The T2FD (English Tilted Terminated Folded Dipole, translated as: inclined completed folded dipole ) is a shortwave antenna, which was developed in the late 1940s by the United States Navy. It works reasonably well over a wide frequency range with respect to adaptation and radiation pattern.
Although a T2FD like a worse performance than optimized for a particular frequency antenna. Away from this, usually very narrow frequency range may be the but quickly reversed. The relatively small size, good overall performance, low cost and wide bandwidth makes the T2FD particularly attractive for professional radio services. Especially for modern frequency hopping or ALE devices the large bandwidth is advantageous.
The T2FD is an interesting antenna for shortwave listeners, because it works equally well over a wide frequency range, and is fairly easy to build. The terminator is a normal carbon or metal film resistor in this case. The often traded as a disadvantage power loss in the terminating resistor is in the receive mode without importance, since the noise temperature of the atmosphere is in the shortwave range much higher than the noise temperature of a good shortwave receiver. However, the question whether for receive-only purposes an active antenna is not the better solution - simply because it is much smaller.
The broadband is also the biggest problem for the reception: Modern receivers have input usually only high - and low-pass filters which let more of the shortwave spectrum. Thus, ( eg 21 - m radio broadcast band and 20-m amateur radio band ) easily overwhelm the receiver broadcasters from neighboring frequency bands. An additional preselector is therefore recommended.
The biggest problem for the radio amateurs homemade is the terminator. The one hand must have good high-frequency characteristics, on the other hand, he must have been in a 100 W transmitter for at least 30 W power dissipation are designed - commercially available components to meet these demands rarely.
The wide range is mainly in the 80 - m band is advantageous, because no resonant antenna can cover the entire range from 3.5 to 3.8 MHz.
A T2FD usually has the following characteristics:
- Total length 1 /3 of the longest wavelength
- Distance between the two wires 1 /100 of the wavelength by non-conductive spreaders
- Two spacers on the outer ends that hold the wire halves apart and allow an attachment with one rope.
- Fed into the center of the lower wire with an impedance of about 300 Ω symmetrical, usually through a balun 6:1 with a standard 50 Ω coaxial cable.
- In the upper wire can be a low-inductance terminating resistor of approx 1/3 of the transmit power is safely absorb needs. Are usual here 390 Ω at 50 W. satellite earth station antennas, the performance is not critical.
- For a good all-round radiation the T2FD is stepped down obliquely at an angle of 20 ° to 40 °.