Build a T2FD Antenna — Terminated Tilted Folded Dipole
The T2FD — Terminated Tilted Folded Dipole — is one of the most genuinely broadband HF antennas an amateur can build. Originally developed for military and maritime use where operators needed a single antenna covering the entire HF spectrum without retuning, the T2FD achieves SWR below 3:1 across 2–30 MHz from a single feedpoint using a terminating resistor that deliberately absorbs a portion of the RF energy to flatten the impedance curve. The result is a no-tune, all-band HF antenna with predictable performance, no ATU required, and a radiation pattern that changes with frequency in a controlled way. This guide covers the theory, terminating resistor design, wire construction, balun selection, support configuration, and complete installation for a standard HF T2FD.
The Folded Dipole as a Starting Point
The T2FD begins with a folded dipole — two parallel conductors connected at both ends, fed at the centre of one conductor. A standard folded dipole at resonance presents approximately 300 Ω at the feedpoint (four times a standard dipole's 73 Ω), which a 6:1 balun steps down to 50 Ω. But a resonant folded dipole is still narrowband — it only presents a low SWR at its resonant frequency and harmonics. The T2FD solves this by adding a terminating resistor and tilting the antenna:
Why the Terminating Resistor Broadens Bandwidth
This is the key physics of the T2FD and the source of its primary trade-off. The terminating resistor at the centre of the lower wire converts the antenna from a resonant structure to a travelling-wave structure — RF energy travels along the antenna and is partially absorbed by the resistor rather than fully reflected from the ends:
Antenna Dimensions and Frequency Coverage
The T2FD's physical length determines the lowest usable frequency. The upper end of the coverage range is limited only by the feed system. Most HF T2FDs are designed to cover from the 80m band (or 40m) through the top of the HF spectrum at 30 MHz:
Tilt Angle — Practical and Performance Considerations
The "tilted" in T2FD refers to installing the antenna at an angle rather than horizontally. Tilt is both a practical convenience (only one high support needed) and a performance characteristic:
- Tilt angle range: 10°–30° from horizontal is the standard range. Below 10° the antenna behaves essentially as a horizontal antenna; above 30° it begins to lose the horizontal radiation component and the impedance match can shift slightly.
- Practical tilt setup: the feedpoint end (upper wire centre) is the high end, supported at 40–50 ft. The far end (lower wire centre with terminating resistor) is supported at 20–30 ft. The difference in height over the length of the antenna gives the tilt angle.
- Radiation pattern effect: a tilted T2FD radiates with both horizontal and vertical polarisation components. The tilt introduces an asymmetry — the antenna radiates somewhat better off the low end and somewhat less off the high end. In practice the pattern is broad enough that orientation is rarely critical.
- 15° tilt — the practical sweet spot: a 15° tilt is achieved naturally with a high support at 40 ft and a low support at 20 ft for a 114-ft antenna. This tilt is barely visible but measurably improves low-angle radiation compared to strictly horizontal.
- Horizontal installation: a T2FD installed with equal-height supports (zero tilt) works — it simply becomes the BBTD (Broadband Tilted Dipole) without the tilt component. Performance is slightly inferior at low angles but acceptable.
| TX power | Resistor dissipation (worst case) | Resistor value | Configuration | Resistor type | Notes |
|---|---|---|---|---|---|
| QRP — 5W | ~2.5W | 560 Ω or 680 Ω | Single resistor | 5W wirewound or carbon film | Any non-inductive resistor works at QRP |
| 25W | ~12W | 560 Ω | 2× 270 Ω in series, 15W each | Wirewound non-inductive | Derate to 50% — use 25W+ total combined rating |
| 100W | ~50W | 560 Ω | 4× 2.2 kΩ in parallel (550 Ω net), 25W each | Non-inductive wirewound | 100W combined rating minimum; enclose and ventilate |
| 100W | ~50W | 680 Ω | 2× 1.5 kΩ in parallel (750 Ω net), 50W each | Non-inductive wirewound | Slightly high but acceptable; 100W combined rating |
| 500W | ~250W | 560 Ω | 8× 4.7 kΩ in parallel (587 Ω net), 50W each | Non-inductive wirewound, heatsunk | 400W combined minimum; forced air cooling recommended |
| 1500W | ~750W | 560 Ω | Dummy load resistors in combination | High-power non-inductive, oil-cooled or heatsunk | Complex assembly — use commercial high-power T2FD for legal limit |
Materials for a 114-ft T2FD covering 80m through 10m at up to 100W
Building the 80m–10m T2FD
This guide builds a 114-ft T2FD for 3.5–30 MHz coverage at up to 100W. Build the terminating resistor assembly and feedpoint balun first — these are the most critical components. The wire and spreader assembly follows. Work methodically and weatherproof every outdoor connection before raising.
