Electromagnetic waves can be emitted into the open space by an antenna; it is called free propagation of waves, a process that is widely used in telecommunication systems [1]. When the frequency and / or the power increases, the use of transmission lines is very limited by the appearance of higher order modes that occur as soon as the transverse dimensions of the structures used are no longer negligible compared to the wavelength and also by increasing dielectric losses in the insulators which also often causes an important attenuation [2] in this case, it becomes necessary to use structures called generally "waveguide" for the transmission of this electromagnetic energy. It's about a hollow conductor in which electromagnetic waves are propagated by successive reflections on the internal walls. The metal waveguides are the basis for the design of many microwave devices such as filters, transformers, adapters, polarizers.... In this chapter we'll have a detailed overview on the use of waveguides (filters in general), their characteristics, advantages, disadvantages compared to other guiding structures.
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