Microwave guiding structures. Propagation and geometry

Microwave devices such as filters, amplifiers, antennas, couplers, etc. are generally connected or powered via lines, cables or waveguides. These structures have the property of guiding waves that carry or transfer energy to or between devices. They must do so optimally, i.e. with minimum losses, minimum signal dispersion and adaptation to the load and generator, over the frequency bandwidth useful to the application.

Uniform guiding structures, which may have different cross-sections but are invariant in the longitudinal direction of propagation, operate according to a common basic mechanism: transmitted waves are guided by bouncing off metal walls and/or interfaces between different media. The interference between these waves forms equiphase field planes in the guide, moving in the longitudinal direction of the guide, which is assumed to have an invariant cross-section. The distance between planes of the same phase (modulo 2π) defines the guided wavelength, which is often different from the wavelength of the infinite medium. The longitudinal displacement velocity of these planes defines the phase velocity. The peculiarity of the waveguide problem is that there are an infinite number of solutions that can propagate independently. These, called modes, each have their own phase velocity and wavelength, and the dispersion diagram illustrates the dependence of these parameters on signal frequency. Finally, a mode can only propagate if the signal frequency is higher than its cut-off frequency, below which the amplitude of its fields attenuates exponentially. No active power is propagated in the guide by this mode. The guide's problem is therefore mainly to find the dispersion diagram for the useful modes and extract the associated cut-off frequencies and propagation coefficients. For lossy guides, the latter are complex, and their real part gives the mode's attenuation coefficient. Perturbation methods can be used to calculate these attenuations, provided losses are low, which is compatible with the practical objectives of the guide.

A wide variety of guiding structures have been proposed, depending on the application and operating frequency band. The latter two parameters influence the geometry, size, materials used and, consequently, the technology employed. For example, a high-power application, such as radar, will require the use of hollow metal guides, generally with a rectangular or circular cross-section, or coaxial cables for lower-power applications. On the other hand, low-power applications allow the use of planar technologies such as microstrip lines printed on dielectric substrates, or new technological approaches like Surface Integrate Waveguide (SIW) or Non Radiating Dielectric (NRD). Last but not least, some special lines offer interesting propagation...