The End-Fed Half-Wave (EFHW) Antenna

The End-Fed Half-Wave (EFHW) antenna is a resonant half-wave wire fed at one end using a high-ratio impedance transformer (commonly 49:1). It's beloved by portable operators and homeowners because it offers excellent performance with minimal supports and no heavy matching networks in the shack.

This article keeps all the practical details you expect — transformer windings, enclosure tips, capacitor tweaks for 10 m — while reorganizing the content so search engines and AI models can extract answers quickly and reliably.

Understanding the EFHW Concept

The EFHW is a wire cut to a half-wavelength of a chosen band (for example, a 40 m half-wave). Because a half-wave is resonant at harmonic frequencies, a 40 m EFHW also shows resonant or near-resonant behavior on 20 m (2×), 15 m (3×/2) and 10 m (2×) — frequently allowing operation without a tuner on those harmonic bands.

Impedance and Matching — Why a 49:1?

At the end-fed feedpoint the impedance of a half-wave wire is very high (often in the ~2.5–3 kΩ range). A 49:1 unun transforms that down to a matchable range for 50 Ω coax. Without a transformer you would see very high SWR and poor power transfer. The transformer also must be mechanically and electrically robust — feedpoint voltages can be hundreds of volts at higher power.

EFHW Quick Reference

Typical Length	40m half-wave ≈ 66 ft (≈ 20 m). Trim for exact resonance as needed.
Feedpoint Transformer	49:1 unun on FT240-43 or T140-43 (or 64:1 variants for specific needs)
Resonant Bands	40m, 20m, 15m, 10m (harmonics)
Feedpoint Impedance	≈ 2.5–3 kΩ (varies with height & environment)
Polarization	Horizontal, sloper, inverted-L, or vertical (depends on deployment)
Typical SWR	1.2–1.5:1 on resonant/harmonic bands with a well-built unun

Improving 10m Performance with a Capacitor

Many EFHW users note higher SWR or tuning difficulty on 10 m. The tried-and-true fix is adding a small high-voltage capacitor (10–30 pF) across the secondary or near the feedpoint terminals. This creates a slight resonant tweak for the upper harmonic without significantly changing lower-band performance.

Capacitor tips:

• Start at 10 pF and increase in small steps while re-checking SWR.
• Use an RF-rated ceramic or NP0 capacitor rated ≥1 kV RF.
• Mount it inside the weatherproof unun enclosure and insulate thoroughly to prevent arcing.

Why the EFHW Is So Effective

The EFHW’s efficiency comes from two things: resonance and low-loss matching. A resonant half-wave radiates effectively, and a properly built 49:1 transformer presents the radio with a usable impedance without forcing lossy networks in the shack. Compared to multi-band verticals requiring traps or tuning, EFHWs are mechanically simpler and often quieter (less loss).

How to Build an EFHW — Parts & Materials

Below is a practical parts list based on field-tested builds:

• Transformer: 49:1 unun wound on FT240-43 or T140-43 toroid — typical winding: 2 primary turns, 14 secondary turns (adjust per core spec).
• Radiating wire: 66 ft (≈20 m) for 40m half-wave (18–22 AWG stranded or enamel).
• Enclosure: Weatherproof box with SO-239 bulkhead. Use silicone sealant and a drip loop at connectors.
• Counterpoise / shield: A short counterpoise or use coax shield; results vary by installation.
• Support: Fiberglass pole, tree branch, mast or attic/balcony routing.

Trim the radiator for exact resonance; measurement in-place yields best results since local environment affects length slightly.

Assembly & Deployment — Step-by-Step

1. Wind the unun: Follow a tested winding diagram for your chosen core. Ensure solid solder joints and varnish or seal turns to prevent movement.
2. Mount in enclosure: Install the unun on a bulkhead with SO-239, secure ground lug, and seal entries.
3. Attach radiator: Connect the radiator to the high-impedance terminal. Add mechanical strain relief (paracord or similar).
4. Install counterpoise: Connect a short counterpoise or ensure coax shield is connected to ground lug as your return path.
5. Position antenna: Deploy as horizontal, sloper, inverted-L, or vertical according to space and desired pattern.
6. Test & trim: Check SWR on target bands. Trim small amounts from the far end to move resonance. Add the 10–30 pF capacitor if 10 m needs smoothing.

