Gabriele Gradoni

ABSORBING CROSS SECTION IN REVERBERATION CHAMBER: EXPERIMENTAL AND NUMERICAL RESULTS

Reverberation chamber (RC) test facility allows to determine the absorbing cross section (ACS) of lossy materials under a random fleld excitation. Measurements are based on the quality factor variation produced by the sample under test presence with respect to the empty chamber condition. Simulations are based on the representation of the RC electromagnetic fleld by means of a random plane wave superposition. A flnite-difierence time-domain code is used to compute the material absorbed power and to recover a numerical ACS. The method sensibility is stressed by application to small size samples. Comparison between numerical and experimental data reveals a satisfactory agreement. Results for difierent materials are presented in the paper: soft foam absorbers, carbon foam sheets, and carbon/carbon sheets.

Electromagnetic Characterization of Composite Materials and Microwave Absorbing Modeling

This book chapter is based on the experimental activities conducted mainly at Sapienza University of Rome: Astronautic, Electric and Energetic Engineering Department in collaboration with University of Maryland, Institute for Research in Electronics and Applied Physics (IREAP). A branch of scientific research about composite materials is focused on electromagnetic characterization and subsequent application of electric conductive polymers. The use of such structures is relevant in aerospace/aeronautics, for electromagnetic (EM) protection from natural phenomena (lightning), and intentional interference with radar absorbing materials (RAM), in nuclear physics for shields adopted in particle accelerators, and for nuclear EM pulses (NEMP) protection, in electromagnetic compatibility (EMC) for equipment-level shielding, high-intensity radiated fields (HIRF) protection, anechoic chambers (for the realizations of wedges and pyramidal arrays), and human exposure mitigation. In this chapter, composite reinforced by carbon nanostructured materials are considered, mainly because of their interesting electromagnetic characteristics, such as high electrical conductivity and excellent microwave absorption. Composite materials as well their absorption capability are analyzed and numerical design of wide frequency band microwave absorbing structures is presented and discussed in details. It is crucial to highlight the need of interdisciplinary research fields to go through nanomaterials: besides nanotechnology, also electromagnetic wave propagation theory, composite materials manufacturing techniques, evolutionary computation algorithms, and use those to design the “quasi perfect absorber” are strongly required. In particular, we propose an inhomogeneous multilayer absorber made of micrometric graphite (at different wt%), and nanometric carbon particles (SWCNTs, MWCNTs, CNFs, at different wt%). At the end, an improvement of the traditional absorbers has been achieved upon optimization through an in-house winning particle optimization (WPO) algorithm, this last appositely conceived for absorbers optimization. Main goal of the presented work is to optimize the absorbers

DSP cement composites for electromagnetic shielding: practice and experimental analysis

In this work, the shielding effectiveness (SE) of a densified-small-particles (DSP) cement composite, with different loading particles embedded in, has been investigated through the nested reverberation chamber facility. Particular effort has been spent in characterizing the SE behavior with the increasing drying and in improving the experimental practice to solve out previously faced measurement troubles. Findings are of interest in architectural shielding, microwave spectroscopy and investigation of physical-chemical properties of DSP materials.

Minimum-value distribution of random electromagnetic fields for modeling deep fading in wireless communications

In this work, we presented a theoretical investigation of the minimum-value distribution inside complex electromagnetic environments. In particular, a statistical model for characterizing the minimum value of the complex-value field or power inside a dynamic mode-tuned or mode-stirred reverberation chamber is presented and discussed. Such an EM environment serves as an emulator of multipath radiowave propagation for indoor/outdoor wireless communication channels. It is found that, for both overmoded and undermoded regimes, the generalized extreme value distribution leads to the reverse Fréchet and Weibull types for complex-value (Cartesian and total) fields and for the total energy (or intensity). These distributions are stable and follow from the convergent behavior of the lower tail for their corresponding parent distribution of the Cartesian field magnitude, namely a χ2. On the other hand, received power exhibits a Pareto-type distribution because of the unbounded left tail of the negative exponential parent distribution.

Ballistic characterization of nanocomposite materials by means of “Coil Gun” electromagnetic accelerator

In this paper the authors present their activity in the field of electromagnetic machine applications for aerospace solutions. A three stage electromagnetic accelerator is under construction to perform ballistic characterization of carbon-based nanocomposite materials for anti-debris application. Preliminary experiments as well as numerical simulation have been performed with promising results in terms of bullets energy. Further implementation are needed in order to come closer the velocity of typical space debris (8km/s).

Theoretical model of transient random fields based on the fluctuation-dissipation theorem

In this paper, we calculate the transient field response of an electromagnetic mode inside a dynamic (mode-stirred) complex cavity. This is carried out on a physical basis through application of the theory of linear systems. The transition between equilibrium (stationary) states of the cavity is viewed as a non-equilibrium occurrence (event) for the partial/resultant field and is modeled by a second-order ordinary differential equation with time-dependent modal coefficients. On application of the fluctuation-dissipation theorem from statistical mechanics, it is possible to write this non-equilibrium evolution as a convolution integral of the linear response function of the cavity mode. A solution is found by using the Greens function technique. It is found that, besides the set of harmonics oscillating at natural and excitation frequencies ωn and ω, respectively, the transient regime exhibits a set of transient harmonics oscillating at frequencies (ωn−ω) and (ωn+ω). This intermediate set decays in accordance with modal damping and shows dependence on the initial time, exhibiting nonstationarity. Analytical results are of interest to mode-stirred reverberation chambers, random fields, as well as in other areas of physics and engineering involving dynamic cavities or random media.

FDTD analysis of the field penetration through lossy materials in a reverberation chamber

This paper presents an electromagnetic analysis of lossy materials with a reverberation chamber (RC) and a case inside it. The geometrical structure is modeled with a multistructural FDTD technique. The domain of the RC is simulated with the classical Hill’s plane wave representation, the material domain is followed by an array of monodimensional FDTD and the case domain is made with the perfect electric conductor (PEC). Physically, the material is placed above an electrically large aperture of the case, that couples considered environments. Thanks to the high conductivity of the material, it is possible to separate numerical domains: only the material lattice is subordinate to the cell contraction. Numerical results are validated with the Fresnel theory and compared with experimental measurements.

Field-to-enclosure coupling in reverberation chamber: numerical and experimental analysis

The paper presents a study of the coupling between an external field and a metallic enclosure with a long aperture in the frequency range which includes several box resonances. A reverberation chamber is considered as a field generation structure. A customized FDTD code is used to calculate the current induced by the external field in a loop placed inside the enclosure. In order to simulate the reverberation chamber the field is represented by a proper superposition of random plane waves. Numerical results are experimentally validated. The proposed method is useful to investigate the performance of the enclosure during its early design stage, before the prototype realization