Complex Permittivity Extraction of Planar Dielectrics Using a Noninvasive Microwave Transmission Line Resonant Technique

This article presents a novel planar resonant method for complex permittivity measurement of planar dielectric substrates. Conventional planar dielectric sensors are composed of resonant structures, such as metamaterials, annular rings, and patch coupled to transmission lines to generate resonance. Here, we demonstrate a simple microstrip line resonator for complex permittivity measurement, and the generated resonant modes in the microstrip line are much like in 3-D conventional microwave cavities. The proposed prototype is distinguished from regular planar resonators due to the presence of a number of distinct electric and magnetic perturbable resonant modes and the absence of additional resonating structures in the design. The use of test samples in the commercially available thickness eliminates extensive sample preparation and possible air gaps or errors introduced due to the stacking of multiple dielectric layers. The microstrip line resonator fabricated on the FR4 substrate shows good agreement in measured permittivity and loss tangent values compared with the manufacturer’s datasheets. The proposed technique is a simple, fast, compact, and cost-effective measurement method for dielectric characterization, and it could be extended for other microwave transmission lines also.