Practical Installation Tips

• Keep the feedpoint high for low-angle DX; lower feedpoints favor NVIS.
• Use insulated supports at high points to prevent arcing on high feedpoint voltage.
• Run the coax away from the operating position and consider a choke at the feed to reduce common-mode currents.
• In tight or HOA-limited spaces, route the wire along fences, eaves, or attic rafters — EFHWs are forgiving when properly matched.

Performance & On-Air Results

Real-world EFHW operators report solid performance across harmonic bands. Typical observations from WSPR, FT8 and SSB users:

• Good DX performance on 20m and 15m when mounted at moderate height.
• Many users report 1.2–1.5:1 SWR on harmonic bands with a well-built 49:1 unun.
• QRP contacts across continents are commonly reported on digital modes with proper deployment.
• Noise performance comparable to dipoles at similar heights; adding a choke and good grounding often improves listening SNR.

Configurations & Use Cases

1. Sloper: Feedpoint low, end elevated — excellent multi-band DX.
2. Horizontal: Use for NVIS when mounted low and parallel to the ground.
3. Inverted-L: Combines vertical low-angle radiation with horizontal elements for wider coverage.
4. Vertical: Mount straight up for omnidirectional low-angle radiation and compact footprint.

Maintenance & Longevity

• Use UV-resistant wire for outdoor installs and check for frayed insulation seasonally.
• Seal the enclosure with silicone and use a drip loop on coax to prevent water ingress.
• Inspect solder joints and toroid windings after severe weather or several seasons of use.

Frequently Asked Questions

Why use a 49:1 unun for an EFHW?

A half-wave end-fed wire presents very high feedpoint impedance, around 2.5 to 3 kilohms. A 49:1 unun steps that down to a level compatible with 50-ohm coax and the radio, which lets the antenna show low SWR on its resonant and harmonic bands without an external tuner. A poorly built transformer is the most common reason an otherwise correct EFHW disappoints on the air.

Will a 40m EFHW work on 20m, 15m, and 10m?

Yes. A 40m half-wave EFHW is also resonant on the second, third, and fourth harmonics, which puts it in the 20m, 15m, and 10m bands. SWR is usually usable across those bands with a well-built unun, though small trimming of the radiator or a capacitor tweak across the secondary often improves 10m performance.

What capacitor value helps an EFHW on 10m?

A small high-voltage RF capacitor, typically in the 10 to 30 pF range, placed across the unun secondary often smooths impedance on 10m without harming the lower bands. Start at 10 pF and step up while watching SWR. Use an RF-rated ceramic or NP0 capacitor rated for at least 1 kV to handle feedpoint voltages.

Can I run 100 watts through an EFHW?

Yes, when the transformer, windings, and enclosure are built for that power. Use a properly sized core such as an FT240-43, thick enamel wire, solid solder joints, and a sealed weatherproof box. Run digital modes at lower power first and monitor the toroid for heat before stepping up to full carrier.

Do I always need an antenna tuner with an EFHW?

Not always. On the design band and its harmonics, a well-built EFHW usually presents a usable SWR straight into the radio. A tuner becomes useful when you want to operate off-harmonic bands like 30m or 17m, or when local environment shifts the resonance enough that you would rather match the antenna than re-trim it.

How do I stop RF in the shack from an EFHW?

Add a common-mode choke at the feedpoint and, if needed, a second one near the shack entry. Make sure your counterpoise or coax shield return is connected at the unun ground lug, route coax away from the operating position, and tidy any long unintended antenna paths along the coax. Type 31 or 43 ferrite mixes are good starting points for HF chokes.

Further Reading & References

• EFHW transformer winding guides and toroid selection references (online ham archives).
• NEC ground and antenna modeling basics for optimizing height and ground loss.
• Local club build guides and on-air test reports for real-world installation ideas.