Build the Terminating Resistor Assembly
The terminating resistor is the most unique component of the T2FD and requires careful construction. The resistor must be non-inductive — standard wirewound resistors have significant inductance that degrades high-frequency performance. Use resistors specifically labelled non-inductive wirewound, or use carbon composition resistors (which are inherently non-inductive but harder to find at high power ratings).
Build the Feedpoint Balun
The feedpoint balun for a T2FD must provide a 6:1 impedance transformation (300 Ω balanced to 50 Ω unbalanced) and act as a current choke to suppress common-mode current on the coax. A 4:1 balun is sometimes used with T2FDs that present nearer to 200 Ω at the feedpoint — the exact ratio depends on the specific wire spacing and antenna geometry. If in doubt, build or purchase a 6:1 current balun and measure SWR across the HF range:
Cut the Wire and Prepare Spreaders
Cut two wires to 115 feet each — 114 ft nominal plus 6 inches per end for connection allowance. These are the upper (fed) wire and lower (terminated) wire. They must be identical in length. Mark both wires at the exact centre point — this is where the feedpoint balun connects on the upper wire, and where the terminating resistor connects on the lower wire.
Prepare 20–24 spreaders from fibreglass rod, acrylic rod, or 1/2-inch PVC cut to 6-inch lengths. Each spreader keeps the upper and lower wires 5–6 inches apart. Drill a small hole through each end of each spreader for a cable tie or wire loop to secure it to both conductors. Space spreaders every 4–6 feet along the antenna length — closer spacing near the ends where the wires converge at the end insulators.
Assemble the Complete Antenna on the Ground
With both wires parallel and all spreaders attached, complete the end connections. At each end of the antenna, connect the upper wire and lower wire together through an end insulator — the two wires are joined at both ends, forming the folded dipole shape. Use a short length of #14 wire to make this connection, soldered to both conductors and passed through the end insulator loop for mechanical support.
At the centre of the upper wire, connect the balanced terminals of the 6:1 feedpoint balun — one terminal to each side of the upper wire (split the wire at the centre point and insert the balun in series). The coax connects to the balun's unbalanced output. At the centre of the lower wire, connect the terminating resistor assembly — one lead to each side of the lower wire, also split at the centre. Solder all connections and weatherproof immediately.
Plan the Tilted Support Configuration
The T2FD is installed with one end higher than the other, creating the tilt. The feedpoint (balun) end is conventionally the high end — this minimises coax run length from the feedpoint to the shack. Plan the support heights to achieve a 15° tilt over the 114-ft antenna length:
Raise the Antenna
Raising a T2FD is more involved than a simple dipole due to the two-conductor construction and attached components. Attach halyards to both end insulators — the upper wire end insulator at the high-end support, and the lower wire end insulator at the low-end support. The antenna is relatively heavy due to the spreaders — use robust 3/16-inch or 1/4-inch Dacron rope for all halyards.
Raise the high end first, pulling the antenna up by the upper wire halyard to the high support. Then pull the low end to its support. The spreaders keep the two wires parallel during raising — the antenna maintains its shape as it goes up. Once both ends are at height, adjust tension so the antenna is taut but not under excessive strain. The balun and resistor enclosures add localised weight at the wire centres — ensure the wire has enough tension to hold these components clear of any obstacles below.
