T2FD Antenna: A Wideband Terminated Folded Dipole for HF

Most wire antennas are a bargain with physics: they work very well on the frequency they were cut for and less well everywhere else. The T2FD antenna tries to break that bargain. It is a terminated folded dipole that presents a reasonably steady impedance over a very wide range of frequencies, so a single wire can be used from the lower HF bands up to the top of the shortwave range without a tuner. It does this by sacrificing efficiency, and understanding that trade-off is the whole story. This guide explains how the antenna is built, what the resistor does, how much signal you should expect to lose, where it makes sense and where a simple dipole will serve you better.

What a T2FD Antenna Is

The name stands for tilted terminated folded dipole. It is also called the balanced-termination folded dipole and, among amateurs, the W3HH antenna after Captain G. L. Countryman, one of the engineers who developed it for the US Navy in the 1940s. The design was first meant for ships, where space is limited and the hull and the sea make an excellent ground plane. Countryman introduced it to amateurs at the start of the 1950s, and as the T2FD antenna article explains, it was a popular design in the mid twentieth century and then faded as the upper HF and VHF bands, which needed small dipoles, became more important, and as low impedance 50 ohm coaxial feedline became the standard.

In outline, the antenna is a folded dipole, which is two parallel wires joined at both ends and fed in the middle of one of them. The special feature is a non-inductive resistor placed in the middle of the other wire. That resistor is the termination, and it is what turns a narrowband resonant antenna into a broadband one.

How the Termination Works

A plain half-wave dipole is resonant on one frequency. Away from it the feedpoint impedance swings quickly, and the SWR climbs. When a signal travels along a wire and reaches an open end, it is reflected. A resonant dipole depends on that reflection to build a standing wave. The T2FD does the opposite: the resistor absorbs the energy that would otherwise be reflected from the far end, so the current on the wires is closer to a travelling wave than a standing wave. A travelling wave antenna has a much flatter impedance curve, and the SWR stays modest across a wide range.

The price is simple. Whatever the resistor absorbs is turned into heat and does not radiate. At the lower end of the design range, where the antenna is electrically short, the resistor takes a growing share of the power. The reference description quotes at least 30 percent of the RF power lost as heat in the resistor, and the resistor must therefore be rated to take a substantial part of the transmitter power. That is not an opinion about a particular brand; it follows from how a terminated antenna works.

It also explains why a low SWR is a poor measure of success here. A dummy load has a perfect SWR and radiates nothing. The T2FD sits somewhere between a resonant dipole and a dummy load, and the useful question is how much signal arrives at the other station, not how flat the SWR meter looks. Our guide to what an SWR meter tells you and what it hides covers the same point from the meter side.

Typical Dimensions and Construction

A classic receiving or low-power transmitting T2FD is built from two parallel wire conductors with the following characteristics, taken from the common published description.

• Span. About half the wavelength of the lowest frequency you want to use.
• Spacing. The two wires are separated by roughly one hundredth of that wavelength, kept apart by insulating spacers. At the ends, the upper and lower wires are joined by short vertical sections that follow the end spacers.
• Feed. The feedpoint is in the middle of the lower wire, with a balanced impedance on the order of 300 ohms. A 4:1 balun transforms that to roughly 75 ohms, which is an acceptable match for common coax.
• Termination. A non-inductive resistor of 400 to 480 ohms, in the middle of the upper wire, rated to absorb at least a third of the applied transmitter power.
• Tilt. To make it roughly omnidirectional, the antenna is ideally strung sloping at 20 to 40 degrees from horizontal. It also works when it is horizontal, provided the wires are kept straight with little sag.

Commercial versions vary. A well known commercial design covers 3 to 30 MHz with a length of about 90 feet (27 metres) and wire spacing of about 18 inches (460 mm). It uses a 16:1 balun that transforms the 50 ohm coax to about 800 ohms at the antenna, together with an 800 ohm non-inductive load. That arrangement allows the feedpoint impedance to swing between 400 and 1600 ohms across the range while keeping the SWR at the transmitter at about 2:1 or lower.

The resistor deserves real attention. It must be non-inductive, must tolerate sustained RF power without changing value, and must be protected from rain. Home builders often make it from a bank of many small resistors in series and parallel to share the power. Using a single small resistor is a good way to ruin the antenna, because the load value drifts or fails when it overheats.

Frequency Coverage and Performance

The antenna is usable across a frequency range of about 1:6. A version around 33 metres (110 feet) long with conductors spaced about a metre apart is intended for roughly 3 to 18 MHz. A shorter antenna of about 20 metres (66 feet), with spacing of about 60 centimetres, covers roughly 5 to 30 MHz. If you cannot fit that span, a shorter antenna still gives reasonable reception down to about half of its lowest design frequency, but transmitting performance falls away quickly.

Careful measurements by J. S. Belrose in 1994 showed that, although the usual length is close to a full-size 80 metre antenna, the T2FD begins to suffer serious losses in both transmission and reception below about 10 MHz. At 10 MHz the 80 metre signals were reduced by 10 dB compared with a reference dipole. A reduction of 10 dB is a factor of ten in power, which is a large cost for the ability to be everywhere at once.

Above that point, on the 30, 20, 17, 15, 12 and 10 metre bands, the behaviour is more forgiving. A broad, smooth response with no pronounced dead spots in frequency or direction is the real appeal. You can change bands as quickly as you can turn the dial, which suits scanning, listening and casual digital work.

