L. Vegni

Scattering properties of antennas residing in cavities filled by inhomogeneous materials via a variational formulation

Inhomogeneous media exhibit many properties that are very useful either in microwave circuits or in radiation components. In this paper a scattering analysis of cavity-backed microstrip patch antennas with inhomogeneous slabs is proposed. An equivalent variational formulation of the electromagnetic problem for this kind of antenna is derived, such that a very efficient Finite Element Method-Boundary Integral code can be straightforwardly applied. Numerical results are compared with those known in the literature showing an excellent agreement with them. The main scattering properties of the inhomogeneous substrates are also pointed out and compared with those of homogeneous substrates.

Method of lines numerical analysis of conformal antennas

This paper presents a generalization of the method of lines (MoL) numerical algorithm for the analysis of integrated structures with a finite local curvature. Starting from the derivation of the generalized transmission line equations in a generic orthogonal reference system and in a bianisotropic material, it is shown the possibility of solving the electromagnetic problem associated to an integrated structure with arbitrary curvature and/or loaded by complex dielectric materials via a MoL approach. The introduction of new algorithms that extend and complete the original method is required in the most general case. A detailed description of this new extended version of the MoL is presented through the paper and its full coincidence with the original algorithm in the case of structures with simple loading materials is also shown. Finally, some numerical results obtained analyzing microwave conformal antennas and resonators are presented and compared with analyzes performed employing other numerical methods and available in the literature.

Polygonal Patch Antennas with Reactive Impedance Surfaces

Here we present an innovative design of microstrip antennas for wide-band mobile communications. In order to obtain reduced back-radiation, while keeping the overall antenna dimensions suitably compact for practical terminals, the proposed radiators are backed by a reactive ground plane, constituted of a suitably designed pattern of metallic patches. Moreover, the operative bandwidth is increased with the use of a proximity coupling feed and the choice of a polygonal patch shape. Some numerical results are reported, obtained through a rigorous Method of Moment numerical tool, to show the effective improvement that this design may offer in the antenna performance for UMTS mobile communication systems.

Metamaterial bilayers for enhancement of wave transmission through a small hole in a flat perfectly conducting screen

In this talk, we theoretically demonstrate the role of two properly selected pairs of metamaterial layers, one located in the front and the other in the back surface of a small sub-wavelength hole in a flat perfectly electric conducting (PEC) screen. In recent years, there have been experiments in the optical regime conducted by others showing how an appropriately designed periodic corrugation on a metallic opaque screen leads to significant increase of the power transmission through a subwavelength hole. Oliner et al. (2003) explained and justified this power enhancement as the result of the excitation of the leaky waves supported by the corrugated screen. In the present work, we suggest a different configuration using metamaterial bilayers covering the entrance and the exit face of the hole in a flat PEC screen. In this setup, one pair of metamaterial layers effectively collects the power of the incident wave and concentrates it at the entrance of the hole, the other pair of layers at the exit face re-shapes the radiation patterns in the receiver side. Conditions for the optimal transmission and certain physical insights are presented, and some numerical results confirming the theory are also shown.

Design of polygonal patch antennas with a broad-band behavior via a proper perturbation of conventional rectangular radiators

The design of polygonal patch antennas with a broad-band behavior (e.g. for UMTS applications) is presented in this contribution. The main idea of this work is based on the possibility of changing the electrical antenna features of a standard rectangular patch by means of proper perturbations of its sides (slope, length, etc.). The patch shape resulting from this perturbation is a convex polygonal radiator, which is analyzed through a spectral domain full-wave formulation and a method of moment (MoM) numerical code employing entire-domain basis functions. Some examples of practical UMTS antenna designs are also presented and compared with similar radiators already proposed in the literature in order to show how a proper perturbation of the patch shape can significantly improve the antenna performance.

