Time2.5–4 hours

DifficultyIntermediate

Cost$30–60

Skills NeededSoldering, toroid winding

📌

More Involved Than a Dipole — Start There If You're New to Wire Antennas

An EFHW needs you to wind a working impedance transformer, which is a genuinely different skill from a dipole's screw-and-solder center insulator. If this is your first wire antenna, our dipole build guide is the gentler starting point — same trim-to-resonance process, simpler feedpoint.

📋 In This Guide

Materials & tools you'll need
Step 1: Calculate length & counterpoise
Step 2: Wind the 49:1 unun
Step 3: Mount the transformer
Step 4: Attach the antenna wire
Step 5: Attach the counterpoise
Step 6: Build & place the choke
Step 7: Mount & raise the antenna
Step 8: Measure & trim to resonance
Common mistakes
Frequently asked questions

🧰 Materials & Tools You'll Need

Wire & Transformer Parts

12–14 AWG antenna wire, stranded copper or copper-clad steel, insulated
FT240-43 ferrite toroid — the standard core for a 49:1 HF unun
14 AWG enameled magnet wire — roughly 1 ft for the primary, 7 ft for the secondary
100–150 pF capacitor (optional) — improves SWR on 15m/10m for some builds

Enclosure & Connectors

Weatherproof project box sized for the toroid
SO-239 chassis connector
Binding post or wing-nut terminal for the antenna wire
Second terminal for the counterpoise/ground connection
PL-259 connector and coax to reach your radio

Counterpoise & Choke

Counterpoise wire — length depends on lowest band (see Step 1)
Ferrite toroid for the choke (FT240-31 or -43) plus several turns of coax
UV-resistant Dacron rope for the radiator's far end and/or mast support

Tools

Wire cutter / stripper, needle-nose pliers
Soldering iron (60–100W), rosin-core solder, flux
Power drill + small bits
Tape measure (long — 70+ ft for a 40m EFHW)
Antenna analyzer or NanoVNA — for tuning in Step 8

📐 Step 1: Calculate Length & Counterpoise

The radiator wire uses the same starting formula as a dipole — 468 ÷ frequency (MHz) — since the wire itself is electrically identical, only the feed point differs. Our EFHW calculator does this plus the counterpoise sizing automatically.

Example — 40m EFHW at 7.15 MHz:
Radiator length: 468 ÷ 7.15 = 65.45 ft, cut at +4% ≈ 68.1 ft to start
Counterpoise length: 0.05 × wavelength ≈ 6.9 ft (about 2.1 m)
Unun ratio: 49:1 (7:1 turns), targeting roughly 2450Ω feedpoint impedance

🌀 Step 2: Wind the 49:1 Unun

This is the step with no dipole equivalent — take your time here, since a miscounted turn changes your impedance ratio.

Cut your two winding wires

Cut a short length (roughly 1 foot) and a long length (roughly 7 feet) of 14 AWG enameled magnet wire. The short wire becomes the primary; the long wire becomes the secondary, continuing where the primary leaves off.

Twist the bifilar section

Starting at one end of each wire, twist the short and long wires tightly together along the entire length of the short wire. This twisted pair forms the primary winding.

Wind the primary (2 turns)

Pass the twisted pair through the toroid's center hole twice, counting each pass through the hole as one turn. Leave a few inches of wire hanging free at the start before the first turn — don't count that tail as a turn.

Continue with the secondary (to 14 total turns)

After the 2nd turn, untwist the wires and let the now-free end of the short (primary) wire hang loose — it's done. Continue winding only the long wire through the toroid for turns 3 through 14, spacing the turns evenly around the core.

Strip the enamel and prepare leads

Enameled wire needs its insulating coating scraped or sanded off at each connection point before it will solder — a sharp blade or fine sandpaper works. Tin each stripped end before soldering to the terminals.

Turns Ratio	Impedance Ratio	Target Impedance
2 primary : 14 secondary	49:1	2450Ω from 50Ω coax
3 primary : 21 secondary	49:1 (same ratio, more turns)	2450Ω — equivalent result

⚡

A Note on Bifilar Windings at High Power

The twisted bifilar primary described above is the standard, widely-used method and is fine for typical QRP through 100W operation. Some experienced builders running amplifiers report concern about the primary and secondary windings being close enough together at high voltage to risk arcing between them, and recommend keeping the windings physically separated instead if you plan to run an amplifier. If you're building for QRP or a standard 100W transceiver, the bifilar method shown here is well-proven and standard practice.

📦 Step 3: Mount the Transformer

Drill and mount the connectors

Mount the SO-239 on one face of the enclosure. Drill and mount a separate terminal for the antenna wire (the high-impedance side) and another for the counterpoise/ground connection.

Wire the primary to the coax side

Solder the primary winding's two leads to the SO-239's center pin and ground lug — this is your 50Ω input.

Wire the secondary to the antenna and ground terminals

The far end of the secondary winding (the long wire, after all 14 turns) connects to the antenna wire terminal — this is your high-impedance output. The shared connection point between primary and secondary connects to the counterpoise/ground terminal.

Add the compensation capacitor (optional)

If using one, solder the 100–150 pF capacitor across the antenna wire terminal and the ground/counterpoise terminal. This resonates with the transformer's leakage inductance and can improve SWR specifically on 15m and 10m.

Secure the toroid and close up

Mount the wound toroid securely inside the enclosure (cable ties or a glued mounting tab both work well) so it can't shift and stress the solder joints. Don't seal the enclosure permanently until you've completed tuning in Step 8.

