Designing and Building a 49:1 End‑Fed Half‑Wave Antenna for HF

End‑fed half‑wave (EFHW) antennas are popular because they combine simple deployment with multiband capability. A single wire roughly a half‑wavelength long at your lowest band can also resonate on harmonics, giving you coverage on several bands without traps or a tuner. The key to a good EFHW is understanding where the high voltage and high impedance live, how to transform that impedance to 50 Ω, and how to manage common‑mode currents so your shack stays quiet and your signal gets out. In this guide, we’ll calculate the wire length, design a 49:1 transformer (unun), and walk through practical build and tune steps—with safety and reliability in mind.

Understanding the Basics

An EFHW is a half‑wave radiator fed at the high‑impedance end. A straight half‑wave wire in free space presents an end impedance on the order of 2–3 kΩ. Most HF transceivers expect about 50 Ω, so we need an impedance transformation of roughly 40–60:1. A 49:1 transformer is a common sweet spot.

• Half‑wave wire length (approximate): L(ft) ≈ 468 / f(MHz) L(m) ≈ 143 / f(MHz) The constant 468 accounts for “end effects” of real wire near Earth. You will trim to final resonance after installation.

• Harmonics: A half‑wave at f0 also resonates at 2·f0, 3·f0, 4·f0… (within limits). A 40 m EFHW (≈66 ft) often works on 40, 20, 15, and 10 m.

• Voltage/current distribution: Current is maximum and voltage minimum at the center; at the wire end the current is low and voltage is high. Keep the end clear of people, gutters, and vegetation (arc and RF‑burn risk), and use good insulators.

Safety: Evaluate RF exposure per FCC §97.13(c). EFHWs can have high end‑voltages. Maintain clearances and use proper hardware.

Key Concepts in EFHW Design

1. Impedance transformation The impedance ratio is the square of the turns ratio: Zratio = (Ns/Np)^2. To transform ~2,450 Ω to 50 Ω, we want (Ns/Np) ≈ √(2450/50) ≈ 7:1. A practical winding is 2:14 or 3:21 turns on a ferrite toroid, yielding ≈49:1.

2. Core selection

   • Mix 43 ferrite (e.g., FT240‑43): broad HF coverage (3–30 MHz), good general‑purpose choice.
   • Mix 52 ferrite (FT240‑52): often runs cooler on higher bands; slightly less inductance per turn.
   • For common‑mode chokes, Mix 31 is effective 1–10 MHz; Mix 43 works well 5–30 MHz.

3. Capacitive compensation A small NP0/C0G capacitor (≈100–150 pF at ≥3 kV) across the primary (50 Ω side) improves high‑band SWR and reduces core heating by compensating leakage inductance.

4. Counterpoise and feedline EFHW systems need a return path. Many designs rely on the coax shield as a short counterpoise. For stability, provide a modest counterpoise (e.g., 0.05λ at the lowest band) and/or add a common‑mode choke at the feedpoint and another 0.05–0.1λ down the line.

5. Shortening for lower bands To reach 80 m from a 40 m‑length wire, use a loading coil and a tail wire beyond the coil. Start values: inductance on the order of 100–120 µH with a 6–8 m tail; iterate in place for resonance.

Practical Application

Let’s design a portable 40 m EFHW that also covers 20/15/10 m.

1. Choose the design frequency and compute length

   • Target: 7.1 MHz (middle of 40 m phone in Region 2)
   • Initial length: L ≈ 468/7.1 ≈ 65.9 ft (20.1 m). Cut to 66 ft; you’ll trim shorter to raise frequency.

2. Wind the transformer (49:1)

   • Core: One FT240‑43 (100 W SSB/CW class). For higher duty cycle or power, stack two cores.
   • Turns: Primary 2 turns; secondary 14 turns (evenly spaced). Use 14 AWG enamel wire for durability.
   • Wiring: Autotransformer style with the coax shield and counterpoise at the cold end; center conductor to the primary; the high‑impedance end to the long wire.
   • Capacitor: 120 pF NP0 across the primary.
   • Enclosure: Weatherproof box, eye bolt on the high‑Z side for strain relief.

