research

When electromagnetic (or acoustic, or elastic) waves affect an object, the so-called diffuse or "scattered" field originates from the interaction between the incident field and the object itself. The characteristics of this field are obviously dependent on the accident field and the shooting object, that is from the geometrical parameters of the scatter, like its position and shape, and its material composition.

With the name of frequency selective surfaces (“Frequency-Selective Surfaces”, FSS) some particular planar resonant structures are commonly indicated, obtained by combining a certain number of dielectric layers with one-dimensional or two-dimensional alignments (“arrays”) of metal elements (or openings on metal screens). This type of surfaces is used in many electromagnetic applications in a frequency range from UHF to infrared. At microwave frequencies, periodic surfaces are widely used as phased arrays, artificial dielectrics, diffraction gratings, frequency selective reflectors for antennas, dichroic surfaces and spatial filters.

The space age has just reached half a century: it was, in fact, October 4, 1957 when the Russian Sputnik 1 rocket detached from the launch pad of the Baikonour cosmodrome with four wire antennas on board for transmission only. Fifty years after the historic event, the antennas mounted on satellites have undergone an extraordinary and continuous evolution. Modern satellites for telecommunications, to fulfil their primary mission of radio links from space, can count on a "farm" of highly specialized antennas and technologically advanced (the term farm, from the technical English "antenna farm", highlights the variety of antennas mounted on the same satellite).

Normally, one of the first steps in designing a filter is to create an equivalent circuit model [1]. The more accurate the model, the more accurate the analyses will be useful for the project. The models currently available in the literature [2] are mainly valid for narrowband applications, but they are unsuitable for broadband applications. The aim is therefore to investigate equivalent circuits for the analysis of structures operating at broadbandand improving both the bandwidth and the out-of-band response, for example by predicting higher order modes. It would be useful, in fact, to have a more accurate analysis of losses, group speed, and high-power behavior. Finally, these analyses would improve the calculation of the desired structure size.

The term "metamaterials" refers to that vast range of electromagnetic materials artificially produced and, consequently, not existing in nature, synthesized inserting conductive structures of particular shape and size in a host dielectric medium. The shape, size, structure, orientation and arrangement of these inclusions are designed to modify the electromagnetic characteristics of the host dielectric and to obtain particular properties for the applications of interest that cannot be achieved with conventional means. The characterization of these materials has become a fundamental need in recent years, mainly due to the fact that production technologies have made it possible to achieve unexpected results only a short time ago.

Electromagnetic band-gap materials are periodic structures of considerable interest for their applications at microwave, visible and infrared frequencies. They are generally referred to by the acronym EBG (Electromagnetic Band-Gap) or PBG (Photonic Band-Gap, most used in the optical field), and they’re also called electromagnetic or photon crystals. In EBG structures, inclusions of a material having a specific dielectric constant are periodically immersed in a host medium: the inclusions have dimensions comparable to the working wavelength, they can be dielectric, metallic, but also magneto-dielectric, ferromagnetic, ferroelectric or active.