Build a W3EWP Terminated Folded Dipole — Broadband HF Antenna
The terminated folded dipole (TFD) is one of the most genuinely broadband HF wire antennas an amateur can build. Covering 2–30 MHz from a single fixed installation with SWR well below 3:1 across the entire range, it eliminates the antenna tuner for most operating and requires no retuning between bands. This guide walks you through the complete construction of the W3EWP design — one of the most widely reproduced and proven TFD variations.
How the Terminated Folded Dipole Works
A standard half-wave dipole is a resonant antenna — it presents low reactance only near its resonant frequency, and SWR rises steeply on either side of resonance. The terminated folded dipole solves this by converting the antenna from a resonant device into a resistively terminated transmission line. A resistor placed at the far end of the folded element absorbs any energy that would otherwise reflect back and create standing waves. This termination fundamentally changes the antenna's impedance behaviour from narrow-band resonant to wideband resistive.
The folded dipole geometry raises the feed point impedance by a factor of four compared to a simple dipole — from approximately 72 Ω to around 288 Ω. The terminating resistor at the far end adds a parallel resistance path. When the two are combined correctly, the antenna presents a relatively flat impedance of 200–450 Ω across a very wide frequency range. A 9:1 impedance transformer (unun) at the feed point steps this down to 25–50 Ω, compatible with standard 50 Ω coaxial feedline.
Advantages Over Resonant Antennas
Covers all HF bands from 80 m through 10 m from a single fixed installation. No retuning required when changing bands. Often usable without an antenna tuner. Accepts the widest range of deployment shapes — straight, inverted-V, sloping, or dog-leg configurations.
The Trade-Off
The terminating resistor dissipates 3–6 dB of transmitter power as heat — effectively wasting half your power or more. The TFD is best suited for 100 W or less operation where broadband convenience outweighs efficiency concerns. At QRP power levels the efficiency loss is negligible in practice.
Where the W3EWP Design Excels
Ideal for apartments, temporary installations, restricted sites, field day, and multi-operator situations where retuning between operators is impractical. The 40 m physical length fits most suburban gardens. Performance on 40 m–10 m is entirely usable for contest and DX work.
Antenna Dimensions & Theory
The W3EWP design uses a total wire length of approximately 28–29 m (92–95 ft), which provides coverage from 160 m through 10 m when used with an ATU, or from 80 m through 10 m without a tuner. The two conductors of the folded dipole are spaced 100–150 mm (4–6 inches) apart using insulating spacers every 1–2 m along the length. The terminating resistor is placed at the far end between the two conductors. The 9:1 unun connects at the centre feed point between the two conductors at one end.
L (m) = 28.0 m (92 ft) — covers 3.5–30 MHz without ATU with 9:1 unun
Rterm = 390–470 Ω, non-inductive, power rating ≥ PTX / 4 at minimum
(for 100 W TX: minimum 25 W — use 50 W+ for safety margin)
Zout = Zin / 9 — e.g. 450 Ω feed point → 50 Ω to coax
The spacing between the two wire conductors is not critical — anywhere from 75 mm (3 inches) to 200 mm (8 inches) works well. Closer spacing slightly increases mutual coupling between the conductors and raises the transmission line impedance; wider spacing lowers it. The 100–150 mm range represents a practical compromise that keeps spacers small and handling easy without seriously detuning the antenna's impedance from its optimum.
Materials required for one complete W3EWP TFD
Resistor type is critical: the terminating resistor must be non-inductive. Standard wirewound resistors have significant inductance that makes them frequency-dependent — use ceramic non-inductive wirewound, metal oxide film, or thick-film power resistors specifically rated as non-inductive. Ohmite, Arcol, and Vishay make suitable types. Two 820 Ω 25 W resistors in parallel also work well and are easier to source.
9:1 Unun — Construction Sequence
The 9:1 unun (unbalanced-to-unbalanced transformer) steps the antenna's nominal 450 Ω feed point impedance down by a factor of 9 to approximately 50 Ω for the coax. It is wound as a transmission line transformer on a ferrite toroid — the core material and size matter significantly for wideband performance. Build the unun first before assembling the wire elements.
Choose the correct core
Use a Fair-Rite Type 43 ferrite (Mix 43) core for coverage from 1.8 MHz to 30 MHz. The FT-240-43 (2.4 inch OD) handles 200 W continuous, making it the correct choice for 100 W stations. The smaller FT-140-43 handles approximately 100 W and is adequate for QRP through 50 W operation. Type 31 material also works well and may provide slightly better low-frequency performance below 3.5 MHz.
Prepare the winding wire
Cut two equal lengths of 1.5 mm enamelled copper wire approximately 1.5 m each. Twist them together uniformly at approximately 3–4 twists per centimetre — this forms the bifilar transmission line that will be wound onto the core. The twist rate should be consistent along the full length. Lightly secure both ends with cable ties to prevent unravelling during winding.
