Salvatore Celozzi

Shielding Effectiveness of Periodic Screens Against Finite High-Impedance Near-Field Sources

The shielding effectiveness of artificial periodic screens is investigated with reference to high-frequency high-impedance near fields produced by arbitrarily oriented electric dipoles. The screens are formed by metallic objects of arbitrary shape displaced in a 2-D lattice. The problem is first studied through a full-wave approach, using the array scanning method in conjunction with a periodic method of moments in the spatial domain. Next, the shielding problem is solved analytically in some characteristic electromagnetic compatibility configurations through the use of approximate low-frequency homogeneous models together with a classical analysis in the spectral domain. Finally, the solutions are compared with those deriving from the use of the so-called transmission-line approximation. The provided results show the suitability of the analytical approaches in dealing with finite sources different from standard plane-wave excitations and give a useful tool for the design of periodic shields.

Shielding Properties of a Wire-Medium Screen

The shielding performance of a planar metamaterial wire-medium (WM) screen under plane-wave illumination is studied. The screen consists of a finite number of periodic layers of thin conducting cylinders embedded in a dielectric matrix; in the low-frequency limit and for waves with the electric field polarized along the wire direction, such a screen can be modeled as a homogeneous slab with a plasma-like dispersive permittivity. In particular, well below the plasma frequency, the effective relative permittivity is negative and very large in absolute value, giving rise to high values of the shielding effectiveness. A comparison with a conventional planar metal screen is reported, showing how the proposed structure can be designed to be advantageous in terms of low density and weight saving. In order to be effective against arbitrarily polarized waves, the original structure is modified, adding a second WM screen with wires orthogonally oriented with respect to the first one. Numerical results are provided that illustrate the validity of the homogenized model in predicting field levels both near to and far from the screen and show the shielding performance attainable with the proposed metamaterial screen in both single- and double-layer configurations at normal incidence.

A Tunable Ferroelectric Antenna for Fixed-Frequency Scanning Applications

In this letter, a novel fixed-frequency scanning antenna is proposed. The antenna consists of a grounded substrate-superstrate configuration covered with a two-dimensional periodic array of slots in a conducting plate, fed by a simple source (e.g., a narrow slot on the ground plane); the superstrate is a thin ferroelectric slab that, under a bias voltage, changes its permittivity allowing for a fixed-frequency electronically controlled scanning process. The operating principles of such an antenna are illustrated and numerical results are provided to show the attainable performance with actual ferroelectric materials

Alternative Definitions for the Time-Domain Shielding Effectiveness of Enclosures

The assessment of the shielding effectiveness of enclosures under transient conditions calls for an accurate analysis of fundamental aspects. Stemming from a correct understanding of the coupling mechanism and from a victim point of view, new shielding parameters are proposed as figures of merit to designate the actual effectiveness of an electromagnetic enclosure. Their suitability is tested in different configurations excited by typical transient electromagnetic threats.

New figures of merit for the characterization of the performance of shielding enclosures

Evaluation and measurement of shielding performance of enclosures and protection structures in general are based on the comparison between the local field values with and without the shield. Apart from some practical problems arising in specific, but relevant, situations like enclosures of small dimensions, such an approach appears rather incomplete and sometime deceitful. Despite this, shielding effectiveness (SE) is a well-established parameter, and it seems that for decades, the attention has been focussed more on how to evaluate and measure the so-called SE, rather on what the goal of any shielding structure is and why each evaluation and measurement should be performed. The main drawback is that SE is a local quantity and its knowledge does not help in the prediction of the real mitigation of undesired effects achieved by means of any shielding structure; undesired effects are mainly due to an integral of an electric or magnetic field, and/or to spatial variations of electric and magnetic field components. In the past, proposals were advanced toward an improved definition and measurement of electromagnetic SE; the proposed new figures of merit were based on the energy (power) penetrating the enclosure, perceived as the key factor for the shielding problem. However, it seems more adequate and correct the direct reference to the mechanism of birth of induced effects, as stemming from Maxwell equations. For these reasons, two new figures of merit are proposed for the comparison of enclosures and shields performance.

Two-port equivalent of PCB discontinuities in the wavelet domain

A novel wavelet-based technique is presented for the extraction of two-port equivalents of common printed circuit board (PCB) discontinuities. The wavelet-transformed scattering parameters of the discontinuity can be included in a wavelet equivalent of TEM wave propagation paths along the PCB in order to obtain an overall equivalent of the whole structure under analysis. The wavelet representation minimizes the CPU time and computer storage requirements while maintaining excellent accuracy, thus, it proves to be a very useful modeling tool, especially in the design stage.

Time-Domain Analysis of Building Shielding Against Lightning Electromagnetic Fields

The shielding performance of different buildings in presence of indirect lightning return strokes are investigated directly in the time domain. To this end, a sensitivity analysis is conducted by varying the channel geometrical characteristics and considering three different buildings. The performance is evaluated by adopting the time-domain shielding parameters recently proposed, which appear to be suitable for shielding studies in presence of lightning electromagnetic pulses. The results confirm the usefulness of introducing different parameters to fully characterize the shielding behavior of buildings exposed to lightning electromagnetic fields.

Dipole Excitation of Periodic Metallic Structures

The excitation of a periodic structure composed of two- or three-dimensional metallic scatterers by a finite dipole source is studied through the application of the Array Scanning Method. The difficulties in the numerical implementation are discussed in details, providing general guidelines and suggesting a useful preprocessing procedure to determine the number of quadrature points needed in the involved spectral integrations to obtain a given accuracy in the field calculation. The problem of transmission and reflection from two typical periodic screens (arrays of apertures in a metallic plate and arrays of metallic spheres) is first addressed in order to show the differences between a finite-source and the conventional plane-wave excitation. Finally, results for the near field of a recently proposed two-dimensional periodic leaky-wave antenna (an array of metallic patches over a ground plane) are also presented examining for the first time its main characteristics.

Analysis of the shielding performance of ferromagnetic screens

We present a method for the analysis of ferromagnetic shields under low-frequency excitation conditions and demonstrate its validity. The source types are arbitrarily oriented current loops or multiconductor lines. We account for hysteresis by means of the Jiles-Atherton model and solve the relevant equations in the time domain by adopting a finite-element time-domain procedure.

Extraction of equivalent lumped circuits of discontinuities using the finite-difference time-domain method

A general purpose technique for the extraction of equivalent circuits from FDTD results is presented. The technique is based on the approximation of the scattering parameters of the discontinuity in terms of rational functions, and the subsequent synthesis of high-order equivalent lumped circuits.