Electric Field Homogeneity Optimization by Dielectric Inserts for Improved Material Sensing in a Cavity Resonator

A TM010 mode cylindrical cavity is employed as the main sensing element to characterize variable density compositions of material streams, for example, paper, textile fiber, or plastic composites. Through-passage holes at both end plates in the middle of the resonator frame provide for the material flow, hence forming a cylindrical channel region that is isolated from the cavity by a thin dielectric insert tube to avoid contamination of the cavity. In relation to the cavity dimensions, the hole size is relatively large to accommodate material flow variations in volume or mass density. The large through-hole geometric feature causes the electric field uniformity to be compromised within the channel region. As a consequence, unwanted frequency shift responses occur due to lateral movements in the material stream. A study related to the electric field homogeneity of a cylindrical cavity is presented. It is shown that the field homogeneity can be improved by placing a tubular dielectric insert inside the cavity. The main aim is to optimize the shape profile and select a feasible material for the dielectric insert. The evaluation of the electric field homogeneity is based on the measure of electric field variations within the channel. Dielectric inserts with conical profile are presented as an improved alternative to cylindrical tubes for uniform field homogeneity inside the sensing region.