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Waveguide Theory

  Waveguide Theory, in electrical engineering, is used to predict and analyse propagation of high frequency electromagnetic waves in a variety of media. It can be thought of as an extension of transmission line theory applied to high frequency electricity, and like transmission line theory was formed from the basis of electromagnetic theory. The impetus behind the research and application of waveguide theory was the need for ever faster and more numerous means of communication, whether it be more telephone lines or higher radio frequencies to permit transmission of more television stations.

Since the 1890s it had been shown theoretically possible to propagate electromagnetic waves inside a hollow tube of conducting material. At this time, however, it was thought a physical impossibility as electricity required two conductors in order to flow, one to allow current to flow to the object and the second to allow the current to return to the source, thereby completing an electrical circuit. Around the 1930s it had been shown experimentally that such propagation was physically possible, at this time along water-filled pipes, and shortly afterwards the theory was devised to characterize these effects more accurately.

The theory predicted a relationship between the frequency of electromagnetic wave propagated and the dimensions of the waveguide or pipe. Whereas conventional electrical wires would conduct from zero frequency to large frequencies, where the signal was attenuated, waveguides were found to propagate waves only from a certain cut-off frequency and upwards, the opposite to convention. The cut-off frequency in most instances was considerably higher than the upper working frequencies of electrical wires. It was the fact that such frequencies could be propagated that offered the possibility of communicating much greater amounts of information and at faster speeds. This form of metal pipe waveguide was microwave communication.

Although a microwave radio link between Dover and Calais was in use in 1934, applications of microwaves were primarily in radar until the end of World War II, because microwave radar used smaller antennae and transmitters than previously, and equipment was therefore lighter and easier to transport and provided greater directional resolution.

The theory of waveguides has produced other propagation media. Coaxial cable (where one conductor is located in the centre of the hollow second conductor the space being filled with an insulator) was also being used in the 1930s. This offered increased performance and greatly reduced interference from neighbouring conductors than conventional parallel wire cables.

The latest use of waveguide theory is in the area of optical fibres, where light, an electromagnetic wave of very high frequency, is propagated along a very small glass fibre. Due to the use of higher frequencies even than microwaves, vast amounts of information can be propagated along a single fibre. Development of practical optical fibre systems has only occurred since the late 1960s following the invention of laser light sources and light-emitting diodes, and with the solution of certain problems in the manufacture of glass fibre. AC



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