✂️ Step 4: Attach the Antenna Wire

Cut your radiator wire to the cut length from Step 1 (with trim margin included). Strip and tin the end that connects to the transformer, form a connection loop, and secure it mechanically to the antenna wire terminal before soldering — the same mechanical-first principle that applies to any wire antenna connection.

⚡ Step 5: Attach the Counterpoise

Don't skip this step. Every EFHW needs a counterpoise — it's just much shorter than what other antenna types need, which is easy to mistake for "not needed at all."

Option A: Dedicated counterpoise wire

Connect a wire of the length calculated in Step 1 to the ground terminal. Run it away from the main antenna wire, let it hang or lay freely, and keep it clear of metal objects.

Option B: Use a measured section of coax shield

If you'd rather not run a separate wire, a deliberately-measured length of coax (matching the same 0.05λ counterpoise length) between the transformer and your common-mode choke can serve the same role — this is exactly what the choke placement in Step 6 needs to account for.

🛡️ Step 6: Build & Place the Choke

Wind the choke

Wind roughly 10–12 turns of your coax through a ferrite toroid (FT240-31 or -43 are both common choices). Keep the winding within the coax's bend radius — winding too tightly can damage the cable's shield.

Place it correctly relative to your counterpoise choice

If you're using a dedicated counterpoise wire (Option A above), the choke can sit close to the transformer. If you're using a section of coax shield as the counterpoise (Option B), place the choke beyond that measured section — not right at the feedpoint — so you don't choke off the very counterpoise the antenna depends on.

📡 Step 7: Mount & Raise the Antenna

Choose your configuration

EFHWs are flexible — sloper, inverted-L, horizontal run, or zig-zagged to fit a tight space all work, since the antenna doesn't depend on a perfectly straight or symmetrical layout the way a dipole does.

Raise the transformer end first

Get the transformer box up to its mounting point, then run the radiator wire out to its far support. For an inverted-L, maximize the vertical section where practical — it contributes more to low-angle radiation than the horizontal section.

Keep clear of metal and out of reach

Same rule as any wire antenna: avoid running close to metal gutters, roofing, or power lines, and keep the far end out of normal human reach.

📏 Step 8: Measure & Trim to Resonance

Same core process as a dipole, with one extra detail worth knowing: if 20m and 15m harmonic coverage matters to you, where you land on the fundamental affects whether those harmonics fall in-band (see the harmonic coverage table on the calculator page).

Calibrate for your band and sweep

Set your NanoVNA's range to your fundamental band, calibrate, then connect and sweep. Look for the SWR dip — that's your current resonant point.

Trim the radiator, not the counterpoise

If the dip is below your target frequency, the radiator wire is too long — trim from the far end. If it's above target, you've trimmed too much or cut too short originally. The counterpoise length from Step 1 generally doesn't need adjustment during this process.

Check the harmonic bands

Once the fundamental is tuned, sweep your other target bands (20m, 15m, 10m for a 40m EFHW) to see where their SWR actually lands. If 15m or 20m landed outside the band edge, you may need to accept a tuner on that band, or slightly re-trim the fundamental toward the low end of its band to pull those harmonics back in range.

Repeat and finalize

A few trimming passes is typical. Once you're satisfied, seal the enclosure and weatherproof the coax entry point.

❌ Common Mistakes

Miscounting toroid turns: Each pass through the center hole is one turn — it's easy to lose count partway through 14 turns. Count out loud or mark each pass.

Skipping the counterpoise entirely: "No counterpoise needed" is a myth — the coax shield will silently take over that role if you don't provide one deliberately.

Choking the feedpoint when relying on coax-as-counterpoise: This removes the antenna's return path right where it needs it most.

Forgetting to strip enamel insulation before soldering: Magnet wire won't take solder through its enamel coating — scrape or sand it off first.

Expecting all harmonic bands to "just work": Only the highest harmonic (commonly 10m on a 40m EFHW) is reliable across the whole band — others are cut-frequency sensitive.

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❓ Frequently Asked Questions

What ferrite core should I use for the unun?

FT240-43 is the standard, broadly-recommended choice for HF coverage from about 3 to 30 MHz and is what most published builds use. A single FT240-43 typically handles around 100W SSB; for higher power or duty cycle, builders commonly stack two cores rather than using a single larger one.

Can I use regular insulated wire instead of enameled magnet wire for the winding?

Enameled magnet wire is strongly preferred for toroid windings because its thin insulation lets you fit many turns tightly on the core, and it's designed to handle the heat and proximity of adjacent turns. Regular insulated hookup wire is bulkier and may not fit the required turns count on a given core size.

Do I really need the compensation capacitor?

No — many working EFHW builds skip it entirely. It's worth adding only if you find SWR on 15m or 10m is marginal after tuning; the capacitor compensates for the transformer's own leakage inductance at higher frequencies and can pull those bands back into a usable range.

My SWR looks fine but the antenna doesn't seem to perform well — what's wrong?

A clean SWR reading only confirms the impedance match, not that power is actually radiating efficiently. Common culprits: missing or undersized counterpoise, common-mode current flowing on the coax shield (add or reposition a choke), the antenna running too low or too close to absorptive objects, or — in rare cases — a "dummy load effect" where losses in the system are masquerading as a good match.

Is this harder to build than a dipole?

Yes, meaningfully so. Winding a transformer correctly, getting the counterpoise right, and placing the choke appropriately are all skills a basic dipole doesn't require. If you're building your first wire antenna, starting with a dipole first is a reasonable way to build confidence before tackling an EFHW.

How to Build an EFHW Antenna