Conception and Design of Tunable Frequency Selective Surfaces

Designing and analyzing the Frequency Selective Surfaces (FSS) is reported in this thesis. FSS structures are organized in the group of resonators structures. The arrangement allows to transmit and reflect and in some cases absorb the electromagnetic waves in a given frequency band. These structures are widely used in radoms, reducing radar cross-section (RCS), etc. In this work it is concentrated to design a FSSs by the incorporation PIN diodes to make three structures tunable. The purpose of using diodes is to overcome the difficulty of FSS structures in terms of reflection/transmission characteristic which is dependent on the incidence angle. Implementing PIN diodes is one way among other solutions, which have been attracted the designers` attention. Using active elements as the PIN diode implies the need of biasing. In this thesis it is decided to use a microstrip line based on biasing network, aiming to reduce its effect on the electromagnetic answer of the initial structure, and because of its reduced complexity. This work is focusing on the analyzing the behavior of the FSS structure for two different incidences: Firstly, TE incidence corresponding to y-directed electric field vector, and secondly a TM incidence (corresponding to x-direction). The initial design is created by two different circles and crossed microstrip lines, which are named as the main structure. It is built on FR-4 substrate with 1.58 mm thickness. Parameter analysis over the main dimensions of the structure has been performed aiming to find the best results in terms of bandwidth. Also, it is tried to analyze the behavior of the FSS in terms of linear to circular polarization conversion, which additionally to the equal amplitude also requires -90° or +90°phase difference between transmission coefficient in both TE and TM incidence cases. By considering this feature the initial structure turns into the ellipsis FSS structure to fulfill this goal.