Numerical investigations on the connection between surface waves and trasmitting properties of tunable periodic structures

Numerical analysis of tunable periodic structure is developed in this thesis through the use of the simulative software CST Studio Suite on the single unit cell. To this end, four different configurations were designed and analyzed. The starting unit cell consists of three different resonators namely an inner, a central, and an outer ring. The resonators, with diverse resonant frequencies, are connected by microstrip lines. The other three configurations are differentiated by the presence, location and number of cuts that were inserted in the connecting lines. All configurations are implemented on FR-4 substrate. On the opposite face of it three microstrip lines are considered, having the role of biasing active elements inserted in the cuts. The connection between the resonators and biasing lines are realized through three metalized vias. The purpose of this thesis is to observe the correlation between the propagation modes in the dielectric and the transmittance of the unit cell for different angles of incidence (0, 15°, 30°, 45°). To this end, ten surface wave modes have been analyzed with four different boundary conditions for each of the four configurations. The results of the simulations are displayed as a Dispersion Diagram (DD), illustrating wave propagation behavior in terms of frequency versus phase difference (ranging from 5° to 175°). For each configuration the transmission coefficients have been determined and reported in terms of S-matrix entries for the above-mentioned incident angles. Then, only the modes inside the dielectric have been considered. Comparing the results obtained with the two different paradigms used, it is possible to identify a clear correlation between the propagation modes and transmission characteristics of the unit cell. More precisely, modes that have a constant frequency throughout the analyzed phase range, i.e., high-Q resonant modes, exhibit a lower shift in frequency of the transmission coefficient with respect to those with higher dispersion. The effect is more pronounced for higher incidence angle and the shift in frequency is strongly correlated to the behavior on the DD. This occurrence has been extensively analyzed and discussed in the thesis.