📐 How EFHW Length & Matching Work

An end-fed half-wave is, electrically, the exact same wire as a center-fed dipole cut for the same frequency — 468 ÷ frequency (MHz) gives the same total length either way. What changes is where you connect the feedline. Feeding at the center gives a moderate, easy-to-match impedance (around 70Ω in free space). Feeding at the very end of the same wire lands you at the opposite extreme: a high-impedance point, empirically measured anywhere from roughly 1800 to 5000 ohms depending on height, ground, and counterpoise — far too high to connect directly to 50-ohm coax with any kind of efficiency.

That's the entire reason EFHW antennas need a special impedance-transforming "unun" (unbalanced-to-unbalanced transformer) instead of a simple 1:1 balun. The length math is identical to a dipole; the feed hardware is what's genuinely different.

📡 Which Bands You Actually Get Without a Tuner

This is the part most EFHW marketing oversimplifies, and it's worth getting right before you buy or build one. A 40m EFHW is genuinely a multiband antenna — but exactly which harmonic bands it covers without a tuner depends on precisely where in the 40m band you cut it.

The 4th harmonic (10m) is the most dependable — it stays inside the 10m band across the entire legal 40m range. The other two are more cut-frequency-sensitive than most calculators let on. The 2nd harmonic (20m) stays inside the 20m band only up to about 7.175 MHz — cut above that (which is still well within the legal 40m band, which runs to 7.3 MHz), and the 2nd harmonic drifts above the 20m band edge. The 3rd harmonic (15m) is even tighter: cut near the bottom of 40m (around 7.0–7.15 MHz), the 3rd harmonic lands at roughly 21.0–21.45 MHz — inside the 15m band. Cut closer to the top of 40m (toward 7.3 MHz), that same 3rd harmonic shifts up to around 21.9 MHz, well above the 15m band edge and out of range without a tuner. The WARC bands — 30m, 17m, and 12m — simply aren't harmonically related to a 40m half-wave at all, regardless of cut frequency, and always need a tuner on a standard 40m EFHW.

A full-size 80m EFHW (cut around 3.5–3.6 MHz) extends this same pattern one band lower, commonly reaching 80/40/20/15/10m — but the same cut-frequency sensitivity applies: cutting toward the higher end of 80m can push the 4th and 6th harmonics (20m and 15m) out of their bands, leaving only 80/40/10m reliably covered.

🔌 The 49:1 Unun, Explained Correctly

Impedance transformation through a toroidal transformer scales with the square of the turns ratio, not the turns ratio itself — a detail that trips up a surprising number of online explanations. A 7:1 turns ratio gives a 7² = 49:1 impedance ratio, which steps 50Ω coax up to 2450Ω (50 × 49) — right in the middle of the typical 1800–5000Ω measured EFHW feedpoint range. That's why 49:1 (built from 7 turns) is the standard, not an arbitrary marketing number.

Some commercial and homebrew units use a 64:1 ratio instead, built from an 8:1 turns ratio (8² = 64, targeting 3200Ω). Neither ratio is universally "more correct" — they're both reasonable points inside the same wide real-world impedance range, and which one performs better on your specific installation comes down to your actual measured feedpoint impedance, not a spec sheet number.

A small 100–150 pF capacitor is sometimes added across the unun's high-impedance terminals. It resonates with the transformer's own leakage inductance at higher frequencies, which can improve the match specifically on 15m and 10m — many working EFHW builds skip it entirely and only add it if SWR on those bands is marginal.

⚡ Counterpoise: The Part Most Builders Skip

Every antenna needs a return path for RF current — even a single-wire EFHW. "No counterpoise needed" is one of the most common pieces of misinformation repeated about EFHW antennas. What's actually true is that the counterpoise can be very short compared to other antenna types, and it's easy to overlook because the coax shield will silently take over that job if you don't provide one deliberately.

The standard sizing rule of thumb: counterpoise length ≈ 0.05 × the longest wavelength you plan to operate on. For an EFHW covering down to 40m, that works out to roughly 2 meters (about 6.5 feet). If your antenna also covers 80m, size it for 80m instead — about 4 meters (around 13 feet). This counterpoise can be a dedicated wire connected to the unun's ground terminal, run away from the main antenna wire, or — if you don't add one — the coax shield itself will serve the same role by default.

🛡️ Common Mode Choke Placement

Because an EFHW depends on a counterpoise rather than a balanced feed, RF current on the outside of the coax shield is a real and common issue — more so than with a center-fed dipole. A common mode choke (several turns of the coax through a ferrite toroid) suppresses that current before it reaches your shack.

Placement matters: don't put the choke directly at the feedpoint if you're relying on the coax shield as your counterpoise — choking the shield right there removes the counterpoise the antenna needs to work against, which can hurt your SWR rather than help it. The common practical approach is a short run of coax (several feet, often matched to the 0.05λ counterpoise length) between the unun and the choke, letting that section of coax serve as the counterpoise while the choke isolates everything beyond it from carrying RF back to the radio. If you've added a separate dedicated counterpoise wire instead, the choke can sit much closer to the feedpoint without that conflict.

⚖️ EFHW vs. Center-Fed Dipole

On the bands where the harmonics line up, an EFHW performs essentially the same as a dipole of the same length and height — the radiating wire is electrically identical either way. The real tradeoff is installation convenience versus feed cleanliness: one support and one transformer (EFHW) versus a self-balanced feed that doesn't depend on getting a counterpoise right (dipole).

❌ Common Mistakes

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