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

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

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

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

A Skeptic's View of Unstirred Components

International audienceIn the practice and research on reverberation chambers, the concept of unstirred field components have often been invoked as soon as empirical distributions fail to comply with goodness-of-fit tests applied to data collected at a frequency of operation far beyond the lowest usable frequency. The current explanation for this phenomenon is that under certain conditions, the field samples generated by means of a stirring procedure are not characterized by a zero average-value, but actually present a deterministic offset term that is linked to a line-of-sight contribution. In this paper, we prove that this practice is not sound, as it does not acknowledge the fact that even a very low but non-zero residual correlation between the samples is enough to put in jeopardy the validity of the limits imposed by most hypothesis-test statistics, and hence their ability in properly detecting any constant contribution. An alternative approach is here proposed, based on the analysis of the variability of the line-of-sight contribution estimate, capable of accounting for the residual correlation in a reverberation chamber. Experimental results are presented to support the validity of our approach, exposing the critical use of goodness-of-fit tests as currently applied

Probability Distributions of Local Modal-Density Fluctuations in an Electromagnetic Cavity

International audienceResults from random-matrix theory are applied to the modeling of random fluctuations in the modal density observed in an electrically large cavity. By starting from results describing the probability distribution of the modal spacing between adjacent frequencies of resonance, or nearest-neighbor spacing, we introduce a simple procedure allowing to pass from the modal spacing to the local modal density as measured over a finite bandwidth. This local definition of the modal density is more consistent with the physics of reverberation chambers, since it has been recently shown that the deviation from asymptotic statistics of field samples is dependent on the number of modes overlapping within a modal bandwidth. It is shown that as opposed to current interpretation, the number of overlapping modes is a strongly fluctuating quantity, and that estimating it by taking the frequency derivative of Weyl's formula can lead to non-negligible errors and misunderstandings. Regarding these fluctuations as second-order effects is therefore not sound from a physical point of view, since the existence of modal depleted scenarios can easily explain the appearance of local anomalies in the field statistics, particularly, but not exclusively, in the lower frequency range of operation of reverberation chambers

Closed-form expressions for the total power radiated by an electrically long multiconductor line

International audienceTwo analytical solutions based on transmission-line theory for the total power radiated by a multiconductor line above a ground-plane are proposed. The line is not assumed to be electrically short or close to the ground-plane, thus making the proposed model suitable for assessing the emission/immunity of actual transmission-lines employed in industrial contexts such as in the automotive domain, in railway lines and power-distribution lines. The model allows an imperfect ground plane to be considered through the complex-image approximation, together with propagation losses. Numerical and experimental results are provided as a validation, while an empirical rule to assess the accuracy of the results is proposed. The two expressions aim at allowing fast parametric analysis of radiation during the design phase of the electrical and geometrical configuration of an unshielded MTL

GOODNESS-OF-FIT TESTS IN RADIATED SUSCEPTIBILITY TESTS

International audienceGoodness-of-fit (GoF) tests are commonly used to assess the degree of perfection of a reverberation chamber to fulfill the overmoded condition. One may expect the outcome of GoF tests to provide some hints on the value of practical interest. The present paper shows that no correlation is found between the outcome of GoF tests and the range of values taken by the maximum power which is of practical interest in radiated susceptibility tests

Multiple-Source Time-Reversal Transmissions in Random Media

International audienceThe ability of time-reversed signals in reproducing a given time-dependence of the electromagnetic field within random media is investigated. A general setup consisting of multiple sources cooperating in providing the best transmission is considered, where the constructive interference of their individual contributions is meant to improve the quality of the field generation with respect to a single-source setup. The medium response is described by means of tools from random-process theory, for the case of stationary media complex enough to ensure a large number of multi-path contributions. It is shown that even a very weak spatial coherence in the medium is sufficient to significantly hinder the improvement expected from the use of multiple-source scenarios. Experimental results obtained in a reverberation chamber support the validity of the proposed theory. Direct applications of these results can be found in recent proposals about the potential benefits of time-reversed signals used in wireless communications, imaging techniques, as well as in pulsed-field generation devices based on energy compression through dispersive media

Analysis of Time-Reversal-Based Propagation for Spatial focusing and Multiplexing

4 pagesInternational audienceTime-Reversal (TR) technique is well-known for itsability of focusing waves. A common claim, based on this focusing ability, supports that TR technique may allow spatial multiplexing and secure transmissions. In the present paper, measurements campaigns were performed in a reverberation chamber and in a standard office to assess the viability of TR technique. It is shown that the results obtained do not support the common claim, especially in standard indoor environment where the focusing ability is strongly degraded