End Fed Half Wave (EFHW) FAQ's

We have pulled together some of the most answered questions about End Fed Half Wave antennas into this simple FAQ section.

What is an EFHW antenna, and why does it need tuning?

An End-Fed Half-Wave is a single wire cut to half a wavelength on your lowest band and fed at one end through a matching transformer. Because it is a resonant wire rather than a tuner-fed random length, its length sets where it works. Tuning simply means adjusting that length until the SWR minimum sits where you want it to be.

Why does one wire cover five bands?

A half-wave on 80m is a full wave on 40m, and the pattern keeps doubling up through 20m, 15m and 10m. A properly tuned 80m EFHW therefore works on all five without traps or a tuner. It also means every band is locked to the others — you cannot move one without moving all of them.

What does the 49:1 transformer actually do?

At the fed end of a half-wave wire the impedance is very high, around 2,450 ohms. The 49:1 transformer steps that down to roughly 50 ohms so it matches ordinary coax and your rig. It is a matching device, not a tuner — it will not fix a wire that is the wrong length.

How long should the wire be to start with?

Use L (metres) = 143 ÷ f (MHz), or L (feet) = 468 ÷ f (MHz). That gives about 40.3m (132ft) for 3.550 MHz, 20.1m (66ft) for 7.100 MHz and 10.1m (33ft) for 14.200 MHz. Always cut around 4% long — roughly 41.9m, 20.9m and 10.5m respectively — because it is far easier to shorten a wire than to lengthen one.

Where should the 80m dip sit, and why 3.550 MHz?

Aim to land the fundamental between 3.520 and 3.580 MHz, ideally 3.550 MHz. That is not about 80m alone: it is the point at which all five harmonics fall inside their respective band edges. Push the fundamental too high or too low and 10m, the eighth harmonic, walks out of the band entirely.

Do I need an antenna analyser, or will the rig's SWR meter do?

An analyser or NanoVNA makes it far quicker because it shows you the dip directly. But a rig with an SWR meter is perfectly workable — you read SWR at five points across the band and look at the slope rather than hunting for an exact minimum. If SWR falls as you tune downwards the wire is too long; if it falls as you tune upwards it is too short; a V-shape with the bottom in the band means you are there. Use CW, FM, AM or TUNE at 5–10 watts and keep to two or three second bursts.

Should I measure at the rig or at the transformer?

At the transformer, if you possibly can. Coax loss eats reflected power on the way back and flatters the reading at the shack end, and the effect gets worse the higher the band. A pretty 1.3:1 at the rig can be a poor match at the antenna — the gap between the two readings is your feedline loss.

Do I need to switch the ATU off?

Yes. Switch off the internal tuner and any external one. With a tuner in circuit you are measuring what the tuner is doing, not what the antenna is doing, and you will chase your tail all afternoon.

Do I need a counterpoise or a choke?

You need one of two arrangements, and readings will wander until you have one. Either fit a counterpoise wire of about 4m (0.05 wavelength at your lowest band) to the ground side of the transformer with the choke at the transformer, or place your first choke 4m down the coax and let that measured length of braid act as the counterpoise, with a second choke at the rig. A counterpoise wire, or a defined braid length plus a choke — but not neither.

Does height affect the tuning?

Considerably. Resonance shifts with height and with whatever is nearby, so measure at the final height, in the final position, every single time. If you lower the antenna to trim it, hoist it fully back up before you re-measure — readings taken at head height are worthless.

How much wire equals how much frequency change?

On an 80m EFHW, roughly 1cm of wire per 1kHz on 80m — about half an inch per kHz. Removing 10cm lifts 80m by about 9kHz, 40m by 18kHz, 20m by 35kHz, 15m by 53kHz and 10m by 70kHz. On a 40m EFHW it is nearer 0.3cm per kHz. To work out a specific trim: wire to remove = current length × (frequency shift needed ÷ current frequency).

My dip is below the band — do I cut the wire?