Broad-band U-slot patch antennas loaded by chiral material

The analysis of wide band microstrip/slot antennas residing in cavities with arbitrary patch shapes and in presence of both isotropic and chiral materials is proposed in this paper. The theoretical approach is based on the variational formulation and the numerical solution is carried out by applying a combined Finite Element-Boundary Integral (FE-BI) Method. Such an approach already known in the literature only for isotropic and anisotropic media is extended also to the case of biisotropic dielectrics, including chiral materials. A numerical code based on the new theory here presented has been developed and it is employed in order to investigate the main features of chiral materials as substrates for cavity backed microstrip antennas. This numerical code allows to study also antennas loaded by isotropic substrates as particular cases when imposing the chiral admittance to vanish. To test this capability the design of an U-slot microstrip antenna printed on a conventional isotropic dielectric for wide band applications is proposed. Finally, it is shown how the employment of chiral substrates instead of isotropic ones reduces the antenna size for a fixed working frequency and improves the impedance bandwidth, without decreasing too much the other radiating properties.

Broadside radiation enhancement using a spiral resonator MNZ metamaterial substrate

The broadside power radiation enhancement has been tested for the case of a magnetic dipole in presence of an MNZ metamaterial substrate. In order to assess the theoretical estimations, a slot antenna on a ground plane has been fabricated and an AMC composed of 1 layer square spiral resonators has been used as an MNZ metamaterial slab. The radiation patterns have been measured in the anechoic chamber, achieving a gain in directivity of more than 1 dB. However, it should be taken into account that the thickness of the fabricated metamaterial slab (6 mm) is much smaller than the theoretical optimum thickness (hundreds of mm). The directivity of the antenna might be improved when increasing the number of layers of the metamaterial slab; in fact, in such a case, the leaky waves responsible of directive radiation are expected to be more significantly excited giving rise to the sought broadside power enhancement.

Microstrip disk antennas with inhomogeneous artificial dielectrics

A novel kind of microstrip patch antenna is proposed in this paper. The antenna layout consists of a probe-fed circular patch integrated on a grounded slab of inhomogeneous artificial dielectric. A rigorous full wave analysis of this radiation component is here developed starting from the determination of new expressions for the spectral dyadic Greens function that take into account the inhomogeneity presence. The spectral Greens dyad is also obtained in the Hankel domain and a complete and accurate Method of Moments (MoM) is presented. Finally, several numerical results, obtained by means of a self produced code based on the theory here developed are presented in order to show the main capabilities of the novel antenna. Particularly, the comparison between the radiation pattern and the input impedance of such an antenna and those obtained for a similar one with a homogeneous substrate shows an improvement of both the directivity and bandwidth.

Design of Polygonal Patch Antennas for Portable Devices

In this paper, extending the design technique presented by the authors in a previous work, we propose the study of a new family of polygonal patch antennas for portable devices of communication systems. Such antennas are suitable to be mounted in modern terminals, enabling wideband/multi-frequency operation and new multimedia features. The desired electromagnetic behaviour of the proposed radiators is obtained by adding either shorting posts, properly located between the polygonal patch and the ground plane, or circular slots, drilled at the appropriate position on the patch surface. Circular slots are also useful to easily accommodate a photo-camera in the terminal, in order to enable multimedia services and video calls. Some practical layouts of polygonal patch antennas to be used in: a) modern PDAs and Smart Phones integrating cellular phone operation and wireless functionalities; b) UMTS terminals integrating also GSM functionalities, are, flnally, presented. The efiectiveness of the proposed designs is conflrmed through proper full-wave numerical simulations.

Design of inhomogeneous slabs for filtering applications via closed form solutions of the reflection coefficient

—A novel efficient design technique for microwave inhomogeneous media components (filters and matching lines) is developed in this paper. The inhomogeneity here considered can be modeled via a continuous spatial variation of the permittivity along the power flow direction and can be taken into account in the transmission line equivalent circuit representation in terms of a longitudinal line impedance and a transverse line admittance varying along the same direction. In order to design a filter or a matching line layout with such a material, new analytical expressions of the reflection and transmission coefficients for nonuniform transmission lines are found. Some examples are also presented in order to illustrate the validity of this technique for an efficient realization of proper layouts involving inhomogeneous layers which satisfy prescribed filtering characteristics in the frequency domain.