Build an End-Fed Half-Wave HF Antenna for Ham Radio

An end-fed half-wave (EFHW) antenna is one of the most practical HF antennas a ham can build: it's fed from one end instead of the center like a traditional dipole, which means you can run a single long wire from your operating position out to a tree or mast without needing to get a feedline up into the middle of the span. Pair that with a small 49:1 impedance-matching transformer and it'll load up cleanly on a coax feedline across multiple HF bands with a tuner, or on its fundamental band with no tuner at all. This is a genuinely different build than a receive-only setup like the ATS Mini V4's stock or aftermarket antennas — this is a transmit-capable antenna for an actual amateur radio station, and it requires a valid amateur radio license to legally transmit with once it's built.

Why End-Fed Half-Wave

A half-wave dipole needs feedline support at its center and open space on both sides — awkward in a suburban yard or a shop with limited outdoor real estate. An EFHW is fed at one end, so the entire structure can run in a single direction: out a window, along a fence line, or up into one tree, with just the transformer box near your station. The tradeoff is a very high feedpoint impedance — thousands of ohms at a true half-wave resonant point — which is why these antennas need an impedance-matching transformer rather than being fed directly with 50-ohm coax.

Parts List

14 AWG stranded copperweld or PVC-insulated antenna wire — length depends on target band (see table below)
FT240-43 toroid core for the matching transformer
26 AWG enameled magnet wire for the transformer windings
SO-239 chassis-mount connector for the transformer box input
Weatherproof project enclosure to house the transformer
Egg or dog-bone end insulators (two — one at the far end, one for a short counterpoise if used)
1000pF, 3kV-rated capacitor for the transformer's resonating capacitor
Paracord or Dacron support line for the far-end tie-off
RG-8X or RG-58 coax feedline, length to match your run to the radio
Antenna analyzer (NanoVNA or equivalent) for tuning and SWR verification

Wire Length by Band

The classic EFHW length formula is 468/f(MHz) feet for a half-wave, same as a center-fed dipole — the difference is entirely in how it's fed, not how long the wire is.

BandApprox. Wire LengthNotes 80m~130 ftNeeds real space — most home installs use this as a multi-band antenna instead, loaded with a tuner on 80m 40m~66 ftCommon "all-band" EFHW length; works reasonably on 40/20/15/10m harmonically with a tuner 20m~33 ftGood compact option; loads cleanly on 20m and several harmonics 10m~16 ftSmall enough for a fence-line or attic install; narrower usable bandwidth

Winding the 49:1 Transformer

On the FT240-43 core, wind the primary: 4 turns of magnet wire, evenly spaced around roughly a third of the core.
Wind the secondary over the same section: 28 turns, keeping turns snug and evenly spaced. The 4:28 turns ratio squares to approximately 49:1 impedance transformation, matching a ~50-ohm coax feed to the antenna's high end-feed impedance.
Connect the primary winding across the SO-239 center pin and ground lug.
Connect the 1000pF/3kV capacitor in parallel with the secondary winding — this resonates out stray capacitance and noticeably improves bandwidth and SWR on the upper HF bands.
Connect the antenna wire to the "hot" end of the secondary, and a short (roughly 8–17 foot) counterpoise wire or ground connection to the SO-239 shield/ground side — this counterpoise matters more than most builders expect; skipping it often shows up as RF feedback into your equipment.
Mount the wound toroid and capacitor inside the weatherproof enclosure with the SO-239 through one wall for the coax and a stress-relieved cable gland or insulator through another for the antenna wire.

Deployment and Tuning

Run the wire from the transformer box up and away at whatever angle your space allows — sloped, inverted-L, or as close to a straight horizontal run as you can manage; EFHWs are notably forgiving about being bent around obstacles compared to a dipole, though a straight run performs best. Tie off the far end through the egg insulator to a tree branch, mast, or fence post with enough support line for the wire itself to hang without direct tension on the wire-to-insulator solder joint.

Before transmitting, sweep the antenna with your NanoVNA or antenna analyzer across your target band and confirm the SWR minimum lands near your intended frequency. If the dip is noticeably below your target frequency, the wire is slightly long — trim a few inches at a time from the far end and re-check, since you cannot "un-trim" wire once it's cut. If the dip is above your target frequency, the wire is short and needs a small extension spliced in.

Safety Notes

Treat the far end of an outdoor HF wire antenna as a lightning hazard — install a lightning arrestor or disconnect and ground the feedline at the entry point to your shack whenever the antenna isn't actively in use, especially if it runs near or above roofline height. Keep the antenna wire clear of power lines by a wide margin during installation; a wire antenna coming into contact with overhead electrical service is a fatal hazard, not just an equipment risk. Never key up on a band or frequency you aren't licensed for, and confirm your license class covers the bands you intend to use — a Technician license, for example, has only limited HF phone privileges compared to General or Extra.

Wrapping Up

A well-built EFHW is genuinely one of the best value-for-effort antennas in the hobby — a single afternoon of winding a transformer and running a wire can put you on the air across several HF bands. If you already run a Raspberry Pi–based digital mode station or an RTL-SDR receive setup in the shop, this antenna is a natural next step toward actually transmitting instead of just listening.

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