire, transformer, coax shield, choke location, station bonding and surrounding objects. Moving the choke or changing feedline length can change feed impedance, common-mode current, efficiency and pattern. IEEE work on OCF feedline-current suppression likewise shows that feedline radiation is a design variable, not an invisible ideal conductor.

Worked Example: A Loss Budget Is Not Yet EIRP

For an illustrative loss L, remaining power from a 100 W reference is P = 100 × 10−L/10. The table demonstrates that arithmetic for two hypothetical loss budgets. These values are not measured product results, total radiated power or directional EIRP.

Even with measured loss inputs, the table would not establish a directional EIRP difference. That also requires the gain pattern of each complete installation. Equal wire length and height do not guarantee equal gain on the higher bands: multi-half-wave wires develop additional lobes, and feedpoint location, slope, ground and common-mode current can move those lobes. One antenna may provide more realized gain in one azimuth and elevation angle while another leads elsewhere.

A product comparison must name the exact EFOC or EFHW variant, feedline, choke boundary and pattern used for every band. EFOC17 and EFOC29 are distinct antenna systems; data from one cannot be silently assigned to the other.

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. Map current along the feedline and repeat after changing choke position and grounding.
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.

Bottom Line

A low SWR is useful operating information, but it is not a complete efficiency measurement. RF.Guru's EFOC architecture combines a lower-ratio matching network with an intentional coax-counterpoise and choke boundary; an EFHW uses a different high-impedance feed and return-path arrangement. Each can be effective within a suitable installation.

Select and compare these systems through measured matching loss, feedline loss, return current, radiation efficiency and directional pattern. Use system realized gain or calibrated EIRP at a declared reference plane, and keep total radiated power, ERP, EIRP and SWR in their proper roles.

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