T2FD
The T2FD ( English Tilted Terminated Folded Dipole , roughly translated: inclined, closed folded dipole ) is a shortwave antenna that was developed by the United States Navy in the late 1940s . It works reasonably well over a wide range of frequencies in terms of adaptation and radiation pattern .
A T2FD has a poorer performance than an antenna optimized for a certain frequency, but apart from this, usually very narrow, frequency range this can quickly reverse. The relatively small size, good overall performance, low cost and large bandwidth make the T2FD particularly attractive for professional radio services. The large bandwidth is particularly advantageous for modern frequency hopping or " Automatic Link Establishment " devices.
The T2FD is an interesting antenna for shortwave listeners because it works consistently well over a wide frequency range and is quite easy to build. In this case, the terminating resistor is a normal carbon or metal film resistor . The power loss in the terminating resistor, which is often seen as a disadvantage, is irrelevant in reception mode, because the noise temperature of the atmosphere in the shortwave range is much higher than the noise temperature of a good shortwave receiver. However, the question is whether an active antenna is the better solution for pure reception purposes - if only because of its smaller size.
At the same time, broadband is the biggest problem for reception: modern receivers usually only have high and low pass filters in the entrance, which allow a larger part of the shortwave spectrum to pass through. Thus, broadcasters can from adjacent frequency ranges (eg. 22-m broadcast band and 20-m - amateur radio band easily overwhelm the receiver). An additional preselector is therefore recommended.
The biggest problem for radio amateurs when building their own is the terminating resistor. On the one hand, it must have good high-frequency properties; on the other hand, it must be designed for at least 30 W power loss even with a 100 W transmitter - commercially available components only rarely meet these requirements.
The large bandwidth is particularly advantageous in the 80 m band because hardly any 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 or 1/2 of the longest wavelength
- Distance between the two wires 1/100 of the wavelength through non-conductive spreaders
- two spacers at the outer ends, which keep the wire halves at a distance and allow attachment with one rope each
- fed in the middle of the lower wire with an impedance of around 300 Ω symmetrically, mostly through a 6: 1 balun with a standard 50 Ω coaxial cable .
- In the upper wire there is a low-inductance terminating resistor that must be able to safely absorb approx. 1/3 of the transmission power . 390 Ω at 50 W are usual here. For pure receiving antennas, the power is irrelevant.
- For good all-round radiation, the T2FD is stretched down at an angle of 20 ° to 40 °.
Individual evidence
- ↑ An Experimental All-Band Nondirectional Transmitting Antenna by Gil L. Countryman, W1RBK, (W3HH), QST, June 1949, p. 54.
- ↑ PDF at www.bwantennas.com ( Memento of the original from October 11, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Modeling the T2FD , by LB Cebik, W4RNL, http://www.cebik.com/content/a10/wire/t2fd.html