Wind 9 bifilar turns through the toroid
Thread both wires simultaneously through the toroid hole — one pass counts as one turn. Wind 9 turns, spreading them evenly around the core. Each turn passes through the toroid's centre hole and loops around the outside. Keep the winding tension consistent and avoid kinking the wire. Leave 80–100 mm of free wire at each end for termination connections.
Identify and connect the trifilar connections
Label the four wire ends: start of wire A (A1), end of wire A (A2), start of wire B (B1), end of wire B (B2). For a 9:1 unun, connect them as follows: the primary (50 Ω) side connects between A1 (centre conductor) and B1 joined to A2 (ground/braid). The secondary (450 Ω) side connects between B2 (antenna hot wire) and B1 joined to A2 (antenna return/ground). Scrape enamel off the wire ends carefully before soldering.
Verify with an ohmmeter before potting
With all connections made, check: primary to secondary should read open circuit (no continuity between coax centre and antenna terminal across the transformer — only through the turns ratio). Check for shorts between the SO-239 centre pin and outer body — should be open. A DC short here means a wiring error before RF is ever applied.
Mount in weatherproof enclosure
Drill the enclosure for the SO-239 chassis connector on one face and a 10 mm cable gland on the opposite face for the antenna wire entry. Mount the toroid inside using a cable tie through the enclosure base. Apply a bead of silicone sealant around the SO-239 mounting flange and cable gland thread. Do not pot the transformer in epoxy — heat dissipation from the ferrite is important and potting can cause failure at high power.
W3EWP TFD Antenna Calculator
Wire Elements & Antenna Assembly
With the unun built, cut the wire conductors, install the spacers and terminating resistor, then hoist and test the antenna.
Cut and prepare both wire conductors
Cut two equal lengths of insulated wire, each 14.0–14.5 m long (for the standard 28 m W3EWP design). Strip 30 mm of insulation from both ends of each wire. Tin the stripped ends with solder. Mark one wire as conductor A and the other as conductor B with a cable tie colour or a permanent marker — keeping track of which is which prevents wiring errors later.
Fabricate or source the spacers
Cut 15–20 spacers from 10 mm round PVC conduit, each 120 mm long. Drill a 3 mm hole 10 mm from each end. Thread a 200 mm length of UV-resistant cable tie through each hole and loop it around the wire, pulling snug. Space the spacers every 1.2–1.5 m along the antenna length. The two parallel wires should now run side by side, 120 mm apart, held by the spacers. The first and last spacers should be 300–500 mm from the ends of the antenna.
Install the terminating resistor at the far end
At the far end of the antenna (opposite the feed point), connect the non-inductive resistor between the stripped ends of conductor A and conductor B. Use marine-grade ring terminals crimped to the wire ends and bolted to the resistor terminals using M4 stainless hardware. The connection must be mechanically robust — this joint is at the end of the antenna where it is most exposed to wind load. Encapsulate the resistor assembly in a short section of 32 mm PVC conduit filled with silicone sealant, or mount it inside a small weatherproof junction box with the wire entries sealed. Apply self-amalgamating tape over the entire assembly for additional weatherproofing.
Install end support hardware
At the far end, attach a strain relief — a short length of nylon rope or fibreglass rod connects the end of the antenna to the support point (tree, mast, wall bracket). This rope/fibreglass section carries the mechanical tension and prevents the antenna wire from taking the full weight. A length of 300–500 mm is typical. At the support end, use a stainless eye bolt and stainless carabiner or S-hook for easy removal and re-hanging.
Connect the unun at the feed end
At the near end (feed point), mount the unun enclosure on the support structure — a wooden spreader, PVC cross-piece, or directly to a mast bracket. Connect conductor A to the antenna hot terminal of the unun and conductor B to the antenna return terminal. Both connections should use ring terminals and stainless hardware. Attach the coaxial feedline to the SO-239 connector. Apply self-amalgamating tape from the coax jacket up to and over the SO-239 connector body.
Hoist and tension the antenna
Hoist the far end to its support point first, then adjust the near end height. The antenna can be deployed as a horizontal flat-top, inverted-V (apex up), or sloping wire — all configurations work acceptably. For an inverted-V, aim for an apex angle greater than 90° (wider than 90° between the two arms) for best pattern symmetry. Tension the wire moderately — enough to prevent excessive sag but not so tight that thermal expansion in summer causes the supports to flex. A slight catenary sag of 300–500 mm is normal and desirable.
Initial SWR test at low power
Connect the radio and transmit at 5–10 W maximum. Sweep across 3.5–30 MHz with an antenna analyser or SWR meter and record readings every 500 kHz. Expect SWR of 1.5–2.5:1 across most of the range. Values above 3:1 at specific frequencies usually indicate a resistor connection fault, an unun wiring error, or a very unusual deployment geometry. Do not transmit at full power until the SWR sweep confirms the antenna is working correctly.
