The Challenges of Multiband End‑Fed Half‑Waves and Ferrite Losses
Updated August 21, 2025
Multiband End‑Fed Half‑Wave (EFHW) antennas are popular for their simplicity and coverage. But expecting one ferrite‑core transformer to deliver high efficiency from 80 m (3.5 MHz) through 10 m (28 MHz) is wishful thinking. Material physics, winding losses, and operating geometry impose hard limits. This article breaks down where the losses come from and why “one core to rule them all” is a compromise at best.
The Ferrite Core’s Job in EFHW Transformers
EFHWs need a high‑ratio unun (commonly 49:1 or 64:1) to transform ~2.5–5 kΩ down near 50 Ω. Ferrite toroids provide the magnetizing inductance and coupling to make that happen. The catch: ferrite mixes are frequency‑selective — permeability and loss tangent vary with frequency and temperature. No single mix is truly “wideband HF” without trade‑offs.
Frequency‑Dependent Losses in Ferrite Cores
Low bands (80/40 m):
- Higher magnetizing currents due to low reactance → more core heating (hysteresis + eddy loss).
- Turns count grows to keep inductance high → longer wire → higher copper loss (I²R).
- Permeability shifts with temperature can detune the transformation under load.
High bands (20–10 m):
- Stray capacitance/leakage inductance erode ideal transformation; insertion loss rises.
- Skin effect shrinks the effective conductor area → higher AC resistance.
- At QRO, core approaches flux density limits on peaks → non‑linear loss and compression.
High Power Factors: Core and Copper Loss
Core saturation & heating: Low‑band operation at high power pushes flux density; once near saturation, hysteresis loss spikes and efficiency collapses.
I²R in windings: More turns = more resistance. At HF the AC resistance rises further from skin and proximity effects. Result: conductor heating and extra dB lost before power reaches the wire.
Common Ferrite Mix Trade‑Offs (Indicative)
Ranges below are rule‑of‑thumb for EFHW transformers; exact behavior depends on core size, turns, and layout.
| Ferrite Mix | Favored Range | Strengths | Weaknesses |
|---|---|---|---|
| Fair‑Rite 43 | ~3–10 MHz | Good μ for 80/40 m inductance; easier matching at low bands | Noticeable loss on 15/10 m; gets warm at QRO low‑band duty |
| Fair‑Rite 52 | ~7–30 MHz | Lower loss above ~20 m; better high‑band behavior | Less inductance for 80 m unless turns and size increase |
| Fair‑Rite 61 | ~10–30 MHz | Best for upper HF efficiency | Poor 80 m performance; needs large cores/turns to compensate |
Why Single‑Core “80–10 m” EFHWs Disappoint
- Low bands: Core heating from high magnetizing current + long, resistive windings.
- High bands: Stray C and leakage L degrade transformation; skin effect boosts copper loss.
- Everywhere: High turns ratios magnify imbalance sensitivity and voltage stress, inviting “magic capacitors” that fix SWR while hiding loss.
Practical Paths to Better Efficiency
- Core strategy: Stack or hybridize mixes (e.g., 43 + 61) sized for thermal headroom; keep turns minimal for target bands.
- Band segmentation: Use separate EFHWs or switchable transformers for low vs high HF. Dual‑band (½λ + full‑wave) pairs are the sweet spot.
- Winding optimization: Tight but not inter‑winding capacitive; short paths; consider bifilar/trifilar only where beneficial; prefer thicker wire or litz where heating is observed.
- Installation‑aware ratios: Choose 49:1 for high horizontal installs; consider 68–70:1 for inverted‑L/vertical EFHWs that present 4–6 kΩ+ at the feed.
- Choking & returns: Provide a defined return (counterpoise) and a high‑CMR choke at the feed to keep loss and pattern drift in check.
Conclusion
Multiband EFHWs are convenient, but a single ferrite core cannot be simultaneously optimal for 80 and 10 meters. Frequency‑dependent ferrite behavior, transformer copper loss, and voltage stress make “full HF with one core” a compromise — often with hidden dBs lost as heat. If you want consistent performance, split the problem: tailor core mix and turns to target bands, or segment antennas. You’ll gain efficiency, stability, and reliability — especially at QRO.
Mini-FAQ
- Why does my “80–10 m” EFHW run hot? — Core and copper losses. Low‑band magnetizing current and high‑band stray capacitance both add heat.
- Will a “magic” capacitor help? — It can make SWR pretty, but often hides loss and distorts current distribution.
- Can I make one core work better? — Use a larger core (thermal headroom), minimize turns, and pick a mix for your priority bands.
- What’s the efficient EFHW sweet spot? — Dual‑band pairs (½λ + full‑wave) with a transformer ratio chosen for the actual install height and geometry.
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