Novel Microstrip-Based Simplified Approach for Fast Determination of Substrate Permittivity

This paper presents a simple and fast microstrip transmission line method for the broadband radio frequency characterization of dielectric substrate materials. The proposed method utilizes only the magnitude of the S-parameters of the microstrip line at certain discrete frequency points. The technique is thus free from the phase uncertainty problem, which is the main concern in conventional broadband microwave characterization techniques. Additionally, the proposed method does not require the 50-Ω microstrip line to be designed on the test sample, and hence prior knowledge of the approximate substrate permittivity is not required, unlike earlier proposed planar methods. The scheme presented in this paper is based on the derivation of a novel closed-form relationship to determine the real part of the substrate permittivity from the measured reflection coefficient. The loss tangent of the substrate material is obtained from the numerical model based on the transmission coefficients of designed microstrip line. The proposed method is validated numerically using the 3-D electromagnetic solver CST Microwave Studio. For the experimental validation, the microstrip structure of arbitrary impedance is fabricated on the test substrate, and its complex permittivity is retrieved using the proposed scheme in terms of S-parameters measured with the network analyzer. The extracted permittivities of standard samples using both simulated and the experimental data agree well with their reference values available in the literature.