Loading coil

Coil-loaded lines are lines that have been used in the field of telecommunications. They exist in the original form of addition periodically inserted into the line electrical coils, to increase the inductance of the cable artificially. Under certain circumstances, these additional inductance allows longer distances to distortion as possible transmission of low-frequency signals, ie preventing a height reduction. Simultaneously, the capacitive charging current of longer lines when switching is significantly reduced.

History

The first ideas and the development bespulten lines go to Oliver Heaviside and the transatlantic Telegrafiekabel between Europe and North America from 1887. Heaviside found that by additionally inserted coils in submarine cables decreased the signal distortion. The mathematical description of the effect is known as the Heaviside condition that describes the circumstances under which a signal via an electrical line can be transferred as possible without distortion. Sometimes uncoiled cables are also referred to as Pupinleitungen, which go back to the physicist Mihajlo Pupin, the basis of the preparatory work of Oliver Heaviside this technique in 1894 filed for a patent, but not the actual inventor of this method.

For submarine cables, the increased range was first significantly due to the greater length over laid on land telegraph cable runs. The regular array of discrete coil box along the submarine cable is cumbersome, however constructive, so the inductance of the cable was increased by constructive measures in the following years. In this case, the electrical conductor made ​​of copper in the form of special permeable metal belts is wrapped, of iron, which are continuously incorporated into the cable sheath. Initial work to these efforts in 1900, the Danish telegraph engineer Carl Emil Krarup, who developed the eponymous Krarupkabel.

Improvements resulted in the following years by the replacement of the steel straps by higher permeable alloys mu-metal ( permalloy ). 1923 corresponding cable in Bermuda from AT & T were tested, the first submarine cable regularly used with mu-metal for coil-loading has been taken in the Azores by AT & T in operation in the following year between New York and Horta. Improvements in the metal alloys led to various patents as in 1923 by Western Union, a former competitor of AT & T.

Beginning of the 20th century played uncoiled lines even when on land laid telegraph lines and later in telephone lines between individual local exchanges a role. In the German telephone network were used with an inductance of 80/50 mH earlier in the distance (S) of 1700 m ( 1680 to 1720 m and the outlet box (S / 2) 840-860 m) coils. They were largely replaced in the field of wired communications technology first by coaxial cable and fiber optic cable in a row. Coil-loaded subscriber lines are unsuitable due to the bandwidth limitation for DSL technology.

The principle is used today in different areas application, but not to achieve a greater range. A related application in the field of high frequency technology is the most ideal coupling of short antennas at the transmitter output stage; these are antennas that are shorter than 1/ 4 of the wavelength and therefore behave capacitively.

Description

An electric line can be dx by their equivalent circuit of a line section with the infinitesimal length and describe the data related to the length coverings inductance L ', the capacitance C', the resistance per unit length R 'and the conductance per unit length G'. For a minimal signal distortion, the Heaviside condition must be met. This is the case if:

Applies. In this case, there occurs a signal attenuation, but no signal distortion. In a conventional line usually applies:

The capacitance results from a geometric structure, the dielectric used, the shunt conductance and series resistance by the non-ideal isolation and the imperfect conductivity of the copper conductor. These values ​​can be structurally impossible or difficult to change. However, the inductance can be increased by additional coils or wraps of high permeability of the pipe.

The increased inductance occurs at the same time to a bandwidth limitation, since the Bespulen the line acts as a low pass filter. The inductance L of the coil additionally introduced and the distance D along the line can be determined to be:

Here, the characteristic impedance of the not bespulten line and the cutoff frequency.