End-Fed Half-Wave Antennas

The end-fed half-wave, or EFHW, has become one of the most popular wire antennas in amateur radio. You run a single wire from a small box at one end, hang it from whatever is handy, and feed it with ordinary coax. No center insulator, no heavy feedline pulling the middle down, and often no tuner on its main bands.

It is not magic, though. Electrically it is the same half-wave radiator as a dipole, just fed at a different point, and that change explains most of its quirks.

Why the End Is High Impedance

On a resonant half-wave wire, current is greatest in the middle and falls to nearly zero at the ends. Voltage does the opposite: lowest in the middle, highest at the ends. Impedance is voltage divided by current, so it is low at the center, which is why a dipole is fed there, and very high at the ends.

How high depends on the wire, its height and what is around it, but a few thousand ohms is typical. The ARRL describes the end of its own EFHW kit as around 2,500 ohms. That is far too high to connect straight to a radio that expects 50 ohms.

High impedance also means high RF voltage. Both ends of the wire, and the wire near the feed box, can give you a painful RF burn while you transmit. Keep the ends well out of reach of people and pets.

The 49:1 or 64:1 Transformer

The box at the feed point holds a transformer wound on a ferrite toroid. A transformer changes impedance by the square of its turns ratio. Seven turns to one gives 49:1, which turns 2,450 ohms into 50. Eight to one gives 64:1, which suits 3,200 ohms. Both fall within the range a real EFHW presents.

The core is also the antenna’s weak point. Ferrite turns some of the power into heat, and a small core that is fine at a few watts can overheat at 100. Digital modes such as FT8 and RTTY transmit at full power for the whole transmission, so they heat a core far more than SSB voice at the same setting. A high SWR adds more heat still.

• Follow the maker’s power rating, and check whether it is for voice or for full-duty modes.
• If the box comes down warm after a session, you are near its limit. Turn the power down.

One Wire, Several Bands

A wire that is a half wave long on 40 meters is a full wave on 20 meters, one and a half waves on 15 and two full waves on 10. On every one of those bands, the end of the wire still lands at a voltage peak, so the impedance stays high and the same transformer can match it. The ARRL’s kit uses about 66 feet of wire for exactly those four bands.

The fit is close, not perfect. The ham bands are not exact multiples of each other, so you will usually trim for the best compromise across the bands you use most, and a tuner may still help at the band edges. Bands that are not harmonics of the base band, such as 30, 17 and 12 meters for a 40 meter wire, generally will not match without a tuner.

To find a starting length for any band, use our Antenna Length Calculator, then cut a little long and trim.

Counterpoise and Common-Mode Current

Every antenna needs something to push against. A dipole has its other half. An EFHW draws very little current at its feed point, but very little is not zero, and that return current has to flow somewhere. If you give it nothing else, it will use the outside of your coax shield, your radio and possibly you.

The usual signs are RF in the shack, an SWR that changes when you touch the radio or move the coax, and extra receive noise. There are two common fixes, and many installations use both:

• Attach a short counterpoise wire to the ground side of the transformer, so the return current has a deliberate path.
• Put a common-mode choke on the coax a short distance below the transformer. The short stretch of shield above the choke then acts as the counterpoise, and the rest of the line stays quiet.

Our article Ferrites and Common-Mode Chokes explains how chokes work and how to build one that is effective on HF.

Portable Use

The EFHW really shines in the field, which is why it is so common at Parks on the Air and Summits on the Air activations. You need only one support: a tree branch, a telescoping fiberglass pole or a fence post at the far end. The feed point sits near you, so there is no coax to hoist and only a short run to the radio.

You can string it as a sloper, an inverted V or an inverted L to fit the site. Keep the wire away from power lines, keep it clear of trails where someone could walk into it, and remember the high voltage at the ends. For batteries and power in the field, see Portable Amateur Radio Power.

Compared With a Random Wire

Not every wire fed at the end is an EFHW. A random wire is deliberately not resonant. You pick a length that avoids being a multiple of a half wave on any band you plan to use, because at those lengths the impedance climbs so high that most tuners cannot match it.

A random wire is usually fed through a 9:1 unun, which steps 50 ohms up to 450. That brings the wire’s widely varying impedance into a range an antenna tuner can handle, but you always need the tuner. In return, a random wire can work on almost any band, including the ones an EFHW skips.

The two transformers are not interchangeable. A 49:1 belongs on a resonant half-wave; a 9:1 belongs on a non-resonant wire with a tuner.

Getting Started

1. Choose your lowest band and cut the wire a little longer than the calculator suggests.
2. Put it up in the shape and height you will actually use, since both change the tuning.
3. Check SWR at low power on the lowest band, and trim the far end in small steps.
4. Check the harmonic bands and settle on the best compromise.
5. Add a counterpoise or a choke if you see RF in the shack or an SWR that wanders.

Keep the power within the transformer’s rating and you will have an antenna that goes up in minutes.