T2FD Antenna
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The T2FD (tilted, terminated, folded dipole) is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy.
It performs reasonably well (if not with optimal efficiency) over a wide frequency range and without marked dead spots in terms of frequency or direction.
While in many ways measurably inferior 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 electronic matching to operate with a standard shortwave transmitter, have made it popular in professional shortwave communications. Since the late 1980s also amateur radio operators and hobby shortwave listeners have become aware of its qualities.
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.
- In order to make it roughly omnidirectional, it is strung sloping at an angle of 20 to 40 degrees from horizontal.
Such an antenna is usable for both local and medium-long distance communication in a frequency range of about 1 to 6.
For example, an antenna for the lower portion of shortwave (3 - 18 MHz) will be roughly 33m (110 feet) long, with conductors spaced 1m (3.3 feet).
In order to cover 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).
Many ready made commercial versions are available for the professional / military, ham radio, and hobby listening markets.ja:T2FD