Few antennas have received as much attention in recent years as the EFHW — the End-Fed Half-Wave antenna. Whether for SOTA activations on Austria’s mountain peaks, POTA in national parks, or as a discreet garden antenna: the EFHW has established itself as one of the most popular antennas in HF amateur radio. But why? And what’s really going on technically?
The answer lies in the combination of simplicity and effectiveness. An EFHW essentially consists of a piece of wire and a small impedance transformer — yet it is a full resonant antenna that works on multiple bands. In this article, we’ll look at how the EFHW works, what matters when building and setting it up, and which misconceptions are out there.
What is an EFHW?
EFHW stands for End-Fed Half-Wave — an antenna fed at one end whose length equals one half wavelength at the fundamental frequency. For the 40-metre band (7 MHz), the wire is about 20 metres long; for 80 m, approximately 40 metres.
Unlike the classic half-wave dipole, which is fed at the centre (where the impedance is about 72 Ω), the EFHW is fed at one end. At the ends of a half-wave antenna, there is a voltage maximum and a current minimum — the impedance there is very high. Depending on height above ground, surroundings, and soil conditions, it typically ranges between 1,800 and 5,000 Ω, with free-space simulations yielding values around 2,450 Ω.
This high impedance must be transformed down to the 50 Ω of the coaxial cable — and that’s exactly the job of the impedance transformer.
The 49:1 Impedance Transformer (UnUn)
The heart of every EFHW is the UnUn (Unbalanced-to-Unbalanced Transformer). Unlike a balun (Balanced-to-Unbalanced), the UnUn connects two unbalanced systems: the coaxial cable on one side and the single wire on the other.
The most common transformation ratio is 49:1. This number comes from the square of the turns ratio: with 2 primary turns and 14 secondary turns, the turns ratio is 1:7, and 7² = 49. This theoretically transforms 2,450 Ω down to 50 Ω — an ideal value for many practical situations.
Transformer Construction
The transformer is wound on a ferrite toroid core. Common cores include:
- FT240-43 (Mix 43): The standard core for QRO operation up to 100 W and beyond. Good broadband performance across the entire HF range (3–30 MHz).
- FT140-43: More compact variant, suitable for QRP up to about 20–30 W. Popular for QRP portable operation.
- FT240-52 (Mix 52): Often runs cooler on the upper bands, slightly less inductance per turn.
A small compensation capacitor of 100–150 pF (NP0/C0G, voltage rating ≥ 3 kV) is placed across the primary winding (50 Ω side). This compensates for the transformer’s leakage inductance and significantly improves SWR on the upper bands.
Why Not 64:1?
Some manufacturers also offer 64:1 transformers (turns ratio 1:8). These transform to higher impedances (~3,200 Ω) and can be advantageous for installations with particularly high feed-point impedance — for example, when the wire hangs low above the ground or runs close to buildings. In practice, however, 49:1 works well for most setups.
DIY vs. Commercial
- DIY: 15–25 EUR for ferrite core, wire, SO-239 connector, and enclosure. With a NanoVNA, you can test the transformer after winding.
- Commercial solutions: 45–200 EUR. Well-known manufacturers include QRPguys, MyAntennas, Balun Designs, and HFkits.
Testing tip: Connect a resistor of about 2,700 Ω (e.g., 2K7) between the antenna terminal and ground, then measure with an SWR meter — you should see an SWR below 1.5:1.
Multiband Operation: Understanding Harmonics
A major advantage of the EFHW: it works not only on the fundamental frequency but also on integer multiples — the harmonics. This is because at every multiple of a half wavelength, the ends of the wire again exhibit voltage maxima, meaning the impedance is once again high and matches the transformer.
An EFHW designed for 40 m (7 MHz) with approximately 20 m of wire therefore works on:
- 40 m (7 MHz) — Fundamental (λ/2): Full half wave, typically SWR below 1.5:1
- 20 m (14 MHz) — 2nd harmonic (1 λ): The wire is one full wavelength long. Often SWR below 2:1
- 15 m (21 MHz) — 3rd harmonic (3×λ/2): Very good match, often SWR below 1.5:1
- 10 m (28 MHz) — 4th harmonic (2 λ): SWR variable, usually below 2.5:1
On non-harmonic bands such as 30 m (10.1 MHz) or 17 m (18.1 MHz), the antenna is not resonant — you’ll need an antenna tuner. An 80 m EFHW won’t work on a 40 m wire either, as it’s simply too short.
