One understands by electric lengthening the modification of an aerial which is shorter than the a whole-number multiple of the quarter of the radiated wavelength, by suitable electronic device without changing the length of the aerial in such a way that it corresponds electrically to the next whole-number multiples of the quarter of the used wavelength. Basically a lengthening is only possible to the next whole-number multiples of the quarter of the radiated wavelength. Thus an aerial with a length corresponding to the eighth of the radiated wavelength can be extended, only to a quater wave radiator, but not to a half wave radiator. A half-wave dipole Antenna being driven the naughty way with no balun between the coaxial cable and the dipole A dipole antenna is an antenna with two driven elements. ... A half-wave dipole Antenna being driven the naughty way with no balun between the coaxial cable and the dipole A dipole antenna is an antenna with two driven elements. ...

Advantage

The electric lengthening allows the construction of shorter aerials. It is applied in particular for aerials for VLF, longwave and medium-wave transmitters, because mast radiators of the necessary height cannot be realised economically.

Disadvantage

The electric lengthening reduces the bandwidth, if not suitable measures are met. An electrically extended aerial is less actual, than a not extended aerial.

Technical realization

There are two possibilities for the realisation of the electric lengthening. Either one switches coils in seriel with the aerial, or one switches at the aerial ends metal surfaces which form a capacitator with the earth. One calls these metal surfaces roof capacity. Often both measures are combined. The coil switched in row must be placed sometimes in the middle of the aerial construction. A such construction is the cabin installed in a height of 150-metres on the Blosenbergturm in Beromünster, in which a lengthening coil is installed for the supply of the upper tower part (the Blosenbergturm has in addition also a ring-shaped roof capacity on its top) The Blosenbergturm is a transmitting tower in Beromünster, Canton of Lucerne, Switzerland, built in 1937 for the German radio station DRS and sending at a AM frequency of 531 kHz. ...

Application

Transmission aerials of transmitters working on frequencies below the longwave broadcasting band always apply electric lengthening. Broadcasting aerials of long waves broadcasting stations apply it often, but not always. However, for transmission aerials of NDBs electrical application is extensively applied, because they use antennas which are considerably less taller then the quarter of the radiated wavelength.

See also

• Electrical shortening
• Loading coil