End-Fed Half-Wave Antenna: Unun, Length and Real Performance

The end fed half wave antenna, usually shortened to EFHW, is one of the most popular wire antennas among portable operators and apartment dwellers. It is a single length of wire, roughly half a wavelength long on the lowest band, fed at one end through a small transformer. There is no centre insulator, no ladder line and no radial field to lay out. You hang one end of the wire in a tree, put the transformer box at the other end near your radio, and, with a bit of luck, the antenna works on several bands without an antenna tuner.

That simplicity is why the design appears in so many POTA and SOTA activations, camping trips and small gardens. It is also why it is so often misunderstood. The transformer, the coax and the way you route the wire all change how the antenna behaves. This guide explains how an EFHW works, why the feedpoint needs a 49:1 impedance transformer, how long to cut the wire, which bands you can expect it to cover, and how to avoid the common problems.

What an End-Fed Half-Wave Antenna Actually Is

A half-wave antenna is a conductor that is about one half of a wavelength long at the operating frequency. The familiar centre-fed half-wave dipole is fed in the middle, where current is highest and voltage is lowest. As the overview of the dipole antenna explains, that gives a feedpoint impedance of about 73 ohms resistance, which is a convenient match for 50 ohm coax.

An end-fed half-wave turns that around. The same half-wave length of wire is fed at one end instead of the middle. At the end of a resonant half-wave conductor, current is at a minimum and voltage is at a maximum, so the impedance there is very high. That is the whole story of the EFHW: it is a resonant half-wave radiator with its feedpoint moved to the high-impedance end, plus a matching transformer to bring that impedance down to something a transceiver will accept.

Because the antenna is resonant, it does not depend on a ground system to work, unlike a quarter-wave vertical. That is the main attraction. The wire itself is the whole radiating element, and the coax is not supposed to take part, although in practice it often does, which we come back to below.

Why the Feedpoint Impedance Is So High

The exact impedance at the end of a half-wave wire depends on wire diameter, height, nearby objects and frequency, so there is no single figure. As a rule of thumb it is in the range of a few thousand ohms, and it varies noticeably with the installation. That is far from the 50 ohms your transceiver expects, and a direct connection would produce a very high SWR.

This is where the transformer comes in. An impedance transformer with a ratio near the real feedpoint impedance, divided by 50, converts the end impedance into a value the radio can use. The most common choice is a 49:1 impedance ratio, which is a 7:1 turns ratio because impedance scales with the square of the turns. A 49:1 transformer maps roughly 2,450 ohms at the wire end to 50 ohms at the coax connector. Some builders prefer 64:1 (an 8:1 turns ratio), which maps about 3,200 ohms, because they find it a better compromise across several bands. Both are widely used, and neither is magic: the real impedance varies from band to band and from installation to installation, so the SWR you measure will vary as well.

Unun, Balun or Transformer: What Is in the Box

You will see the matching device called a 49:1 unun, a 49:1 balun, an impedance transformer or simply a matching unit. Strictly, “unun” means an unbalanced-to-unbalanced transformer, which is what an EFHW needs: one side is the coax, the other side is a single wire and the return path. If you want a deeper look at the terminology, our guide to baluns and ununs covers what each type does and where it belongs.

Inside a typical EFHW matching unit you will find:

• A ferrite toroid. The transformer is wound on a ferrite core, commonly a mix designed for HF such as type 43 or type 61. The core must be large enough for the power you plan to run, and cheap, small cores can overheat on a high duty cycle mode such as FT8 or RTTY.
• A primary and secondary winding. In a 49:1 design the primary has a small number of turns and the secondary has seven times as many, wound so that coupling is tight.
• A small compensation capacitor. Many designs add a small capacitor on the high-impedance side to flatten the SWR on the higher bands.
• A connector and a wire terminal. Usually an SO-239 or BNC for coax and a binding post or bolt for the antenna wire.

Quality varies widely between commercial units, and so does power rating. Check the manufacturer’s rating for the mode you actually plan to use, and keep power modest until you know the unit stays cool.

How Long Should the Wire Be?

