On the radiation of antennas within a subwavelength-separated wire distribution, super-localization and time-reversal
Résumé
Electromagnetic time-reversal enabling potential super-resolution in telecommunication systems when operated within complex media –imaging is left aside from now– has been much studied. Yet it remains a controversial issue. Here, one illustrates what could be the behavior of a radiating broad-frequency device when operated in a properly-structured, finitely-extended medium. One attempts to see whether the field observed in that medium (deterministic, no randomness here as sometimes ad-hoc explanation) and outside it, in the far-field or at least far enough (so as evanescent fields in principle do not matter) can be such that super-resolution, or better said super-localization, can be achieved via time-reversal. In practice, an array made of three identical monopole wire antennas a quarter-of-wavelength (at the median operation frequency) long, and apart by a small fraction of this wavelength is inserted within a finite set of regularly distributed, slightly shorter thin metal wires also apart by a small fraction of the wavelength. The antennas in the array play the role of sources and receivers. Time-harmonic or transient fields in the UHF regime due to radiation of each antenna in the array drastically depend upon the characteristics of the wire distribution, both within this distribution (specific resonances emerge) and outside. So, time-reversing if properly performed may enable the localization of the singly emitting antenna. Here, one means that the exterior fields are collected by broad-band antennas along a quarter of circle (due to symmetry) and time-reversed (or conjugated and superimposed within a frequency-diverse regime) to get a time-reversal mirror. A comprehensive numerical analysis illustrates the behavior of the proposed set-up, carefully using the CST software and since both metal wires and antennas in the array are thin enough, the NEC software. It is supported by laboratory-controlled experiments [3] at downscaled frequencies to ensure mechanical reliability. Interest and limitations of the proposed device are then discussed. Linkage with investigations of complex micro-structured media attacked via asymptotic expansion of pertinent Green functions (here a dyad, which however is reached only via simulations) and the traditional Singularity Expansion Method, since an open cavity of a peculiar sort has been created, the behavior of which is linked to the resonances observed in it, is considered as well.