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

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 (ℇ’ and ℇ’’), 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 ℇ’ and 2.67% for ℇ’’). Additionally, it achieves an average sensitivity of 3.11% across a wide permittivity range (ℇ’ = 1–111).