Sweep SWR and Verify Broadband Performance
Connect the NanoVNA to the coax at the shack end and perform a full sweep from 1.8 MHz to 30 MHz. The T2FD's hallmark is a flat, low SWR curve across the entire HF range — this is what confirms the antenna is working correctly. A correctly built and installed T2FD produces:
Once the sweep confirms broadband performance, the T2FD requires no further adjustment — band changes need no retuning. Document the sweep result with a photo or saved NanoVNA trace as a baseline for future troubleshooting.
Where the T2FD Excels
The T2FD is not the right antenna for every station — but for the right application it is outstanding:
- SWLs and receive-only stations: the terminating resistor penalty applies only to transmit. For receive, the T2FD has no efficiency penalty and provides genuinely flat response across the entire HF spectrum — excellent for SDR receivers, monitoring, and general coverage listening.
- Expedition and portable stations: a 40m–10m T2FD (57 ft) fits in a medium-size carrying bag. It operates on every HF band without any ATU — one antenna, one coax, instant operation on any frequency from 7–30 MHz. Invaluable for a portable setup where quick deployment and frequency agility are priorities.
- Stations sharing a single antenna with multiple transceivers: a T2FD feeds directly from any radio without an ATU — no incompatibility between different radios and a shared antenna. Military and maritime installations favour the T2FD for exactly this reason.
- Operators who change bands frequently: no-tune band changes are the T2FD's most practical daily-operating advantage. Calling CQ on 40m, hearing a DX station spotted on 17m, and immediately switching to 17m without touching an ATU is a genuinely different operating experience from any resonant antenna system.
Where the T2FD is Not the Best Choice
The T2FD's 1.5–3 dB transmit efficiency penalty makes it a poor choice in some situations:
- Contest operating at full power: in a competitive contest environment, losing 1.5–3 dB to the terminating resistor is a meaningful disadvantage. A resonant dipole or Yagi on each band always outperforms the T2FD at equal power. Serious contesters use band-specific resonant antennas.
- DX chasing at QRP power levels: at 5W, a 3 dB loss means 2.5W effective radiated power. That matters when the contact is marginal. A resonant EFHW or dipole for the specific band is a better QRP DX antenna than the T2FD.
- 160m primary stations: the T2FD's length for 160m coverage is approximately 200 ft — a significant structure. And the resistor loss on 160m is particularly painful since every dB matters on that band. A dedicated 160m antenna (inverted-L, loaded vertical) is a better choice for serious 160m work.
- HOA/stealth situations: the T2FD requires two parallel wires with visible spreaders — it is not a stealthy antenna. A single-wire solution (EFHW, mag loop, indoor antenna) is more appropriate for a restricted installation environment.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| High SWR across all bands — no flat region | Wrong balun ratio or terminating resistor significantly off value | Measure resistor value with ohmmeter before installing; verify balun is 6:1 not 4:1 | Replace balun with correct ratio; rebuild resistor assembly to correct 550–680 Ω value |
| SWR rises sharply above 15 MHz — low bands flat, high bands poor | Terminating resistor is inductive — standard wirewound used instead of non-inductive | Measure resistor impedance with NanoVNA across frequency — inductive resistor shows rising impedance vs frequency | Replace terminating resistors with genuinely non-inductive wirewound or carbon composition types |
| SWR flat but 3 dB higher than expected signal reports | Normal T2FD transmit efficiency — resistor absorbing 30–50% of power | Compare signal reports vs known resonant dipole at same height | This is normal operation — T2FD sacrifices efficiency for broadband coverage. Accept the trade-off or switch to resonant antenna for transmit. |
| Resistor enclosure hot after short TX session | Normal — resistor is designed to dissipate 30–50% of TX power | Check temperature after 5-minute digital mode transmission at full power | If enclosure is uncomfortably hot (over 60°C), add ventilation slots on underside; do not seal enclosure completely. Resistor warmth is expected and normal. |
| SWR good initially, degrades over months | Moisture ingress at resistor or balun connections; wire corrosion at centre split points | Inspect all outdoor solder joints and enclosure seals visually | Re-weatherproof all connections with fresh self-amalgamating tape; re-solder any corroded joints; reseal enclosure entries with silicone |
| One or two specific bands have high SWR peaks in otherwise flat curve | A spreader conductor touching one wire, creating a short at that point; or one wire broken | Walk along the antenna visually inspecting each spreader; check DC continuity of both wires full length | Insulate any spreader contact with wire by adding a small PTFE sleeve; repair any broken wire conductor |
| RF in the shack on certain bands | Common-mode current on coax — balun not providing sufficient choking impedance | Touch coax chassis — RF tingle confirms common-mode current | Add a second common-mode choke (6–8 turns RG-8X through FT-240-31) at the shack coax entry point |
How much signal do I lose to the terminating resistor?