Important: Low SWR does not automatically mean good radiation! Especially with broadband transformers and compensation capacitors, an apparently good SWR can result from energy being dissipated as heat in the ferrite core rather than radiated. When in doubt, use a NanoVNA to verify that the antenna is actually resonant.
Counterpoise and Common-Mode Choke
The EFHW is often marketed as an “antenna without a counterpoise” — that’s an oversimplification. Physically, every antenna needs a return path. With the EFHW, the coaxial cable (specifically its outer shield) takes on this role — whether you want it to or not.
Counterpoise
The recommended counterpoise length is 0.05 λ (one-twentieth of the wavelength) on the lowest band. For a 40 m EFHW, that’s about 2 metres; for 80 m, about 4 metres. This short piece of wire is attached to the ground terminal of the UnUn and laid out freely — it doesn’t need to be straight.
Why so short? At 0.05 λ, the impedance at the counterpoise is very high and the current minimal — the system remains stable and purely resistive.
Common-Mode Choke
Without a common-mode choke, common-mode currents flow back along the cable shield to the transceiver. This leads to:
- RF in the shack (interference, microphone hum, computer glitches)
- Inaccurate SWR readings
- Altered radiation pattern
The solution: A common-mode choke (1:1 current balun or line isolator) placed directly at the UnUn or no more than 30 cm of cable away. Alternatively, the choke can be placed at 0.05 λ cable length from the feed point — that’s where a current maximum occurs on the shield, making the choke most effective.
Wire Material and Setup
Suitable Wires
- Insulated stranded copper wire 0.75–1.0 mm²: Ideal for portable operation. 20 metres weigh only about 150 grams.
- Stranded copper wire 1.5–2.5 mm²: For permanent installations. More robust, heavier.
- Tinned or silver-plated antenna wire: Corrosion-resistant, slightly more expensive.
Setup Configurations
- Inverted-L: UnUn near the ground, wire rises vertically (1–3 m) then runs horizontally to the anchor point. The most common configuration — a good compromise between low-angle radiation (DX) and high-angle radiation (local contacts).
- Sloper: UnUn at the top (tree, mast), wire runs diagonally downward. Popular for SOTA, as the wire needs only one suspension point.
- Horizontal: Wire running level between two points. Like a dipole, but fed from one end. At least λ/4 height above ground (10 m for the 40 m band) for good DX radiation.
- V-shape or zigzag: Space-saving, slight efficiency losses, but workable in practice.
High Voltages at the Feed Point
A point many underestimate: very high voltages occur at the end of a half-wave antenna. At 100 watts transmit power and an assumed impedance of 2,500 Ω, the voltage is calculated as U = √(P × Z) = √(100 × 2500) = approximately 500 volts. Even at 5 watts QRP, it’s still over 100 volts.
This has practical consequences:
- Compensation capacitors must be adequately voltage-rated (≥ 3 kV for 100 W)
- Insulation at all connection points must be impeccable
- The wire itself should not be touchable while transmitting
- Caution in wet weather: reduced dielectric strength
Portable Operation: Packing List and Tips
The EFHW is the queen of portable antennas — it’s by far the most popular antenna for SOTA and POTA activations. Here’s the typical packing list:
- 49:1 UnUn in enclosure (~50–80 g)
- 20 m antenna wire on winding spool (~150 g)
- 2 m counterpoise wire (~20 g)
- Throw bag with 15 m line (~100 g)
- 3–5 m coaxial cable RG-174 or RG-316 (~100 g)
- Ground wire/stakes optional
Total weight: under 500 grams! Compare that with a vertical antenna with radials or a magnetic loop with variable capacitor.
Setup Tips
- Height is king: Better to hang the wire high than to stretch it perfectly horizontal. Even 5 m hanging height makes a big difference compared to 2 m.
- Keep your distance: At least 2–3 m from metal fences, masts, and buildings.
- Don’t over-tension: In windy conditions, leave the wire a bit slack — a wire break in the field is frustrating.
- Safety: Never set up near high-voltage power lines!
