T2FD Antenna

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Folded dipole antenna

The T2FD (Tilted Terminated Folded Dipole) is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy ^[1]. It performs reasonably well over a broad frequency range, without marked dead spots in terms of frequency, direction, an angle of radiation above the horizon.

Although inferior in electrical terms ^[2] 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 program listening 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 antenna 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 wire conductors:

Length equal to 1/3 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 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.
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.

Such an antenna is usable for both local and medium-long distance communication in 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 the lowest design frequency.

Many ready-made commercial versions of the T2FD are available for the professional / military, amateur radio, and hobby listening markets.

A computer program for designing a T2FD is contained in "Hamcalc - version 100" available as a free download at <www.cq-amateur-radio.com>.

[edit] References

^ "An Experimental All-Band Nondirectional Transmitting Antenna" by Gil L. Countryman, W1RBK, (W3HH), QST, June 1949, page 54.
^ "Modeling the T2FD", by L. B. Cebik, W4RNL, http://www.cebik.com/wire/t2fd.html