Quad-Core EFHW Inverted-Ls: Ratios, Loss and QRO Limits

The transformer is optimised together with the intended 160/80 m, 80/40 m or monoband 40 m Inverted-L geometry. Ratio, magnetising behaviour, ferrite and conductor loss, insulation stress, return current and thermal duty are controlled as parts of one finished antenna system.

The Current EFHW Inverted-L Product Family

RF.Guru currently uses two geometry-specific transformer classes across the low-band Inverted-L family:

The 68:1 design targets the measured several-kilohm end-feed region of the 160/80 m installation. The 70:1 design serves the current 80/40 m and monoband 40 m Inverted-L systems. The former 20:1 EFHW4010 arrangement is outside this current product family; RF.Guru’s product-family notice explains that the EFHW4010 and EFHW8010 are no longer produced.

Use the exact model manual because its ICAS, CCS and FT8/FT4 figures carry different duty and VSWR boundaries. Operating frequency, keying cycle, mean power, ambient temperature, enclosure orientation and complex antenna load remain part of the rating conditions.

Impedance Ratio Is a Turns-Ratio Result

For an ideal transformer, the impedance transformation is the square of the voltage or turns ratio:

A statement such as “three primary turns produces 70:1” is incomplete. The total winding, tap position, connection sense and interleaving establish the transformation. A three-turn low-side section can participate in many ratios.

Magnetizing inductance is a different constraint

The magnetizing branch appears in shunt with the transformed load. Its reactance is approximately XL = 2πfLm. It should be high enough at the lowest operating frequency that magnetizing current and phase error remain acceptable, but there is no universal “far above 50 Ω” threshold that proves low loss or a power rating.

For the same core stack in the small-signal region, inductance scales approximately with turns squared. Moving from four turns at frequency f to three turns at 2f changes the ideal reactance by 2 × (3/4)² = 1.125, not by an automatic factor of two. Real permeability is complex and frequency-, temperature- and field-dependent. The Fair-Rite 17th-edition catalogue explicitly plots the real and imaginary components of permeability against frequency and identifies temperature and bias as important variables.

How the Quad-Core Architecture Supports QRO

The quad-core assembly increases effective magnetic cross-section and thermal mass while distributing linked flux and loss across the transformer. RF.Guru controls core grouping, winding distribution and the enclosure thermal path as a finished assembly so the architecture delivers repeatable ratio, loss and temperature behaviour.

QRO qualification covers more than saturation. Complex permeability, flux swing, waveform, frequency, duty cycle and temperature set core loss; conductor geometry and connections add copper loss; leakage inductance and interwinding capacitance shape the loaded response. The published ratings therefore belong to measured completed products under their declared frequency, load, VSWR and duty conditions.

The precise ferrite selection and winding implementation remain part of RF.Guru’s controlled product design. Public documentation identifies the quad-core architecture and completed-product performance without turning proprietary construction details into a universal winding recipe.

QRO Means Kilovolts and Potentially Serious Heat

For a sinusoid delivering power P into a purely resistive transformed load R, Vrms = √(PR) and Vpeak = √2 Vrms. The following values are ideal reference-plane examples, not predictions of the maximum voltage inside a real autotransformer:

Actual winding voltage depends on the complex antenna impedance, transformer parasitics, standing-wave pattern, reference plane and transient conditions. VSWR alone does not reveal where the worst voltage or current occurs: load phase and electrical length matter. Arcing can occur at terminals, winding crossovers, moisture or contamination paths even when average core temperature looks acceptable.

PTFE-insulated winding conductors provide temperature and dielectric margin inside the transformer. RF.Guru combines them with controlled conductor spacing, PV coating, nylon spacers, a polycarbonate enclosure, 316 stainless-steel hardware and compression sealing; higher-stress designs add PTFE sleeving where required. The completed assembly—not one material in isolation—establishes the model’s RF-voltage and environmental rating.

Interpreting the measured <0.2 dB insertion-loss limit

An insertion loss of exactly 0.2 dB corresponds to 10^(−0.2/10) = 95.50% power throughput. The difference would be about 67.5 W at 1,500 W input, 90.0 W at 2,000 W and 180.0 W at 4,000 W. Because the product says less than 0.2 dB, these are boundary examples—not claimed dissipation values.

The insertion-loss limit and the QRO rating answer related but different questions. RF.Guru assigns the product rating to the completed assembly under its defined power, duty and VSWR conditions; the installed antenna then needs the specified complex-load range, ventilation and common-mode boundary so the laboratory rating remains applicable.

The Inverted-L Is One Bent Radiator

An EFHW inverted-L is not accurately described as a vertical radiator with a horizontal “loading” wire. Both sections carry the standing-wave current and both radiate. On the fundamental half-wave mode, current is small near both ends and reaches a maximum roughly halfway along the total electrical wire. Whether that maximum falls in the vertical or horizontal section depends on the bend position and electrical length.

