Microwave Resonator Techniques for Sorting Dielectric Objects
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MICROWAVE RESONATOR TECHNIQUES FOR SORTING DIELECTRIC OBJECTS
ANDRZEJ W. KRASZEWSKI and STUART 0. NELSON U. S. Department of Agriculture, ARS, Russell Research Center, Athens, GA 30613, U.S.A.
ABSTRACT Microwave resonators have been found to be sensitive devices for sorting dielectric objects of irregular shapes according to their volumes, for detecting voids inside objects of similar shape or for determining moisture content in objects of varying volumes. Fundamental principles of operation as well as experimental results are discussed in the paper.
INTRODUCTION Sorting dielectric objects of irregular shape according to their volumes, detecting voids inside objects of similar shape, or determining moisture content in objects of varying volumes are important in the production, processing or storage of such objects. There are many nondestructive techniques available to accomplish these tasks with optical sensors, balances, mechanical, sensors and X-ray exposures, but they lack the required accuracy, have limited speed of operation or are too susceptible to environmental hazards. Resonant cavity techniques are widely used for determining microwave properties of materials [1] by measuring the shift of resonant frequency and the change in the Q-factor of the cavity when the sample is inserted into the cavity. Parameters of the cavity depend upon the volume, geometry, and mode of operation of the cavity, as well as on the permittivity, shape, dimensions, and location of the object inside the cavity. For a given cavity and material sample of regular shape and well defined dimensions, one can determine the permittivity of the material [2]. In this paper, we consider the possibility of sorting objects with shapes and dimensions that are unknown at the time of measurement. Microwave resonant cavities have been used for evaluating the dielectric properties of geometrically defined samples (2,3] when the cavity is calibrated with dimensionally identical samples of various known permittivities. By measuring a fiber in two resonant cavities, the dielectric constant and diameter of the fiber may be determined [4], or its moisture content may be determined independent of its diameter or density [5]. Finally, a single resonant cavity has been applied for determining moisture content in uniformely shaped seeds by simultaneous measurement of resonant frequency shift and change in the transmission factor [6]. The promising results obtained in that work provided a basis for continuing the research with other objects.
THEORETICAL CONSIDERATIONS Rectangular Waveguide Resonator If a hollow rectangular waveguide is sealed with a conductive wall perpendicular to the direction of propagation, the incident and reflected waves are superimposed and create a standing wave. The tangential electric field and normal magnetic field are zero at this wall and at distances of integral halfwavelengths from it [7,8]. In such a nodal plane, a second conductive wall can be located without disturbing the field distribution along the waveguide, and ther
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