I. Introduction
Excitation problem is of increasing interest at microwave frequencies. Microwave antennas and printed circuit board (PCB) devices are often fed by waveguides (such as rectangular, circular, coaxial, etc.) or microstrip lines [1]. In general, such devices are composite structures that include both conductive and dielectric elements. Thus, a suitable modelling of the waveguide excitation of such structures is of great interest. Such modelling can be performed by introducing a waveguide port intended to launch an incident wave, to absorb a reflected or received wave (in active or passive modes), and to truncate the waveguide mesh [2]–[3].