Not yet. Fold, don't cut. Double the excess back on itself at the far end and secure it with cable ties: folding 30cm removes 30cm of electrical length exactly as cutting does, but you can undo it. Work in halves, take the length off the far end rather than near the transformer, and only reach for the side cutters once all five bands are behaving at full height.

My antenna is too short — how do I add length?

A soldered splice at the far end is the tidiest answer: twist the wires for strain relief, solder, then cover with adhesive-lined heatshrink, or use a sealed crimp. Add generously — say 1.5m — and fold the surplus back. Alternatively, attach an unmeasured tail at the insulator; it can droop or hang at an angle without hurting performance. Lowering the antenna, adding a droop or turning it into an inverted-L will also bring the resonance down a little.

Will a loading coil fix a short wire?

It can. An inductor of roughly 100–120µH placed about two-thirds to three-quarters of the way along the wire, with a short tail beyond it, adds electrical length without adding physical length. The cost is bandwidth and multiband behaviour, so treat it as a solution for a restricted garden rather than a first choice.

Is there a trick for fine-tuning accurately?

Use 10m. It is the eighth harmonic, so any length error is magnified eightfold — a shift you cannot see on 80m is obvious on 10m. Find the 10m resonance, divide by eight, and you have your true fundamental to a resolution you will never get by squinting at an 80m dip.

Why is my 80m SWR worse than my 20m SWR?

That is normal, not a fault. On a typical installation 80m shows the poorest match at around 2:1 while 20m often comes in near 1.15:1. Perfect performance across five bands is a compromise, not an achievable target: pick your priority band, tune for that, and judge the rest as acceptable or not.

My calculator said 40.3m, but it resonates low — why?

Almost certainly the insulation. PVC-insulated wire resonates 2–5% lower than bare wire because the dielectric slows the wave along it. On 80m that alone accounts for being 70–170kHz low against a bare-wire calculation, which is a large part of why you cut long in the first place.

One band is stubbornly high while the others are fine — what should I check?

Start with the choke and counterpoise, because common-mode current is the usual culprit behind a single bad band. Then get the antenna higher — 12m beats 6m by a wide margin — and move the wire away from guttering, fencing, satellite dishes, foil-backed insulation and wet trees. Finally inspect the feedline: corroded connectors, water ingress or tired coax produce a poor match that no amount of trimming will cure.

Can I use 30m, 17m and 12m?

Not resonantly — the WARC bands do not fall on the harmonics of a half-wave wire, so they need an ATU. Equally, 10m sitting at 2.5:1 is perfectly usable with a modern rig and its built-in tuner. Knowing when to accept a match and let the ATU work is part of the job.

Why does SWR change after rain, or drift with the seasons?

Wet weather typically drops 80m resonance by 20–50kHz, so measure on a dry day if you can. A change that does not recover once things dry out usually means water in a connector or joint — dry it, re-seal it, and wrap the tape from the bottom upwards so it sheds rather than collects. Longer-term drift is generally wire stretch or trees growing, so re-measure the physical length.

I have high, flat SWR with no dip on any band — what now?

Something is disconnected. Check in this order: the wire terminal at the transformer, which is the most common failure, then the far-end attachment, then coax continuity using a short known-good jumper, then every connector for corrosion or a pushed-back centre pin, and finally the counterpoise and choke connections. Do not open the transformer — there is nothing user-serviceable inside.

Is it safe to touch the antenna?

Never touch the wire while transmitting. RF voltages at the far end reach several thousand volts. Terminate the wire in a robust insulator at least 2.5m away from people and animals, keep well clear of overhead power lines, lower the antenna before working on it, and disconnect it before a thunderstorm.

What should I write down once it is tuned?

Record the final measured wire length from transformer terminal to insulator, the height at each end and the configuration, the resonant frequency and minimum SWR on all five bands, the wire type and gauge, the counterpoise or braid length, whether you measured at the transformer or the shack, and the date and weather. That baseline saves you tuning the whole thing again after storm damage or a seasonal change.