EFHW Antenna: One Wire, Five Bands, a Single Support Point
Why it caught on
Putting up a full-size dipole usually means two support points and a span of 10 m or more. An end-fed half wave (EFHW) needs exactly one: the transformer hangs from a pole top or a tree branch, the wire runs out from there, and the far end simply dangles.
For city dwellers, that single anchor point is close to decisive. In China, where most hams live in apartment towers, the workable options are a balcony-to-tree slant, a line dropped from a window, or a wire over the roof edge — and an EFHW takes all of them without complaint. It is the natural antenna for tree-to-ground and balcony-to-fence slopers.
The cost of that convenience is a broadband transformer at the feedpoint, and with it three recurring headaches: common-mode current, high voltage at the feedpoint, and the fact that all bands are tuned together. Understand those three and the EFHW becomes a tool instead of a tribulation.
The physics: why 49:1
Current and voltage are distributed in opposite ways along a half-wave wire. At the center, current peaks and voltage bottoms out; at the ends, current vanishes and voltage is maximum.
Center-fed, that puts the feedpoint right at the current maximum, where impedance sits near 73 ohms — close enough to 50-ohm coax. End-fed, the feedpoint lands on a voltage antinode, and impedance climbs into the thousands. Published figures range from about 1,800 to 5,000 ohms depending on height and surroundings, with free-space simulations around 2,450 ohms. Something has to bring that down.
The usual answer is a 49:1 impedance transformer. The number comes from the turns ratio squared: 2 primary turns and 14 secondary turns give 1:7, and 7 squared is 49. Build it on an FT240-43 ferrite toroid for 100 W, or the smaller FT140-43 for QRP. Core material, turn count, and stray capacitance decide bandwidth and loss; a bad core or a miscounted winding shows up as heat, narrow bandwidth, or phantom resonances at odd frequencies. A 64:1 version (1:8 turns) exists for installations where the feedpoint impedance runs unusually high.
One thing worth stating plainly: this transformer is a matching device, not an antenna tuner. It converts 2,450 ohms to 50 ohms. It does not fix a wire that was cut to the wrong length.
Cutting the wire
Length in meters equals 143 divided by f(MHz); in feet, 468 divided by f(MHz). Take the lowest band you intend to cover:
- 40 m band: about 20 m (66 ft)
- 20 m band: about 10 m (33 ft)
- 80 m band: about 40 m (132 ft)
Start roughly 4 percent long — for a 40 m antenna, cut near 21 m — then trim down in small steps. Shortening is easy; lengthening means a splice, and every splice is a future failure point.
Where the multiband coverage comes from
A half-wave wire also resonates at harmonic frequencies, which is the entire basis of EFHW multiband operation:
- A 40 m half wave is a full wave on 20 m and resonates again on 15 m and 10 m — four bands
- An 80 m half wave covers 80, 40, 20, 15, and 10 m — five bands
- Non-harmonic bands such as 30, 17, and 12 m generally need a tuner
Harmonic operation brings a hard rule: every band is chained to the fundamental. Move one and you move them all.
The standard trick is to park the fundamental where all harmonics land inside their band edges. For an 80 m EFHW, aim for a fundamental between 3.520 and 3.580 MHz, ideally 3.550 MHz. The point is not 80 m itself — it is that only around there do all five harmonics fall within their allocations. Ten meters is the 8th harmonic and the most sensitive: nudge the fundamental up or down and 10 m drops out of the band entirely.
Also accept that performance is not equal across bands. The higher the harmonic order, the more the pattern and efficiency shift from what you measured on the fundamental.
Common-mode current: where EFHW installs go wrong
Because the feedpoint sits at the end, common-mode current is larger than on a center-fed dipole. Left alone, the outside of the coax becomes part of the antenna: it radiates, and it drags household switching noise back into your receiver.
Pick one of two schemes. Both are legitimate, but you must do one:
- Scheme 1: a counterpoise wire of about 4 m (roughly 0.05 wavelength at the lowest band) at the transformer ground point, with the choke placed at the transformer
- Scheme 2: the first choke about 4 m down the feedline, letting that section of shield act as the counterpoise, plus a second choke at the radio
Hedging between the two produces readings that wander forever. Doing neither produces RF in the shack.
Setup and tuning in practice
Common shapes are the slant, the inverted L, the horizontal, and whatever angle a portable activation allows. Get the feedpoint as high as you can — it helps the low-angle radiation — but remember it is also the high-voltage point. Keep it out of reach and leave the insulation room to work.
The tuning routine:
- Bypass the tuner; internal and external, both off
- Start at the lowest band and measure SWR at your target frequency
- Trim 2 to 5 cm at a time and re-measure
- Always cut from the far end of the wire, never at the feedpoint
- Measure at the antenna end if you can, so feedline loss does not flatter your numbers
- Measure at final height and final position — readings taken before the wire is up are fiction
If 10 m never behaves, a common builder’s fix is a small high-voltage capacitor across the secondary or feedpoint end, tens of picofarads, stepped up gradually while watching SWR. Some published designs instead place 100 to 150 pF across the primary winding to flatten the upper bands. Either way, small steps.
EFHW or dipole?
If you have two supports and the room, a center-fed dipole still wins on noise and efficiency, and its common-mode problems are smaller.
But for “one support, fits in a backpack, up and down in fifteen minutes,” the EFHW is the most balanced option available. It is not perfect. It is what you put up when a perfect antenna is not on the menu — and it keeps you on the air until one is.
References
- OERadio, “EFHW: The End-Fed Half-Wave Antenna — simple, effective, versatile” (accessed 2026)
- Johnson’s Techworld, “How to design an EFHW for 80m to 6m” (accessed 2026)
- edufortes, “EFHW Transformer Turns Ratios” (accessed 2026)
- Wikipedia, “Dipole antenna” (for the 143/f length formula) (accessed 2026)