The tilted terminated folded dipole (T2FD or TTFD) or balanced termination, folded dipole (BTFD) - also known as W3HH antenna - is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy. It performs reasonably well over a broad frequency range, without marked dead spots in terms of either frequency, direction, or angle of radiation above the horizon.
Although inferior in electrical terms (up to 30% of the RF power is converted to heat in the resistor ) to antennas specifically designed for given frequency bands, or optimized for directionality, its all-around performance, relatively modest size, low cost, and the fact that it does not require any complex electronic matching to operate with a standard shortwave transmitter, have made it popular in professional shortwave communications.
Also, since the late 1980s, amateur radio operators and hobby shortwave listeners have 'rediscovered' this antenna, especially for broadcast receiving and for amateur two-way modes such as Morse code and PSK31 where brute force performance is not as important as a 'steady' signal. There have also been (disputed) claims that this antenna is comparatively insensitive to man-made radio interference, making it useful in urban environments, where a low noise floor is often more beneficial than high received signal strength. The T2FD is useful for hidden indoor systems, or where several optimised frequency-specific antennas cannot be accommodated. For example: an indoor antenna only 7 metres long will allow operation on all amateur HF bands above 14 MHz on transmit, and down to 7 MHz on receive.
A typical T2FD is built as follows, out of two parallel wire conductors:
- Span equal to ~1/2 of the lowest required wavelength.
- Distance between upper and lower conductors equal to 1/100 of the wavelength. This distance is maintained by a number of insulating dowels.
- Two dowels at the ends are tied to non-conducting ropes, in turn tied to supports.
- Upper and lower conductors are connected at the ends, by wire sections that follow the two end dowels.
- Fed in the middle of the lower conductor, with an impedance in the order of 300 Ohm, balanced, through a standard 4:1 balun. This provides an acceptable all-frequency match to commonly available 75-ohm coaxial cable.
- Terminated in the middle of the upper conductor with a 400- or 480-Ohm non-inductive resistor, rated to safely absorb at least 1/3 of the applied transmitter power. The resistor absorbs a growing portion of the RF power (either captured from the air or supplied by a transmitter) as the operating frequency nears the lower limit of the design range. The resistor can be built of 3 pairs of 10 serial 1,6kΩ 1W resistors.
- In order to make it roughly omnidirectional, it is ideally strung sloping at an angle of 20 to 40 degrees from horizontal, but will also function satisfactorily if mounted horizontally, as long as it is pulled-out in a reasonably straight line.
- The commercially available B&W AC3-30 antenna varies from the above to cover 3 to 30MHz with a 90 foot length with a 18 inch spacing of the wires. The balun is a 16:1 ratio, thereby transforming the 50 ohm coax to an 800 ohm feed at the antenna. The resistor load is also 800 ohm, non-inductive. This allows the antenna impedance to swing from 400 to 1600 ohms over the frequency range intended and thus keep the SWR at the transmitter 2:1 or lower.
Such an antenna is usable for both local and medium-long distance communication across a frequency range of about 1:6. For example, an antenna for the lower portion of shortwave (say, 3–18 MHz) will be roughly 33m (110 feet) long, with conductors spaced 1m (3.3 feet). For the higher portion of shortwave (5–30 MHz), this antenna will be roughly 20m (66 feet) long, with a spacing of 60 cm (24 inches). If such long spans cannot be accommodated, smaller antennas will still give adequate receive-only performance down to about half of their lowest design frequency. Transmit performance, however, degrades rapidly below a certain point. Tests done by Dr.John Belrose VE2CV, and posted in the May 1994 issue QST magazine (Page 88), showed that a T2FD, though it has a length close to a full size 80 meter or 4MHz antenna, starts to suffer serious signal loss (both transmit and receive) below 10MHz, with the 80 meter band signals -10db down from reference at 10MHz.
As a broadband antenna, the T2FD will normally display a reasonably low standing wave ratio across its entire frequency range. However; at some frequencies there may be a moderately reactive element within the loading, so the use of an antenna tuner may be beneficial when using modern, solid-state transmitters at anything approaching their rated power output. Also, LOW SWR does not mean high antenna efficiency. This antenna is not recommended for those wanting to make serious weak signal contacts. A dipole cut for the lowest frequency wanted, and fed with ladder line, and matched with an antenna tuner would perform better than the T2FD over the HF region of frequencies.
Many ready-made commercial versions of the T2FD are available for the professional / military, amateur radio, and hobby listening markets.
- "An Experimental All-Band Nondirectional Transmitting Antenna" by Gil L. Countryman, W1RBK, (W3HH), QST, June 1949, page 54.
- "Performance of the Terminated Folded Dipole" by Capt. G. L. Countryman, (W3HH), CQ, November 1951, page 28.
- "Modeling the T2FD", by L. B. Cebik, W4RNL, http://www.cebik.com/content/a10/wire/t2fd.html
- Antennen für die unteren Bänder 160 – 30 m: Technische Unterlagen für den Selbstbau in praxisnaher Darstellung, Pierre Villemagne
- Practical Wire Antennas – Effective HF Designs for the Radio Amateur (J. Heys, G3BDQ)
- Yaesu YA-30 Broadband HF Antenna http://www.yaesu.co.uk/files/YA_30.pdf