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.

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.

FAQ — Quick Answers

Why does the EFHW need a heavy duty transformer?

Because feedpoint voltages at the end of a half-wave can reach hundreds of volts at higher power. A properly built 49:1 unun on a suitable ferrite core prevents saturation and loss and handles the voltage safely.

Do I always need a tuner with an EFHW?

Not always. On harmonic bands the EFHW will often present a usable SWR without a tuner. If you operate off-harmonic or require precise matching, an external tuner is useful.

What if I get RF in the shack?

Add a common-mode choke at the feedpoint or inside the shack, ensure a proper counterpoise/ground, and tidy coax routing. Ferrite beads (mix 31/43) or a 1:1 choke often cure RF in the shack.

Can I make a 20m or 80m EFHW instead?

Yes. Cut the radiator to the half-wave of your target band (20m half-wave ≈ 32.5 ft, 80m ≈ 131 ft) and re-evaluate your transformer choice and counterpoise. Long radiators may benefit from heavier duty cores and different winding ratios.

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.