3. Provide a counterpoise and choke

   • Counterpoise: 8–10 ft (≈0.05λ at 7 MHz) insulated wire connected to the transformer ground.
   • Choke 1: At the transformer, several #31 beads on the coax or 10–12 turns of RG‑316 through an FT240‑31.
   • Choke 2: 10–15 ft down the coax (≈0.05–0.07λ) to tame residual common‑mode current.

4. Deploy the wire

   • Sloper: Feedpoint ~6–8 ft above ground, far end 25–35 ft high on a tree or mast.
   • Inverted‑L: Vertical rise then horizontal run; good for lower takeoff angles.
   • Keep the wire and especially the end at least a few feet from metal objects and reachable areas.

5. Trim for resonance

   • Measure with an analyzer. If resonance is below 7.1 MHz, trim 1–2 inches at a time. Expect final length ~1–3% shorter than the initial cut depending on height and surroundings.
   • Check 20/15/10 m: Harmonic resonances should land in‑band; small shifts are normal.

6. Optional 80 m extension

   • Insert a ~110 µH loading coil ≈60–70% of the way from the feedpoint, then add a 6–8 m tail.
   • Trim the tail to bring 80 m resonance into your preferred sub‑band. Re‑check 40/20 m; minor interactions are normal.

7. Station checks

   • With 100 W SSB, the FT240‑43 should run warm but not hot. If it overheats, add turns, use a larger/stacked core, improve compensation, or reduce duty cycle (digital modes stress cores most).
   • Verify RF exposure using HamCalc’s RF Safety tool: enter frequency, power, antenna gain (~2 dBi baseline), duty factor, and distance to occupied areas.

Example: 20 m monoband EFHW

• Target: 14.2 MHz → L ≈ 468/14.2 ≈ 33.0 ft (10.06 m). Use same 49:1 transformer; a short 0.05λ counterpoise (~3.5 ft). Expect a low SWR near 14.2 and useful bandwidth across the band with the primary compensation capacitor.

Common Mistakes to Avoid

• Using the wrong core mix: Powdered iron (e.g., T‑200‑2) is not appropriate for this broadband transformer; loss and heating will be excessive. Use ferrite (43 or 52 for the unun; 31/43 for chokes).
• Too few turns: Insufficient turns increase magnetizing current and core heating. Start with 2:14 or 3:21; adjust only with a clear reason and measurement.
• No common‑mode control: Relying solely on the coax shield often causes RF in the shack, touchy tuning, and RFI. Add a counterpoise and at least one choke.
• Ignoring strain relief: The transformer’s high‑Z lug should not carry wire tension. Use an eye bolt and proper end insulators to keep mechanical and electrical functions separate.
• Trimming in the wrong place: Trim only at the far end. Shortening near the feedpoint can alter the voltage distribution and mechanical layout.
• End too close to people/metal: High voltage at the end can arc to gutters or give painful RF burns. Maintain clearances and use ceramic or UV‑resistant polymer insulators.

Tips and Recommendations

• Weatherproof thoroughly: Seal enclosure lid, SO‑239, and wire exits with gasket and UV‑stable sealant. Use drip loops on coax and wires.
• Measure as you build: Use a resistive 2.7–3.3 kΩ dummy load on the high‑Z side while testing the transformer; you should see ≈1:1 SWR if the 49:1 is behaving.
• Model before cutting: Use EZNEC or HamCalc’s EFHW estimator to predict resonances at your installation height and soil.
• Keep notes: Record lengths, coil turns, and choke placements. Small details matter and will help you reproduce success.
• Duty cycle awareness: For FT8/RTTY or AM, reduce power or upgrade the core stack to avoid overheating.

Conclusion

A well‑built EFHW offers an efficient, compact, and flexible HF solution. Calculate the half‑wave length, use a properly wound 49:1 transformer with good ferrite, provide a modest counterpoise and chokes, and trim in place. With careful construction and a few measurements, you’ll have a multiband wire that’s easy to deploy and gets you reliably on the air.