• beam-steering
• Characteristic modes
• inductive excitation.
• pattern reconfigurable antenna
• radiation pattern control

The goal of this thesis is to prove the breakthrough on pattern control capability offered by the Characteristic Modes Theory (CMT). In particular, useful design guidelines are proposed to achieve pattern reconfigurable antennas by exploiting all the potentialities of orthogonal current modes supported by different platforms. The organization of this thesis is shown in the navigation map of Figure 1.
Chapter 1 is mainly focused on the mathematical formulation of the Characteristic Modes (CMs) and the feeding structure to efficiently stimulate a desired CM over the investigated structure. Moreover, a preliminary examination of the CMT on a Perfect Electric Conductor (PEC) circular disk is carried out to illustrate the CMs analysis and introduce the preliminary concepts useful to obtain an efficient radiator.
In Chapter 2, a rectangular conductive plane with the dimension of a portable device has been exploited to achieve a null-steering antenna feature in the principal plane by a suitable excitation of two current modes over the investigated plate. To this aim, the asymmetric excitation paradigm has been introduced to alter the Modal Weighting Coefficient (MWC) of different orthogonal CMs over the object. As a proof of concept, a prototype of the antenna has been realized by resorting to a discrete phase shifter employing several pin diodes. Thanks to the shifter, it is possible to alter the phase difference between two capacitive exciters and thus scan the null of the pattern in the principal plane. In the same Chapter 2, a novel design strategy for realizing a three-dimensional (3-D) null-scanning radiator by applying the CMT was described in detail. An accurate feeding strategy to stimulate the desired current modes over the plate has been selected thanks to the CMT. By manipulating both the amplitude and phase difference among the exciters above the rectangular plate, an optimal current distribution can be generated on the PEC plate, which is able to guarantee a pattern null in any desired direction (,).
The following Chapter 3 is devoted to the Characteristic Modes Analysis (CMA) of a three-dimensional platform. In particular, the multiple CMs excitation technique is addressed by showing its capability of pattern control. Specifically, the CMA of a rectangular box is performed and afterward some orthogonal current modes have been properly combined to satisfy the imposed far field requirements. As proof of concept, both null-steering and beam-steering capability was achieved by accurately stimulating four CMs over the investigated rectangular box. Afterwards, an innovative Balanced Inductive Exciters (BIEs) composed of two symmetric half loops is introduced to strongly improve the radiation efficiency of mounted-on-platform radiators. This radiation efficiency improvement is obtained by increasing the modal excitation purity.
The following Chapter 4 is devoted to Multiple-Input-Multiple-Output (MIMO) antenna systems. More in detail, the performance enhancement due to the adoption of orthogonal Circularly Polarized (CP) radiators instead of orthogonal Linearly Polarized (LP) ones is addressed in this chapter for MIMO systems. The results of this analysis proved that CP radiators are capable of obtaining greater eigenvalues, as a function of the MIMO antenna orientation, than LP ones. For this reason, the achievable channel capacity of the CP radiators outperforms the LP ones when the MIMO antennas are not perfectly aligned. Moreover, CP MIMO allows obtaining better performance from a statistical point of view provided that the 3 dB axial ratio of radiators is guaranteed for angular sectors greater than 40 degrees. Finally, the channel matrix in LOS environment (HLOS) has been evaluated with a rigorous mathematical approach whereas, in the multipath propagation scenario, the MIMO performance have been assessed by both numerical simulations and measurements campaign.