Trap line

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Trap line of the long wave transmitter Solec Kujawski in Poland
Feeding of a trap line

A trap line is a special type of overhead line for feeding antennas of large transmitters in the long and medium wave range .


A trap line consists of outer conductors arranged in a circle , which are connected to the earthing system . Inside the arrangement, the conductors of the inner conductor, separated from the outer conductors by insulators, are attached. The trap line has similar structural features to the bundle ladder , but is different physically and in terms of purpose.


The functional principle of a trap line corresponds to that of a coaxial cable , except that this line is designed as an overhead line and the electrical conductors consist of individual conductor cables and are not designed as hollow tubes. As with high-performance coaxial cables, outer and inner conductors are kept at a constant distance by spacers. This spatial distance and the ratio of the diameter of the inner and outer conductors essentially determine the line impedance of a trap line.


Due to the large spacing between the individual conductors in the outer and inner conductors, trap lines are only suitable for low frequencies in the long and medium wave range up to frequencies of a few MHz .

Bundle of conductors with spacers in the Felsberg-Berus transmitter

As with a coaxial line, the high-frequency power is transported in the field space between the outer conductor and the inner conductor. Since this space between the conductors, apart from the small insulators to ensure a constant distance between the conductors, is filled with air and air has only a low loss factor , large high-frequency power up to the megawatt range can be transmitted with trap lines. One example is the Wachenbrunn transmission system in Thuringia, which was fed with a transmission power of 1 MW via a trap line until 2003  .

New plants

The antenna feed lines in some new transmission systems are also designed as traps , such as the Atlantic 252 long wave transmitter in Clarkestown , Ireland , and the Polish radio long wave transmitter in Solec Kujawski .

Web links

Commons : Trap lines  - collection of images, videos and audio files


  • Otto Zinke , Heinrich Brunswig: Textbook of high frequency technology . 1st edition. Springer, 1965, ISBN 978-3-662-00476-0 , Chapter 4.7 The coaxial trap line.