Improved microwave sensor with enhanced sensitivity for complex permittivity characterization of liquids

Abstract The article presents a microwave sensor with enhanced sensitivity for characterization of complex permittivity by the use of a CSRR resonator excited by a two-port microstrip line. The sensitivity is improved by the use of a modified microstrip line having an interdigital capacitor (IDC) structure between the open gap of the 2-port microstrip line. The modified feedline structure results in better E-field confinement in the sample, which is placed above the CSRR sensing region. The E-field confinement due to feedline modification not only results in significant improvements in the Q-factor and S21 notch levels, but also leads to a larger resonant frequency variation corresponding to the change in sample permittivity. To determine accurate and reliable values of complex permittivity ( ε ′ and ε ″), a non-linear least square curve fitting method is applied to the measured parameters, resulting in low percentage measurement errors. The proposed structure operates in frequency range between 4.2–6.15 GHz, has planar design and is easy to fabricate, as there are no holes, vias, or substrate engravings for holding the sample. For sample testing, the sample is placed in a pre-designed cylindrical-shaped holder with optimized dimensions. Liquid samples can be conveniently loaded using a pipette, thus eliminating the requirement for any external complex auxiliary apparatus, ensuring operational ease and portable testing. For Ethanol–Water mixture testing, the proposed sensor demonstrates a maximum sensitivity of 2.05% and low prediction errors (1.13% for ε ′ and 2.67% for ε ″). Additionally, it achieves an average sensitivity of 3.11% across a wide permittivity range ( ε ′ = 1–111).