In the previous Chapters we have introduced the complete spectrum of open waveguides, considering how to compute both guided modes and continuum. Once the spectrum is derived we are in the position to solve a wide variety of problems: in abstract terms we can say that, once the complete spectrum is known, we have knowledge of the Green's function, and thus a complete characterization of that particular structure.

Form a practical viewpoint, the knowledge of the complete spectrum allows us to derive simple equivalent circuits of waveguide discontinuities in much the same manner as in closed waveguides. In this way we can, for example, characterise an open microstrip discontinuity; or we can provide an equivalent circuit for a microstrip step discontinuity, etc. This possibility of producing rigorous and effective equivalent circuits of open waveguide discontinuities is, of course, of paramount importance for CAD of microwave circuits.

Moreover, the same methodology can also be applied to antenna problems. As an example a microstrip patch antenna can be considered as a step discontinuity interacting with an open end microstrip. If both discontinuities are characterised by their multi-mode equivalent circuits we can rigorously describe the radiation process taking place in this structure.

As a matter of fact, it is quite impossible to try to describe all the most significant applications of the complete spectra of open waveguides in just one Chapter. As a consequence we have chosen to describe just a few of these applications with the purpose of providing at least...