An air-spaced coaxial-fed patch antenna to be used as a microwave sensor for non-destructive porosity measurements in lowloss dielectric materials is presented. The variation of the patch resonant frequency when it is put on the surface of the material under test is used to estimate the dielectric permittivity. Then, a standard two-phase mixture model is used to derive the material degree of porosity from the above permittivity estimate. Measurements at the 2.4 GHz ISM frequency band have been carried out on a set of polymeric samples with artificially induced porosity. The specific choice for the operating frequency is related to the final goal of applying the above microwave sensor for quality control of ceramic materials during the production process.
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