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Transmission Line Theory

  Transmission Line Theory, in electrical engineering, allows the analysis and design of electrical communications and power transmission systems, for example telephone lines or electric pylon lines. The theory permits transmission systems to be created that are efficient with respect to integrity of information exchanged and ensure maximum power transfer between transmitter and receiver.

The foundation of transmission line theory lies in the workings of electromagnetism as detailed by James Maxwell in 1873. By this time telegraphy (communication by a series of electrical pulses) was well established and growing; indeed a telegraph undersea cable had been laid across the Atlantic Ocean as early as 1858. From the telegraphers\' point of view, long-distance transmission only affected the transmitted signal by attenuating it, consistent with elementary circuit theory, and could be tolerated. However, following the discovery and introduction of telephony in the 1870s using the existing electric cables, long-distance telephony could not be achieved due to severe distortion of the transmitted signal making speech unintelligible.

The solution to this problem appeared following the recognition and use of inductance in long cables by the Englishman O. Heaviside. Previously W.T. Kelvin had identified that a cable had a certain capacitance distributed along its length, the effect of which was to distort an electrical pulse. At the time of its discovery the effect of capacitance did not greatly perturb the telegraphers, but caused despair among those developing long-distance telephony. Heaviside\'s work correctly identified that proper use of inductance could cancel the effect of the capacitance and greatly reduce distortion of speech signals. This led to M. Pupin receiving the patent in 1899 for the use of inductive coils as Heaviside had suggested, to increase the distances that telephony could be achieved over.

These newly acquired electrical concepts of capacitance and inductance were to influence many other areas of electrical engineering. Circuit theory was updated to include these two components and high frequency circuits could now be correctly analysed. Electrical filters could be produced from these two new components and were widely used in modulation and multiplexing schemes later used in radio and other communication systems.

The theory of transmission lines also introduced new mathematical methods to the study of electrical engineering, such as complex numbers and Laplace transformations. These proved to be useful in studying electric power transmission lines, defining conditions to achieve maximum power transfer while maintaining stability of the electrical generators, and implying that efficient transmission of large amounts of power should be done at high voltage. Results such as these have led to today\'s high voltage power network, or ‘supergrid’, which uses large pylons holding high voltage wires to transmit huge quantities of power from one end of the country to another.

Transmission line theory and the use of Maxwell\'s equations also led to other transmission systems such as microwave communications and the use of other waveguides. In essence this theory helped create the communications age of today, where a person takes for granted the technology to talk with someone on the other side of the planet, or indeed far out in space. AC



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