A correctly built W3EWP TFD presents the following typical SWR when measured at the coax end of the feedline (after the 9:1 unun), referenced to 50 Ω. These values assume a horizontal deployment at 8–12 m height over average suburban ground:
| Frequency (MHz) | Band | Typical SWR | Notes |
|---|---|---|---|
| 3.5–4.0 | 80 m | 2.0–3.0:1 | Most radios accept with ATU assist; some need no ATU |
| 7.0–7.3 | 40 m | 1.5–2.2:1 | Typically no tuner needed at most points |
| 10.1–10.15 | 30 m | 1.3–1.8:1 | Excellent — well within radio's self-protect range |
| 14.0–14.35 | 20 m | 1.2–1.7:1 | Best band for TFD — very flat SWR |
| 18.068–18.168 | 17 m | 1.3–2.0:1 | Good — no tuner needed |
| 21.0–21.45 | 15 m | 1.4–2.2:1 | Acceptable — within most radio limits |
| 24.89–24.99 | 12 m | 1.5–2.5:1 | Acceptable — ATU may help at band edges |
| 28.0–29.7 | 10 m | 1.5–3.0:1 | Variable — deployment geometry has most influence here |
80 m performance: the 28 m TFD is electrically short on 80 m — the nominal half-wave for 3.65 MHz is 41 m. This means the SWR is higher on 80 m than on higher bands, and an ATU is generally needed for full-band 80 m coverage. The antenna is electrically viable on 80 m with a good ATU and low-loss feedline.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR above 4:1 on all bands | Fault in the 9:1 unun wiring or terminating resistor connection | Disconnect the coax and use an ohmmeter to check the unun: the antenna terminal should show continuity to the resistor via the antenna wire, and there should be no short circuit between the coax centre pin and the unun chassis | A short here means the primary winding ends are crossed — re-check and correct the winding connections |
| SWR flat on all bands at exactly 1:1 or very low | Terminating resistor circuit not connected to the antenna | An SWR reading far too flat on every frequency almost always means the 50 Ω unun output is being measured into an open circuit that the meter interprets as matched | Verify the resistor is actually installed and connected by measuring continuity from the far-end conductor A to conductor B — should read approximately 390–470 Ω |
| Good SWR from 14 MHz up but very high below 7 MHz | Unun core material lacks sufficient permeability at lower frequencies | Classic symptom of using Type 61 or Type 52 ferrite (optimised for VHF) instead of Type 43 or Type 31 (optimised for HF) | Replace the core with an FT-240-43 or FT-240-31 toroid and re-test; also verify you have 9 full bifilar turns — fewer turns reduces low-frequency permeability |
| SWR varies dramatically when touching the coax | Common-mode current on the coax shield | The TFD is technically an unbalanced antenna and common-mode current is not unusual | Add a 1:1 current choke (several turns of coax wound into a 150 mm diameter coil) immediately below the unun to prevent the feedline becoming part of the antenna |
How much power does the terminating resistor waste?
Approximately 30–50% of transmitter power is dissipated in the terminating resistor, depending on frequency. At 100 W input, 30–50 W goes into the resistor as heat. This is the fundamental efficiency trade-off of any terminated broadband antenna design. For casual operating and DX work the broadband convenience is worth it; for maximum-efficiency contest operation a properly matched resonant antenna or Yagi is more appropriate.
Does the orientation of the TFD (horizontal vs. sloping) matter?
Yes, moderately. A horizontal deployment maximises broadside gain in the two directions perpendicular to the wire. An inverted-V slightly reduces gain but improves the low-angle radiation toward all compass directions. A sloping TFD develops some directivity toward the lower end. For general operating, any of these orientations is entirely satisfactory — the TFD is not primarily a directional antenna.
Can I use a 4:1 balun instead of a 9:1 unun?
A 4:1 balun would present the antenna's nominal 450 Ω impedance as 112 Ω to the coax — a 2.2:1 SWR before the antenna is even a factor. It will work, but the 9:1 unun provides a better impedance match to 50 Ω coax across the full frequency range. Some builders use a 6:1 unun (75 Ω output) and run 75 Ω CATV coax, which is also a valid approach.
What is the difference between the TFD and a G5RV?
The G5RV is a resonant antenna that achieves multi-band operation through a specific feedline matching section — but it is still resonant on each individual band and does not cover the full HF spectrum uniformly. The TFD is genuinely non-resonant, using termination to provide a flat broadband impedance. The G5RV has higher efficiency (no terminating resistor loss); the TFD has wider flat-SWR coverage and accepts any deployment shape.
Can I increase the TFD to cover 160 m?
Yes — increase the total length to approximately 41 m (the standard half-wave for 3.65 MHz) to bring 80 m SWR into the no-tuner range and allow usable 160 m operation with an ATU. The terminating resistor and unun values remain the same. The practical constraint is having 41 m of space for the antenna — most suburban gardens cannot accommodate this comfortably.
What coax should I use for the feedline?
Use RG-213 or LMR-400 for the feedline. Because the TFD presents near-matched SWR on most bands, feedline loss operates close to the matched-line figure — typically 0.5–1.0 dB for a 20–30 m run of RG-213 on 20 m. Avoid thin coax (RG-58) for feedlines over 10 m due to its higher loss. The feedline length is not critical — unlike matched stubs, the TFD's broadband nature makes it relatively insensitive to feedline electrical length.