Determination of dielectric material properties using passive MMW measurements for security applications

Abstract

Microwaves can be used to detect hidden objects behind optically opaque materials. Hence, the penetration capability through such materials is of fundamental importance. In order to characterize a material of interest in the microwave region, its permittivity should be known besides its physical structure. In many cases the permittivity is unknown, inaccurately known, or known for only specific frequencies. Also very often the range of values given in the literature can have a large variability for a specific situation. In this paper we describe a procedure to determine the permittivity from radiometric free space measurements of nearly arbitrary materials. A combined and advanced method of passive transmission and reflection measurements is outlined. The corresponding measurement assembly including the material sample and the necessary reference targets for almost environmentally independent measurements is illustrated. The advantage of this method is that large material samples like brick or wooden plates, and materials like textiles, which are hard to mount in a defined way in a waveguide, can be studied. An earlier presented method has been improved to obtain more accurate results. Some representative results for those MMW measurements are shown. The first attempts showed a satisfying performance.