The wire length sets the lowest band. A half-wave in free space is about 150 divided by the frequency in megahertz, giving a length in metres. A real wire needs to be cut roughly five percent shorter than that because of end effects and the insulation on the wire, which is where the familiar 142.5 divided by the frequency comes from, and an end-fed wire is also affected by the transformer. Treat any published length as a starting point, cut it a little long, and trim it with an antenna analyzer. Our guide to the SWR meter explains how to read the numbers while you tune.

As a rough guide, an EFHW cut for the lower part of 40 metres is around 20 metres long, one for 20 metres is about 10 metres long, and one for 80 metres is about 40 metres. Those figures are indicative only. Always trim for your own wire, your own height and your own surroundings.

A practical approach is to cut the wire for the lowest band you want, hang it where you plan to use it, measure the resonance on each band with an analyzer, and trim in small steps. Because the antenna is resonant on harmonics, trimming for the lowest band moves all the others, so it is a compromise. Do not expect every band to land exactly on its centre frequency.

Which Bands Does It Cover?

A half-wave wire is also resonant at multiples of its fundamental frequency, so an EFHW cut for 40 metres is naturally resonant on 20 metres, close to 15 metres and close to 10 metres, as the harmonics of 7 MHz fall near those bands. The same wire fed through a matching unit therefore covers several bands without an antenna tuner. The pattern in practice looks like this:

• 40 metre wire: 40, 20, 15 and 10 metres.
• 80 metre wire: 80, 40, 20, 15 and 10 metres, although the long wire is hard to fit in most gardens.
• 20 metre wire: 20 and 10 metres, a short and light portable option.

The WARC bands at 30, 17 and 12 metres do not fall at harmonics of the common fundamentals, so a plain EFHW is not resonant there. Some commercial designs add a trap, a coil or an extra element to cover them, and an antenna tuner can often make the plain wire usable on them, but the efficiency and the radiation pattern are less predictable. If those bands matter to you, read our overview of the WARC bands before choosing an antenna.

The Radiation Pattern Changes With the Band

On its fundamental band, a horizontal EFHW behaves much like a half-wave dipole, with broadside radiation and nulls off the ends of the wire. On the harmonic bands the wire is several half-waves long, and the pattern breaks up into multiple lobes with nulls in between. For a long-wire pattern the main lobes move closer to the line of the wire as the wire gets longer in wavelengths.

In practice this means that you cannot assume the antenna will work equally well in every direction on every band. A wire that is a good performer to Europe on 20 metres might have a deep null toward a country you want on 15 metres. That is one reason many operators change the wire’s angle slightly between activations, or keep a second antenna for difficult directions. The height of the wire above ground also matters on every band, as explained in our article on antenna height and takeoff angle.

How to Install an EFHW

Because the antenna is just a wire, it can be put up in many ways, and each has its own trade-offs:

• Inverted-V. The centre of the wire is raised on a mast or tree and both ends slope down. It saves the need for two high supports, and the transformer sits near the radio. Many portable operators use a lightweight fibreglass pole for this.
• Sloper. One end is high and the other low. This is easy to set up from a single support, although the pattern is less predictable than with a horizontal wire.
• Horizontal. The wire is supported at both ends at a useful height. This gives the cleanest pattern on the fundamental band, but you need two supports.
• Vertical or near vertical. Hung from a pole or a tree with the transformer at the bottom. The pattern changes, and ground and nearby objects affect it more.

Whatever shape you choose, keep the wire as clear of trees, metal gutters and walls as you can. Foliage and nearby structures detune the antenna and absorb signals, especially on the higher bands. Keep the high-impedance end of the wire away from people and objects, because the voltage there can be considerable at higher power, and use an insulated end rope.

Taming Common Mode Current

This is the single most important practical point. A resonant half-wave end-fed wire has no built-in counterpoise, so the return current has to flow somewhere. The easiest path is the outside of the coax shield, which makes your feedline part of the antenna. The effects can include RF in the shack, a noisy receive signal, SWR that changes when you touch the cable, and a pattern that is different from what you expected.

