NEAR-FIELD MILLIMETER WAVE AND MICROWAVE TOMOGRAPHY IMAGING

摘要

A near-field millimeter-wave tomography system has been designed and built for imaging of hidden objects located in subsurface areas of different materials and fabrics. In our investigations we used conceptions of near-field microscopy and subsurface tomography. Tomographic algorithm [1, 2] is employed for imaging of vertical and horizontal slices of subsurface space containing investigated objects. The algorithm is based on the integral representation of the scattered field at the probing line above a lossy half-space containing a cylindrical object of arbitrary cross-section and electrical properties in the case of plane wave incidence. The object function represents the normalized polarization currents under surface in perpendicular plane containing the probing line. The object function is related to 1D Fourier transform of the measured scattered field by the tomography integral equation. The solution of the tomography integral equation is found for each operating frequency. The reconstructed object function is sum of the obtained solutions. In the inverse problem absorption losses of environment were taken into account. This procedure improved sufficiently the reconstructed images. For image reconstruction of objects a part of the spatial spectrum of the scattered field due to nonuniform or evanescent waves was employed. For improvement of the subsurface object image it is also used a compensation procedure of the field caused by the final sizes of researched area. The developed tomography setup contains vector network analyzer of N5230A type for the frequency range from 10 MHz to 40 GHz; 2D mechanical scanner supporting the scanning with a step in the range from several millimeters to 0.25 mm; desktop or portable computer equipped with data acquisition unit; scanner control unit; antenna system (probe). Photo of the setup is presented in Figure 1.