|
The characterization of dielectric materials is of great importance for many applications, being for instance quality control during product fabrication or status control of outside constructions over time. In many outside situations the objects of interest have limited accessibility, and the investigation has to be done without destruction of any part of the object and without any health risks for an operator. Hence remote sensing from stand-off position is often desirable, and the use of microwaves, millimeter-waves or THz waves offers some penetration capability into matter, depending on its chemical and physical decomposition and of course frequency and polarization. Many objects of interest consist of a dielectric coating or enclosure, which can electromagnetically be treated as a dielectric layered structure or a dielectric slab surrounded by air. In former investigations a linearly polarized Ka-band radiometer was connected via an electronically steerable polarization rotator to the antenna of a near-field scanner. Using this setup four linear polarization states were realized in time multiplex and the corresponding brightness temperatures of close objects were measured for each scene point. Then an estimate of the real part of the permittivity was extracted from those values using radiometry specific polarimetric modeling for incident radiation on tilted dielectric surfaces and corresponding retrieval computation. For new experiments the polarizer was replaced by simple waveguide sections providing the four required linear polarization states. Furthermore a far-field scanner was used in order to allow the imaging of large scenes of various interesting content. In this paper the theoretical background of the approach is briefly outlined and the new measurement results are discussed.
|