Who Should Use a T2FD

The reference material adds a realistic warning. The antenna is not recommended for those who want to make challenging long distance contacts with limited power, whether the legal limit is 1000 W or 1500 W. On the shortwave bands, a dipole cut for the longest wavelength you use, fed with ladder line and matched with an antenna tuner, delivers more of your power to the air than a T2FD does. That is the main competitor, and it is worth comparing it with the doublet fed with ladder line before you buy or build.

Feedline, Balun and Tuner

Because the antenna is a balanced structure, it needs a good balun. The 4:1 or 16:1 ratio depends on the design, and it is the single most important component after the resistor. A low-quality balun may heat up, saturate or add loss that wipes out the benefits of the broadband design. Our explanation of baluns and ununs shows what to look for. Put a common mode choke on the coax near the balun as well, because a long wire antenna near the shack will happily put RF onto the shield.

The match is not perfect at every frequency. The feedpoint can be moderately reactive at some frequencies, and with modern solid-state transmitters running near their rated power an antenna tuner can still be needed. If your radio folds back power as the SWR rises, plan on a tuner and see our note on choosing an external antenna tuner. A tuner will not make a lossy antenna efficient, but it keeps the transmitter comfortable.

Comparing the T2FD with Other Multiband Wires

If you only need two or three bands, a resonant design often wins. A fan dipole or a trap dipole offers real resonance on each band, so efficiency is much higher on those bands. A trap dipole uses parallel tuned circuits to isolate sections of wire, and its coverage is limited to the bands the traps were designed for. A T2FD gives you continuous coverage but with a loss on every band. The decision comes down to whether you value convenience across the whole spectrum or efficiency on a handful of bands.

A lot of the appeal of the T2FD is in receiving. Noise on HF is often higher than the signal level of the weakest stations you can copy, so a few dB lost in the antenna reduces signal and noise together. There have been claims that the antenna is less sensitive to man-made noise, but these are disputed, and you should treat them with caution. If you intend to use the antenna in a noisy urban environment, test it against a simple dipole before you trust the claim.

Practical Installation Tips

• Keep the wires parallel and straight. Spacers must stay in place, because the characteristic of the antenna depends on the spacing. Use light, UV-resistant insulating spacers and non-conducting rope at the ends.
• Mount the resistor in a weatherproof housing. The termination can be hot after a long transmission, so use a ventilated but rain-protected enclosure, and keep it away from flammable materials.
• Give it height and clearance. As with any wire antenna, being well clear of buildings, gutters and metal objects reduces losses and unwanted coupling.
• Slope it if you can. A slope of 20 to 40 degrees helps fill in the pattern, and it can be a convenient way to use one high support and one lower support.
• Use a power rating with a margin. If you transmit with SSB only, the average power is much lower than PEP, but data modes and carriers heat the resistor continuously.
• Check it with an analyzer. A plot across the range reveals the pattern of the SWR and any reactive points that may call for a tuner.

Common Mistakes

The first mistake is to expect resonant performance from a broadband antenna. If you compare it on the air with a half-wave dipole on 20 metres, the dipole will normally win. The second is to ignore the resistor, using one that is too small, inductive or not weatherproof. The third is to forget about the feedline. Long, loose coax and a poor balun can lose as much signal as the antenna itself. The fourth is to put a T2FD up in place of a proper low-band antenna and then wonder why the 80 metre band sounds dead. It is not a replacement for a full-size dipole or vertical on the bands where length matters.

Finally, avoid chasing numbers. Do not claim a certain gain or an exact efficiency for your build unless you have measured it. The available evidence says that efficiency varies with frequency, falls markedly at the low end of the range, and depends on the quality of the balun and the resistor.

The Bottom Line

The T2FD antenna is an elegant solution to a specific problem: how to cover a wide range of HF frequencies with one wire and without constant retuning. It achieves a steady match by absorbing energy in a termination resistor, which means that a meaningful share of your transmitter power never leaves the antenna, and that the loss is worst on the lower bands. For listening, low power operation on the upper HF bands and for stations that need simple, all-band coverage, it can be a sensible choice. For serious DXing and contesting, especially on 80 and 40 metres, a resonant dipole, a doublet with ladder line or a well-matched vertical will deliver more signal. Decide what you value, build the balun and resistor carefully, measure the result, and let the contacts tell you whether it was worth it.

Frequently Asked Questions

What does T2FD stand for?

It stands for tilted terminated folded dipole. It is also known as the balanced-termination folded dipole or the W3HH antenna, and it was developed for the US Navy in the 1940s.

Does a T2FD need an antenna tuner?

Often not, because the match is broad. At some frequencies the feedpoint can be moderately reactive, so a tuner may still be needed when a modern transceiver is operated near its rated power.

How much power does the resistor waste?

The published description quotes at least 30 percent of the RF power lost as heat in the resistor, and the proportion grows as you operate closer to the lower end of the design range.

Can I use a T2FD on 80 metres?

You can, but performance is poor compared with a resonant antenna. Measurements showed 80 metre signals reduced by about 10 dB against a reference dipole, and efficiency worsens below about 10 MHz.

Why is the antenna tilted?

A slope of about 20 to 40 degrees makes the radiation pattern more nearly omnidirectional. It also works when it is horizontal if it is held straight with little sag.

Is a T2FD better than a fan dipole?

It depends on the goal. A fan dipole is resonant on a few chosen bands and is more efficient there, while the T2FD covers a continuous range with a loss on every band.