Typically 1.5–3 dB compared to a resonant dipole at the same height on the same band. The exact loss depends on the band — on bands where the unterminated antenna would present a moderate impedance (near resonance), the resistor absorbs less; on bands far from resonance, it absorbs more. In practice, 3 dB is the worst case on most HF bands for a well-designed T2FD — this is a one S-unit penalty on a receive meter, which is noticeable but not catastrophic for most operating. Running 200W into a T2FD is equivalent to roughly 100–150W into a resonant dipole on most bands, which is a workable operating condition for all but the most marginal DX contacts.
Can I use the T2FD for digital modes at 100W?
Yes, with attention to resistor cooling. Digital modes (FT8, RTTY, PSK31) transmit continuous carrier at full power duty cycle, which means the terminating resistor dissipates its full share (30–50W at 100W TX) continuously. The resistor assembly must be rated for this continuous dissipation, not just peak power. Use a resistor combination with at least 100W combined continuous rating, ensure the weatherproof enclosure has adequate ventilation on the underside, and monitor the enclosure temperature after extended digital mode sessions. A well-built resistor assembly handles this without issue, but an undersized or poorly ventilated assembly will overheat.
Does the T2FD work on 160m?
An 80m T2FD (114 ft) on 160m is approximately 0.17 wavelengths — very short. SWR will typically be above 5:1 at 1.8–2.0 MHz, outside the antenna's useful broadband range. For 160m coverage from a T2FD, you need the longer 200-ft version designed for a 2 MHz lower cutoff. Even then, 160m efficiency is reduced by the resistor. If 160m is a priority, a dedicated 160m antenna is a better solution — the T2FD's strength is 3.5–30 MHz coverage, not 160m.
What resistor value gives the best performance?
The theoretical optimum terminating resistance for a folded dipole with 5-inch wire spacing is approximately 390–560 Ω. In practice, values from 390 Ω to 820 Ω all produce usable broadband performance — the SWR curve shifts slightly with resistor value but remains below 3:1 across the HF range for any value in this range. The most commonly used value is 560 Ω, which is available as a standard resistor value and produces good results across a wide range of antenna geometries. If the NanoVNA sweep shows SWR consistently above 2:1 across the board, try increasing the resistance toward 680 Ω or decreasing toward 470 Ω — one of these adjustments typically flattens the curve for a specific installation.
Is the T2FD better than a G5RV or doublet for general HF use?
It depends on your priorities. The doublet with open-wire feedline to a balanced ATU has lower loss on every band — it wastes nothing to a terminating resistor. The G5RV with an ATU similarly uses all available power. Both require an ATU and retuning between bands. The T2FD is genuinely no-tune with direct coax feed — band changes are instant. For operators who value operating convenience over maximum efficiency, the T2FD is the better daily-use antenna. For operators running high power on specific bands where every dB matters, a resonant or near-resonant antenna on each primary band is more appropriate than the T2FD.
How do I know if my resistors are truly non-inductive?
The safest approach is to buy resistors specifically sold as non-inductive wirewound — Ohmite OY series, Vishay AC series, and Arcol HS series are all genuinely non-inductive. Avoid any resistor described only as wirewound without the non-inductive qualifier. If you have resistors of uncertain type, test them with a NanoVNA: measure impedance from 1–30 MHz. A non-inductive resistor shows a flat impedance versus frequency — the curve does not rise with frequency. An inductive resistor shows rising impedance with frequency — the classic inductive signature. If the sweep is not flat, the resistors are not suitable for the T2FD termination.