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One of the most widely used techniques for dielectric material characterization is the partially filled circular cavity. The exact analysis of this structure is well known in the bibliography. But in practice when the measured material is introduced through a circular pipe below its cut off frequency a problem arises. Traditionally the tube effect is neglected, but then some errors appear, whose magnitude depends on the material properties and the cavity and tube dimensions. In some cases, the errors introduced are noticeable and the tube effect should not be neglected. Some authors have tried to evaluate in a simple way this effect, but their equations are valid only in a restricted range of practical configurations. In this paper we present a model of this effect based on a circuital analysis of the full structure. A simple monomode equation valid for any material property and cavity dimensions is given and a saturation effect, not reported before, is analyzed.

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A simple and fast expression based on perturbation method is described and used to characterize high temperature material in real-time. The expression considers the effect of the sample holder that contains the material and the effect of the insertion hole on the top.