Nondestructive method for measuring dielectric constant of sheet materials

Abstract

In many applications including RF, Microwave dielectric constant (ε r ) of the material is a key design parameter. Present methods for measurement of ε r are time-consuming and require a sample piece of the material on which some processing has to be done (destructive methods). This paper presents a nondestructive and simple method for determining ε r of sheet dielectric materials. The method is nondestructive in the sense that cutting of the material or any processing on it is not required. In this method resonant frequency ( f r ) of a pre-fabricated antenna is first measured with a known material and then again measured with the Material Under Test (MUT) as dielectric part of the microstrip antenna. The parameter ε r is then determined from the ratio of these resonant frequencies and the thickness of MUT. The experimentation is arranged so that errors due to measurement in patch dimensions and electrical connections to the feed line are eliminated. The method can be used for on-line quality control during production of dielectric sheets. An aperture coupled microstrip antenna has been designed and fabricated with f r = 8.4 GHz. A simple jig has been constructed to hold the antenna and connecting cable. It provides an easy means for changing the MUT and brings different areas under the patch. Measurements validate the design and simulation results.