T2FD Antenna

T2FD Antenna

The T2FD (Tilted Terminated Folded Dipole) is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy ["An Experimental All-Band Nondirectional Transmitting Antenna" by Gil L. Countryman, W1RBK, (W3HH), QST, June 1949, page 54.] . 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 ["Modeling the T2FD", by L. B. Cebik, W4RNL, http://www.cebik.com/content/a10/wire/t2fd.html] 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 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 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.

References

External links

* [http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html "T2FD - The Forgotten Antenna"]
* [http://www.simonmason.karoo.net/page498.htm A large T2FD spotted in use at the US Consulate in Jerusalem]


Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • Antenna (radio) — Whip antenna on car …   Wikipedia

  • T2FD — Eine 20 m lange T2FD Antenne, für den Frequenzbereich 5 30 MHz. Die T2FD (Tilted Terminated Folded Dipole, übersetzt etwa: geneigter abgeschlossener gefalteter Dipol) ist eine Kurzwellenantenne welche in den späten 1940er Jahren durch die United… …   Deutsch Wikipedia

  • Microstrip antenna — In telecommunication, there are several types of microstrip antennas (also known as printed antennas) the most common of which is the microstrip patch antenna or patch antenna. A patch antenna is a narrowband, wide beam antenna fabricated by… …   Wikipedia

  • Parabolic antenna — A parabolic satellite communications antenna at Erdfunkstelle Raisting, the biggest facility for satellite communication in the world, in Raisting, Bavaria, Germany. It has a Cassegrain type feed. A parabolic antenna is an antenna that uses a… …   Wikipedia

  • Dipole antenna — A schematic of a half wave dipole antenna connected to an unbalanced coaxial cable. Better practice is to connect the balanced dipole to the unbalanced line with a balun. A dipole antenna is a radio antenna that can be made of a simple wire, with …   Wikipedia

  • Whip antenna — on car A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane.[1] The bottom end of the whip is connected to the radio receiver or transmitter.… …   Wikipedia

  • Discone antenna — Mounted discone antenna designed for VHF and UHF coverage …   Wikipedia

  • Loop antenna — A shortwave loop antenna A loop antenna is a radio antenna consisting of a loop (or loops) of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line. Within this physical description there are two very …   Wikipedia

  • Monopole antenna — Mast radiator monopole antenna used for broadcasting. AM radio station WARE, Warren, Massachusetts, USA. A monopole antenna is a class of radio antenna consisting of a straight rod shaped conductor, often mounted perpendicularly over some type of …   Wikipedia

  • Rubber Ducky antenna — on a transceiver The Rubber Ducky antenna (or Rubber Duck aerial) is an electrically short monopole antenna which functions somewhat like a base loaded whip antenna and is sealed in a rubber or plastic jacket to protect the antenna.[1]… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”