Broadband Dielectric Measurement of PCB and Substrate Materials by Means of a Microstrip Line of Adjustable Width

This letter presents a new method for broadband dielectric measurement of printed circuit board/substrate materials. The method is based on standard two-port transmission/reflection measurements of a microstrip transmission line that is printed over the material under test. The novelty consists in the use of a single line that has an adjustable width. Determination of the dielectric constant requires measurements of this line for at least two different widths. The change in the width (preferably narrowing) is feasible due to step discontinuities of mirror symmetry that are paced at both ends of the line. The proposed method has several advantages. It does not need any additional calibration, does not depend on impedance matching of the line, and has no errors due to inaccurate connector assembly as well as a deviation in the connector's parameters. The method was tested for low-loss materials and proved to be useful in the frequency range of 1-8 GHz. The estimated error is ≤5% with reference to the split-post dielectric resonator method. The accuracy can be improved (error below 1.5%) when a more accurate model of step discontinuities is used.