Unbalanced line

In electrical engineering, an unbalanced line is a transmission line, often coaxial cable, whose conductors have unequal impedances with respect to ground; as opposed to a balanced line. Microstrip and single-wire lines are also unbalanced lines.

Any line that has a different impedance of the return path may be considered an unbalanced line. However, unbalanced lines usually consist of a conductor that is considered the signal line and another conductor that is grounded, or is ground itself. The ground conductor often takes the form of a ground plane or the screen of a cable. The ground conductor may be, and often is, common to multiple independent circuits. For this reason the ground conductor may be referred to as common.

The earliest use of unbalanced transmission lines was for electric telegraph communications. These consisted of single wires strung between poles. The return path for the current was originally provided by a separate conductor. Some early telegraph systems, such as Schilling's experimental needle telegraph (1832) and the Cooke & Wheatstone five-needle telegraph (1837) used by British railways required multiple code wires. Essentially, they were parallel bus coding. In these systems the cost of the return conductor was not so significant (one conductor in seven for Schilling's earliest needle telegraph and one conductor in six for the Cooke and Wheatstone telegraph) but the number of coding conductors was progressively reduced with improved systems. Soon only one coding wire was required with the data being transmitted serially. Important examples of these single-wire systems were the Morse telegraph (1837) and the Cooke & Wheatstone single-needle telegraph (1843). In such systems the cost of a return conductor was fully 50 per cent of the cable costs. It was discovered that a return conductor could be replaced with a return path through the Earth using grounding spikes. Using earth return was a significant cost saving and rapidly became the norm.

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