The usual cures are:

• Add a common mode choke at the feedpoint and, if the coax is long, again a short distance down the cable. Our guide to common mode chokes explains how to choose and build one.
• Give the transformer a short counterpoise wire that is a fraction of the wire length. Many commercial units include a terminal for this. It provides a more deliberate return path than the coax shield does.
• Lay the coax away from the wire and avoid running it parallel to the antenna wire for any distance.
• Keep the coax length sensible. A feedline that happens to be a quarter or half wavelength long on one band can behave badly there. If you see strange SWR on a single band, try changing the coax length slightly.

An EFHW that looks excellent on an analyzer but performs badly on the air is very often a common mode current problem rather than an antenna problem.

EFHW Compared With Other Wire Antennas

The EFHW is one of several choices for a simple multiband wire. The table below sets it beside the options you are most likely to compare it with.

If you are still choosing, our overview of the first HF antenna options compares these designs for a new station, and the guide to the off-center-fed dipole explains the closest alternative in detail.

Portable and Field Use

The EFHW has earned its reputation in the field. Wire and transformer together weigh little, set-up takes minutes, and a single fibreglass pole is often enough to lift the middle of the wire. For parks and summits, many operators carry a short EFHW cut for 40 and 20 metres, and a longer one when space allows. The antenna also plays well with a small tuner at the radio for the in-between frequencies, if the radio has one built in.

A few habits help:

1. Carry spare cord and a few pegs so that you can adjust the angle of the wire quickly.
2. Wind the wire onto a winder so that it does not tangle, and keep the transformer box dry.
3. Measure before you operate. A small analyzer saves a lot of guessing about which band is usable, and a modest SWR on a band is better than a perfect SWR on the wrong frequency.
4. Carry a short common mode choke for the coax. It is light and often decides whether the station is quiet or noisy.

Buying or Building an EFHW

Building your own is a satisfying project: a toroid, enamelled wire, a capacitor, a weatherproof box and a length of wire. The materials are cheap, but the details matter, so follow a published design, check the core’s rating and test the unit on a dummy load before you connect it to a wire. Buying a unit saves time and gives you a tested design with a known power rating, which is useful if you plan to run higher power or digital modes.

Whichever route you take, remember the general principles. Keep the wire away from obstructions, add a choke, trim for your own installation, and treat the SWR curve as information rather than a score. The ARRL antenna resources are a good place to read further about wire antenna design and safe installation, including keeping wires well away from power lines.

The Bottom Line

An end fed half wave antenna is a resonant half-wave wire fed at the high-impedance end through a transformer, most often 49:1. It is light, simple to deploy and, cut for 40 metres, naturally covers 40, 20, 15 and 10 metres without a tuner. It does not remove the need for a good installation: trim the wire with an analyzer, keep it clear of obstructions, use a common mode choke and a counterpoise, and accept that the pattern splits into lobes on the higher bands. Done properly, it is one of the best value antennas for portable and small-space operating.

Frequently Asked Questions

What is an end fed half wave antenna?

It is a wire roughly half a wavelength long on its lowest band, fed at one end through a matching transformer. The end of a half-wave wire has very high impedance, and the transformer brings it down to about 50 ohms for the radio.

Why does an EFHW need a 49:1 unun?

The impedance at the end of a half-wave wire is in the region of a few thousand ohms. A 49:1 impedance transformer converts roughly 2,450 ohms to 50 ohms, which is close enough to give a usable SWR on several bands. Some builders use 64:1 for the same reason.

Which bands will a 40 metre EFHW work on?

A wire cut for 40 metres is also resonant near 20, 15 and 10 metres, because those bands fall close to the harmonics of 7 MHz. The 30, 17 and 12 metre bands do not fall on harmonics, so a plain EFHW is not resonant there.

Do I need radials or a ground for an end-fed half-wave?

No ground system is required, because the wire is a complete resonant radiator. You do need to control common mode current on the coax, which is done with a choke and, often, a short counterpoise wire at the transformer.

How long should the wire be?

Start from the half-wave length for your lowest band, cut slightly long, and trim with an antenna analyzer. Roughly 20 metres for 40 metres and 10 metres for 20 metres, but your own installation determines the final length.

Can an EFHW handle high power?

That depends on the transformer. Small ferrite cores can overheat on long transmissions or digital modes, so use a unit with a power rating that suits your mode and start at modest power while you check that the box stays cool.