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International audience; This paper presents detection and imaging capabilities of a simple time-reversal focusing algorithm, applied to experimental data that are obtained under various radar scene configurations (single- or multiple-target in free-space or through-the-wall). Data are collected with the ultra-wideband surface penetrating radar SIMIS (Synthetic-Impulse Microwave Imaging System, designed in LEAT) which operates almost from dc up to 18 GHz. The algorithm propagates time-reversed received signals in a supposedly known medium (either free-space or through a wall with known parameters) using a simple dipole model for the antennas. At each time step, an image of the scene is obtained, corresponding to the instantaneous electric-field energy in each pixel. A focusing criterion, looking for a minimum of entropy combined with a maximum of energy in the image, automatically selects the optimum frame, shown here. Two free-space (dielectric target, dielectric and metallic target) and one through-the-wall (dielectric target) configurations are investigated, showing the efficiency of the method.