High sensitivity sensor for differential measurement of thickness of thin substrates using double T-shaped resonators

Non-destructive thickness measurement is an important procedure in industries such as aerospace, automotive, and construction. In this paper, we propose a simple yet high-performance method for measuring the thickness of thin layers using symmetrical double T-shaped resonators, which provide enhanced electric and magnetic field concentration in the sensing region, resulting in improved sensitivity and resolution. The proposed structure is based on a dual-band pass filter (DBPF) operating at 1.5 GHz and 2.47 GHz, designed to achieve intrinsic differential-mode operation without the need for external circuitry. In this microstrip structure, the thickness variation of the material under test (MUT) modifies the electric field intensity, which affects the inductance and capacitance values, and the sensing mechanism operates by detecting the resulting shift in the resonant frequency. In this configuration, the first transmission pole (TP) responds solely to ambient variations, while the second TP is sensitive to both thickness and ambient changes, providing a dual-function resonance mechanism that automatically compensates for environmental effects and improves measurement accuracy. A comprehensive equivalent circuit model is developed to establish the relationship between thickness and resonance behavior, and the parametric and lift-off analyses confirm the stability and repeatability of the design. The proposed sensor exhibits a Frequency Detection Resolution (FDR) of 307 and a sensitivity of 12.45%, outperforming most previously reported microwave sensors. Fabricated on an RO4003C dielectric substrate, the sensor demonstrates excellent agreement between simulation and experimental results. The overall size of the fabricated sensor is 97.55 mm × 24.24 mm × 0.508 mm, making it compact, cost-effective, and suitable for precise, robust, and non-destructive thickness measurement applications across materials science and engineering domains.