On the harmonic band, the wire supports additional current maxima and minima, so the pattern acquires more lobes and nulls. Height, bend angle, ground parameters, wire slope, buildings, trees, feedline and return conductors affect feed impedance, polarization and elevation pattern. The in-force ITU-R BS.705-2 makes its ground and current-distribution assumptions explicit when calculating HF patterns, and IEEE 145-2025 keeps directivity, gain, efficiency and impedance as distinct antenna terms.

Consequently, no fixed feed height creates an “ideal impedance plateau,” and low SWR does not prove efficiency or a desired take-off angle. Model the complete geometry over the actual ground, then validate it with feedpoint impedance, common-mode current, transformer temperature and field-strength measurements.

Return Current Must Be Designed, Not Assumed

An end-fed antenna still needs a return path. Current can divide among a deliberate counterpoise or ground system, the outside of the coax, mast and bonding conductors, and capacitance to the surroundings. A counterpoise length near 0.02 wavelength may be a useful experiment, but it is not a universal electrical requirement and it does not guarantee low common-mode current.

Likewise, “place the choke 0.05–0.1 wavelength away” is a tuning recipe, not a law. Moving the choke changes the length and impedance of the coax section allowed to participate in the antenna, and therefore can change resonance, loss and pattern. Select the intended RF boundary first, then measure common-mode current and use a choke whose complex impedance, heating and voltage withstand are adequate on every operating band. Steve Hunt, G3TXQ’s measured HF choke work illustrates why resistance, reactance and useful frequency range—not a single headline impedance—matter.

A second choke at the station entry can be a sensible EMC boundary, but it does not rescue an undefined feedpoint return system. Route and bond the station for lightning and fault safety separately from the RF design.

Harmonic Resonance Is Not Automatic Dual-Band Performance

A wire near a half-wave on 160 m is near a full wave on 80 m; an 80 m half-wave is near a full wave on 40 m. “Near” matters because conductor velocity factor, end effects, bends, ground and coupling move both resonances and impedances. The end impedance on the harmonic can be higher or lower and more reactive than on the fundamental.

The transformer then sees the installed impedance through its leakage inductance, winding capacitance, loss and return network. A tuner may make the transmitter see 50 Ω, but it cannot undo transformer dissipation, arcing or unwanted feedline radiation upstream. Validate both bands separately at the intended power and duty cycle.

A Shunt Capacitor Is a Design Choice, Not a Moral Failure

A shunt capacitor can compensate part of the transformer’s frequency-dependent susceptance or parasitic response. It can also carry substantial RF current or voltage, narrow a useful match, heat, or mask a poor operating point if selected badly. Omitting it can be equally valid when measured loaded response supports the decision.

The defensible comparison is not “capacitor versus no capacitor” in isolation. Compare complex input impedance, insertion loss, temperature rise, voltage/current stress, common-mode current and repeatability across the intended antenna loads, bands, power and environment.

Applying the Product Rating to an Installation

1. Identify the exact model. Record the RF.Guru model and assembly revision, published transformer ratio, conductor and insulation system, terminals and enclosure.
2. Define the loads. Measure the installed antenna’s complex impedance at the transformer terminal on every claimed band and across the intended segment; include plausible environmental movement.
3. Measure loss correctly. Use a calibrated method with fixture correction and uncertainty. Test representative resistive and complex loads rather than inferring efficiency from SWR.
4. Run thermal tests. State carrier or waveform, PEP and mean power, on/off cycle, duration, ambient temperature, enclosure orientation, wind/solar conditions and temperatures at cores, winding, connector and terminals.
5. Check electric stress. Establish RF withstand and partial-discharge/arcing margin for the complete contaminated, humid assembly—not merely the insulation material.
6. Control the current path. Measure coax common-mode current and transformer/choke heating with the intended counterpoise, grounding, bonding and cable routing.
7. Verify antenna performance. Separate accepted power, loss, radiation efficiency, gain, realised gain, pattern and SWR. A match measurement alone establishes none of the others.

Primary Sources Checked

• RF.Guru EFHW16080 current product page
• RF.Guru EFHW8040 current product page
• RF.Guru EFHW40 current monoband product page
• RF.Guru product-family notice — EFHW8010/EFHW4010 discontinued
• Fair-Rite 17th-edition catalogue — ferrite material, frequency, temperature and bias data
• IEEE 145-2025 — Standard for Definitions of Terms for Antennas
• Recommendation ITU-R BS.705-2 — HF antenna characteristics and patterns
• ICNIRP 2020 — RF exposure guidelines, 100 kHz–300 GHz
• IEC 60664-1:2020 — insulation-coordination principles and scope boundary
• G3TXQ — measured common-mode choke resistance/reactance

Questions, antenna-factor records or height trials to share? Contact RF.Guru.