Microwave tomography for the inspection of wood materials: Imaging system and experimental results

A new microwave tomography prototype for the inspection of wood materials is described and experimentally tested in this paper. The system prototype is composed by a set of microwave antennas, used for both illuminating the sample and collecting field measurements, a custom RF hardware, and a hybrid tomographic inversion scheme. The antennas are placed in contact with the outer structure of the sample, e.g., a tree trunk, and are optimized to radiate inside the inspected region. A switching matrix connects the set of antennas with a custom vector network analyzer, which is used for acquiring multistatic and multiview field measurements at multiple frequencies. The measured data are processed by means of a combined qualitative/quantitative inversion method. In particular, in a first step, the field scattered by the inner inhomogeneities or inclusions is estimated, producing also a qualitative image of the sample. Then, these data are utilized by a deterministic inversion algorithm in order to obtain a quantitative reconstruction of the dielectric properties of the structure under test.