Multiband EFHWs: Why Purpose-Built Transformers Work Better

End-Fed Half-Wave (EFHW) antennas are often praised as "plug-and-play" solutions for HF operation. With a single long wire and a transformer—typically 49:1—you can seemingly cover multiple bands with minimal effort. But beneath that simplicity hides a set of compromises that most hams overlook.

One key issue is transformer linearity, which becomes especially problematic as you stretch a single design across many bands.

High Impedance Ratio = Poorer Linearity

The standard EFHW transformer typically uses a 49:1 impedance transformation ratio, derived from a 7:1 turns ratio (7² = 49). This high ratio is necessary to match the high feedpoint impedance of a half-wave radiator—often between 2,000 and 3,000 ohms—to the 50-ohm coax.

But that high transformation ratio comes at a cost:

• Non-linearity increases with turn count, especially when using materials like type 43 or 61 ferrite. The transformer becomes reactive—acting less like an ideal broadband impedance transformer and more like a lossy LC network.
• Parasitic effects such as interwinding capacitance and leakage inductance distort the voltage/current relationship, especially at higher frequencies.
• Core saturation and micro-hysteresis can further reduce dynamic range and increase harmonic distortion, particularly at higher transmit power levels.

In short: the more turns you add, the more you deviate from ideal transformer behavior—and that becomes critical when you’re trying to cover many bands with one box.

Wide Bandwidth, But At What Cost?

Ironically, this non-linearity creates the illusion of a wide usable bandwidth. The transformer begins to behave more like a broadband lossy network than a tuned device. While this can "work" in terms of achieving a match across multiple bands, the price is inefficiency:

• On higher bands (15m, 12m, 10m), insertion losses increase significantly—often exceeding 30% on 10m
• On intermediate bands like 17m, 15m, and 12m, measured losses are still substantial—often in the 12–20% range depending on core material and transformer design
• RF gets dissipated as heat in the ferrite instead of being radiated
• Impedance transformation becomes unstable, especially under varying SWR and load conditions

Measured Transformer Losses

Transformer	Material	Bands	Loss (%)	Loss (dB)	Notes
68:1 EFHW	77	160/80m	8–12%	0.36–0.57	Used in dual-band 160/80m Inverted-L EFHW
70:1 EFHW	77	80/40m	6–10%	0.26–0.46	Dual-band 80/40m, also used in Inverted-L
49:1 EFHW	43	40/20m	4–7%	0.18–0.31	Typical multiband transformer
49:1 EFHW (Mono)	43	20m only	2–3%	0.09–0.13	Monoband transformer optimized for efficiency
49:1 EFHW	43	15m	~16–18%	~0.75	Efficiency drop accelerates
49:1 EFHW (10m)	43	10m	>30%	>1.5	Very poor efficiency; core heating, poor radiation
49:1 EFHW (10m)	52	10m	>25%	~1.2	Slightly better, but still lossy; 52 is no fix-all

Newer material like type 52 has been suggested as a way to improve high-band efficiency in EFHW transformers, especially above 21 MHz. While 52 does shift the optimal range upward, it does not resolve the fundamental linearity and parasitic issues caused by high turns ratios and wideband mismatch. At best, it mitigates loss by a few dB—not nearly enough to justify the complexity of a multiband EFHW system.

Material Matters: Why We Don’t Use One Core for Everything

This is why we choose different ferrite materials depending on the target bands:

• For 160–40 meters, we use type 77 ferrite—a material optimized for low-frequency impedance transformation. It offers high permeability and low loss below 10 MHz.
• For the upper HF range, particularly monoband transformers, we switch to type 43 or 52. Type 43 is well-suited for 20m and below when properly designed. Type 52 can help slightly above 21 MHz, but does not eliminate the inefficiency seen in wideband transformers.

Trying to force one core to cover all bands is like trying to use a single lens to photograph a landscape and a microbe—it just doesn't work well.

Purpose-Built EFHW Transformers Outperform All-Band Designs

All our EFHW transformers are purpose-built and designed to handle 3–4 kW without core saturation or excessive heating. Because they are optimized for specific bands or band pairs, the I²R losses are kept within manageable levels. This ensures not only safe operation at QRO levels but also improved efficiency at lower power levels.

Contrary to popular belief, QRO operation—when matched with an efficient transformer—can actually reduce relative I²R losses, as the system runs closer to its design impedance and magnetic efficiency. Our approach guarantees better performance across the board: higher efficiency, greater thermal margin, and more predictable behavior at any power level.

Given the limitations of broadband EFHW transformers, we believe it makes more sense to tailor your transformer to the band segment you actually use:

• For monoband or dual-band EFHWs, you can optimize turn count, core material, and wire layout for maximum efficiency
• For multi-band operation, a better alternative is often an Off-Center Fed Dipole (OCFD) with a lower transformation ratio (typically 4:1), which shows better impedance behavior across 40–10m or 20–10m

This approach gives you:

• Higher efficiency
• Lower ferrite losses
• Cleaner radiation patterns
• Stable impedance transformation

And yes, often better EIRP than a "multiband" EFHW with a boiling-hot core and a poor radiation pattern.

The Bottom Line

Multiband EFHWs sell well because they’re convenient—but they’re far from ideal. Once you understand the limitations introduced by high transformation ratios and core behavior across frequency, the conclusion is obvious:

If you want efficient multiband coverage, use multiple purpose-built transformers or switch to a better architecture like the OCFD.

A few extra minutes of planning and construction will yield a much more effective station—and a much cooler transformer.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.