Emulating an Anechoic Environment in a Wave-Diffusive Medium through an Extended Time-Reversal Approach

International audienceA generalized time-reversal (TR) technique for the generation of coherent wavefronts within complex media is presented in this paper. Although completely general, this method is primarily considered for testing purposes herein, where an equipment under test is submitted to a series of impinging wavefronts with varying features. Electromagnetic compatibility, antenna testing as well as telecommunications facilities where complex-wavefront schemes (e.g., multi-path configurations) are required, could benefit from the proposed approach. The main advantages and limitations of current standard TR approaches are reviewed in this respect, exposing their inadequacy for this particular context. The proposed alternative technique, named Time-Reversal Electromagnetic Chamber (TREC) is introduced and studied by means of a formal theoretical analysis, showing how a reverberation chamber (RC) supporting a diffused-field condition can be operated as a generator of deterministic pulsed wavefronts. The TREC is demonstrated to be capable of generating arbitrary wavefronts with a remarkable accuracy, allowing to revisit the RC as a deterministic facility: the main advantages of RCs and anechoic ones are merged, leading to a new facility capable of potentially generating in real-time pulsed wavefronts while using low input energies, without requiring neither mechanical displacements nor any special features of the sources

Increasing peak-field generation efficiency of reverberation chamber

International audienceThe use of time-reversal techniques has been shown to allow focusing energy in a spot about half a wavelength wide. The fact of being able to concentrate energy into a reduced volume of space implies higher power densities and, ultimately, higher field levels. The use of this feature for improving the ability of a reverberation chamber in converting energy into high-intensity fields is investigated here. Experimental results are compared to those predicted by a simple asymptotic model, revealing the role played by losses and frequency bandwidth and how the performance of time-reversal techniques depends on these parameters

The Role of Losses in the Definition of the Overmoded Condition for Reverberation Chambers and Their Statistics

International audienceIt is commonly acknowledged that in perfectly stirred reverberation chambers, the energy density of the electric field follows a χ26 law, as long as the overmoded condition applies. This concept, never defined properly, is often confused with the idea of a threshold on the modal density, regardless of the quality factor of the cavity. This interpretation is here proven to be inaccurate, as losses play a fundamental role in the nature of the field statistics and not, as often assumed, just in its scaling. In particular, it is shown how the overmoded condition should be stated mathematically, highlighting how the cavity quality factor and the number of eigenmodes excited cannot be regarded as quantities intervening independently on the field statistics, but should rather be considered jointly. These results are derived by means of a modal analysis, with a limited number of assumptions. A quantitative relationship is established between average modal overlapping and the rate of convergence of the electric energy density towards a χ26 law. Rather than setting an arbitrary threshold on modal overlapping as a necessary condition for an overmoded behavior, the statistical uncertainty due to the limited number of available field samples is shown to affect the very definition of the overmoded condition

La misurazione delle attività di ricerca e sviluppo nei paesi OCSE

L’Europa per cercare di raggiungere i lontani Stati Uniti ha messo in atto una strategia, che però ancora oggi essendo a metà del suo percorso, non ha fatto vedere nessuno dei risultati sperati. Per quanto riguarda l’Italia si parla insistentemente di declino in quanto si collezionano record di risultati negativi; il problema del nostro paese è più quantitativo che qualitativo, infatti si riscontra un’insufficiente quantità di addetti, di risorse finanziarie e di strutture. L’Italia dunque ha scelto la strada della crescita con poca ricerca, adattandosi alle nuove tecnologie prodotte e competendo sulla flessibilità, sul contenimento dei costi e sulla creatività. Complessivamente però non si può parlare di un vero e proprio declino in quanto utilizzando le potenzialità detenute da ogni paese, si potrebbe puntare ad una strategia forte e condivisa che trasformi le tante singole debolezze in una forza unitaria

Source Correlation in Randomly Excited Complex Media

International audienceWhen dealing with modal representations of the Green's function of a complex medium, the modal coefficients are often assimilated to random variables, where statistical independence is justified on heuristic arguments supported by the complexity of the propagation in multiple-scattering scenarios. This letter addresses this assumption when the randomness originates from an uncertain positioning of the sources, proving under what conditions the modal coefficients can be regarded as uncorrelated, showing that this special condition should not be taken for granted, even in complex media

Could vision 2030 reforms destabilize Saudi Arabia?

Last month Saudi Arabia's 31-year-old deputy Crown Prince Mohammad bin Salman (MBS) proposed a bold economic plan to end the kingdom's "addiction" to oil by 2030. MBS' "Vision 2030" would transform the Saudi economy by reforming the bureaucracy, bolstering the private sector, and building the world's largest public investment fund. Vision 2030 is also a promise to the kingdom's youth for greater opportunity and a more efficient and transparent government. With plunging oil prices and a budget deficit expected to reach 13.5 percent of GDP, Saudi Arabia clearly needs bold reforms