Krarup Loading

(also continuous loading), a method of increasing the transmission range of communications cables by means of an artificial increase in their inductance. It was proposed in 1902 by the Danish engineer C. E. Krarup and was a practical way of realizing the theoretical conditions for transmission of electromagnetic waves with minimal loss and no distortion that had been deduced by the English physicist O. Heaviside in 1893.

The essence of Krarup loading is compensation by inductance for the effects of cable capacity and resistance on the cable’s attenuation factor. The inductance of the cable is increased by winding its current-carrying copper wires with one, two, and sometimes three layers of thin tape or wire (with a diameter of 0.2–0.3 mm) made of steel or special alloys whose magnetic permeability is hundreds to thousands of times greater than that of the copper. This increases the cable inductance by an average factor of 10 and reduces the attenuation coefficient by a factor of about 3—that is, it increases the communication range correspondingly.

The first submarine telephone cable (with gutta-percha insulation) with Krarup loading was laid in 1902 between Denmark and Sweden and was 5 km long. This type of loading for submarine cables was found to be more satisfactory than coil loading, which hampered the continuous cable-laying operation because of thickening at the places of attachment of loading coils. During the 1920’s several transoceanic telegraph cables were laid in which Krarup loading was achieved with permalloy and perminvar tapes. The rate of telegraph transmission was thereby increased by factors of 10–100. In the 1950’s, with the advent of line repeaters on submarine cable lines and the provision of simultaneous telephone and telegraph communication on them, Krarup loading lost its importance.