Basics of electromagnetism

Electromagnetic phenomena have long been neglected or even ignored. This is because systems have long operated at relatively low frequencies, for which the wavelength is much greater than their dimensions. It was therefore in the field of electrical machines that the search began for a way to solve the Laplace or Poisson equations governing electrostatic and magnetostatic fields. These equations are the static forms of the wave equation, and enable us to determine the fields (or, more generally, the associated potentials) in structures, and then deduce the charge or current distributions. As far as circuits are concerned, Kirchhoff's lemmas have long since been incorporated into sophisticated software programs, enabling us to solve highly complex circuits. In this case, the elements are localized, and the engineer doesn't need to know anything about electromagnetic phenomena to design the circuit layout and then analyze and predict its performance. The phenomena of radiation and coupling were more the concern of engineers dealing with antennas or wave propagation in various media or structures.

However, device designers were soon faced with problems linked to two constraints. The increase in digital data rates and operating frequencies, and the miniaturization of devices to reduce their size and weight. It was at this point that the first symptoms appeared, calling into question traditional circuit design models. Not only were elements no longer pure inductors, capacitors or resistors, but interconnections became transmission lines, introducing delays, losses and dispersion (signal distortion). In addition, couplings between lines and/or elements must be taken into account to avoid erroneous performance predictions. Finally, signals propagating in devices can no longer be assimilated to a localized voltage (or current), commonly used in circuitry, but rather to waves, for which line theory constitutes a first approximation. More generally, the presence of discontinuities favors the generation of higher modes, leading to reflections and radiation in the case of open structures. All these phenomena need to be taken into account with varying degrees of rigor in many areas of electrical engineering:

• guide characterization (situations where the line concept must be abandoned) ;

• characterization of discontinuities (also taking into account the distribution effect of localized elements) ;

• near-field coupling, electromagnetic interference and EMC (electromagnetic compatibility) ;

• radiation assessment, antennas, SER (radar equivalent surface) ;

• propagation (intra-wall or non-intra-wall radio networks).

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