Performance and Realistic Expectations
With a 40 m EFHW in an inverted-L configuration and 5–10 watts QRP in SSB or CW, Europe-wide contacts are regularly possible on 20 m and 40 m. On the 10-metre band, intercontinental DX is achievable with low power when propagation conditions are good.
But stay realistic: the EFHW is a compromise antenna — like any wire antenna. It’s no replacement for a well-installed beam on a 15 m tower. Its strength lies in the effort-to-results ratio: with minimal material and set up in just a few minutes, it delivers a fully resonant antenna that performs surprisingly well.
Common Mistakes and Misconceptions
- “The EFHW needs no counterpoise” — Technically incorrect. It needs less counterpoise than a quarter-wave vertical, but without any, common-mode currents will flow on the coax shield.
- “SWR 1:1 on all bands” — If SWR is below 1.5:1 on every band from 80 to 10 m, energy is probably being dissipated in the transformer rather than radiated. Some SWR on harmonics is normal and not a problem.
- “The 80–10 m EFHW is the universal solution” — 40 m of wire in a garden is challenging, and the compromise across 8 bands simultaneously is substantial. Better: cut the wire for 40 m or 80 m and work the harmonic bands.
- “A 9:1 UnUn works too” — No. A 9:1 UnUn is designed for long-wire antennas (random wire), not for EFHWs. The impedance of an EFHW is far above what a 9:1 transformer can usefully match.
Common Problems and Solutions
If you’re having trouble with your EFHW, here are the most common causes and remedies:
- High SWR on the fundamental frequency: Check wire length. Even ±50 cm makes a difference. Trim gradually (never too much at once!).
- RF in the shack: Common-mode choke missing or ineffective. Add a counterpoise wire.
- UnUn gets hot: Use a larger ferrite core, reduce power, or limit continuous-duty modes (RTTY, FT8). For FT8 operation, note that 100% duty cycle stresses the core much more than SSB.
- Poor reception despite good SWR: Antenna too low, near interference sources, or the transformer is lossy.
- Wire breaks: Use thicker wire or flexible stranded wire. Don’t forget strain relief at the UnUn.
Sources and Costs
For DIY, you essentially need a ferrite core, some enamelled wire, a capacitor, an SO-239 connector, and a weatherproof enclosure. Total cost: 15–25 EUR.
Ready-made EFHW systems are available from numerous manufacturers:
- QRPguys EFHW Tuner: ~25 USD — minimalist design for QRP
- MyAntennas EFHW-4010: ~90 EUR — 40/20/15/10 m, up to 200 W
- Balun Designs 49:1: ~70–120 USD — transformer only, various power ratings
- HFkits EFHW kit: ~35 EUR — kit with instructions
In Austria, these products are available through specialist retailers or directly from manufacturers via mail order.
Legal Aspects in Austria
To operate an EFHW, you need a valid amateur radio licence. In Austria, this is issued by the Fernmeldebüro (Telecommunications Office, fb.gv.at) — not by the RTR, as is sometimes incorrectly stated. The examination covers technology, operating procedures, and regulations.
An EFHW may be operated on all HF bands authorised for the respective licence class. No special permits are required for the antenna itself, as long as general conditions (transmit power, frequency ranges) are met. On the question of whether you’re allowed to install an antenna on your balcony or in your garden, there’s a separate article on antennas and tenancy law.
Conclusion: The EFHW Deserves Its Reputation
The End-Fed Half-Wave antenna is the most popular portable antenna in amateur radio for good reason. It combines low weight, simple setup, and solid performance across multiple bands. Anyone who understands the physical fundamentals — particularly the role of the impedance transformer, the necessity of a common-mode choke, and the limitations of multiband operation — will get a lot of enjoyment from this antenna.
Whether as your first HF antenna in the garden, as a SOTA companion at 2,000 m altitude, or as a discreet wire antenna in a rented flat: with 20 m of wire, a small transformer, and a bit of experimental spirit, you’ll be QRV on HF.
73 – your oeradio.at editorial team
Transparency Notice
This article was researched and written with the support of AI (Claude, Anthropic). All technical information has been verified against relevant specialist literature and practical experience. The oeradio.at editorial team assumes responsibility for the content.
