Coiled wire

Coiled cables are cables that have been used in the field of communications technology. In their original form, they also consist of electrical coils inserted into the line at regular intervals in order to artificially increase the inductance of the cable. Under certain circumstances, this additional inductance enables longer cable runs for the most distortion-free transmission of low-frequency signals. H. the prevention of a drop in altitude. At the same time, the capacitive charging current of longer lines is significantly reduced when switching on.

Symmetrically coiled cable

history

Pupin coil

"Coiled wire" with a highly permeable MU metal tape

Pupin coils in the PTT Museum in Belgrade

The first ideas and the development of coiled cables go back to Oliver Heaviside and the transatlantic telegraphic cables between Europe and North America from 1887. Heaviside found that by additionally inserted coils in submarine cables, the signal distortions decreased. The mathematical description of the effect is known as the Heaviside condition , which describes the circumstances under which a signal can be transmitted over an electrical line with as little distortion as possible. Coiled cables are sometimes referred to as Pupin cables , which go back to the physicist Mihajlo Pupin , who applied for a patent for this technique in 1894 based on the preliminary work of Oliver Heaviside, but is not the actual inventor of this process.

In the case of submarine cables, the increased range first became significant due to the greater length compared to telegraph cable routes laid on land. The regular arrangement of discrete coil boxes along the submarine cable is, however, structurally cumbersome, which is why the inductivity of the cable was increased through design measures in the following years. The electrical conductor made of copper is wrapped in the form of special permeable metal strips made of iron , which are continuously worked into the cable sheath. The Danish telegraph engineer Carl Emil Krarup , who developed the Krarup cable named after him, did the first work around 1900 .

Improvements were made in the following years by replacing the iron strips with more permeable alloys made of Mu-metal (Permalloy). Corresponding cables were tested by AT&T in Bermuda in 1923 ; the first regularly used submarine cable with mu-metal for winding was put into operation by AT&T the following year between New York and Horta in the Azores . Improvements in the metal alloys led to various patents such as 1923 from Western Union , a competitor of AT&T at the time.

At the beginning of the 20th century, coiled lines also played a role in telegraph lines laid ashore and later in telephone lines between individual local exchanges . In the German telephone network, coils with an inductance of 80/50  mH were used at a distance (S) of 1700 m (1680–1720 m and discharge area (S / 2) 840–860 m) . In the field of wired communication technology, they were initially replaced by coaxial cables and subsequently largely by optical fibers . Coiled subscriber lines are unsuitable for DSL technology due to the bandwidth limitation .

The principle is used today in various areas, but hardly to achieve a greater range. Coiled telecommunication lines can only be found in Germany today at electricity and gas network operators, on motorways, on old railway lines and as old stock on waterways.

description

Equivalent circuit of an electrical line

An electrical line can be described by its equivalent circuit of a line section with the infinitesimal length d x and the length-related coatings inductance coating L ' , the capacitance coating C' , the resistance coating R ' and the dissipation coating G' . The Heaviside condition must be fulfilled for minimal signal distortion. This is the case when:

${\displaystyle {\frac {G'}{C'}}={\frac {R'}{L'}}}$

applies. In this case, signal attenuation occurs, but no signal distortion. For a normal line, the following usually applies:

${\displaystyle {\frac {G'}{C'}}\ll {\frac {R'}{L'}}}$

The capacitance per unit length results from the geometric structure and the dielectric used , the transverse conductance and the series resistance due to the non-ideal insulation and the non-perfect conductivity of the copper conductor. These values ​​cannot be changed constructively or only with difficulty. On the other hand, the inductance coating can be increased by using additional coils or highly permeable wraps around the line.

The increased inductance also results in a bandwidth limitation , since the line being wound acts as a low-pass filter . The inductance L of the additional coils and their distance d along the line can be determined as follows:

${\displaystyle L={\frac {Z_{0}}{\omega _{c}}}}$  and   ${\displaystyle d={\frac {2}{\omega _{c}Z_{0}C}}}$

Here is the line impedance of the uncoiled line and the cutoff frequency . ${\displaystyle Z_{0}}$ ${\displaystyle \omega _{c}}$

context

So-called extension coils are based on the same functional principle but with a different area of ​​application . They are used for the electrical extension of antennas .

Individual evidence

1. ↑ ^{a b} Patent US519346 : Apparatus for Telegraphic of Telephonic Transmission. Registered December 14, 1893 , published May 8, 1894 , inventor: Michael Idvorsky Pupin. ‌
2. ↑ Oliver Heaviside: Electromagnetic Induction and its propagation. In: The Electrician. June 3, 1887.
3. ↑ Oliver Heaviside: Electrical Papers. Issue 1, Boston 1925, pp. 139-140.
4. ↑ patent GB224972 : New and improved magnetic alloys, and Their application in the manufacture of telegraphic and telephonic cables. Filed August 25, 1923 , published November 25, 1925 , inventors: WS Smith, HJ Garnett (also published under US1582353 and US1552769 ). ‌