(also coil loading), a system that increases the transmission range of telegraph and telephone communications over cables by artificially increasing the inductance in the cables. It was proposed by M. Pupin in 1900, and a working example was operated for the first time in 1902.
The Pupin system was the realization of O. Heaviside’s idea concerning the possibility of reducing the energy loss of signals transmitted over a communications cable by finding a certain relationship between the cable’s four basic electric parameters—the resistance R, the inductance L, the capacitance C, and the insulation conductance G—per unit length of cable. The energy losses in the cable, which are characterized by the attenuation factor α, are minimized when RC = LG; in this case, αmin. Use of the Pupin system makes it possible to reduce α (in actual cable structures without Pupin coils, usually RC≫ LG and α ≫ αmin) and thereby increase the communication range by three to five times. Depending on the diameter of the conductors in the cable, communication links can be achieved over distances of 10 to 100 km by means of this system.
In the Pupin system, a cable is loaded at uniformly spaced intervals (0.3-2 km), called coil spacings, with Pupin coils—coils wound with insulated copper wire on closed, ring-shaped cores made of a ferromagnetic material. The coil inductances range between 1 and 140 millihenries, which is several tens of times in excess of the intrinsic inductance of a section being loaded.
The Pupin system is used in the low-frequency lines of local and toll exchange networks, in the low-frequency circuits of coaxial trunk cables for service traffic between attended repeater stations, and in the trunks of intercity telephone networks. A loaded line acts as a low-frequency electric filter with a limited passband (generally 300–3,400 hertz), and this represents a major obstacle to use in multichannel communication. This circumstance and the use of repeaters in cable lines—a more effective means of increasing the communication range—have gradually restricted the system’s application.
D. L. SHARLE