Abstract
A highly sensitive microwave near-field sensor based on electrically-small planar resonators is proposed for highly accurate characterization of dielectric materials. The proposed sensor was developed in a robust complete-cycle topology optimization procedure wherein first the sensing area was pixelated. By maximizing the sensitivity as our goal, a binary particle swarm optimization algorithm was applied to determine whether each pixel is metalized or not. The outcome of the optimization is a pixelated pattern of the resonator yielding the maximum possible sensitivity. A curve fitting method was applied to the full-wave simulation results to derive a closed form expression for extracting the dielectric constant of a chemical material from the shift in the resonance frequency of the sensor. As a proof of concept, the sensor was fabricated and used to measure the permittivity of two known liquids (cyclohexane and chloroform) and their mixtures with different volume ratios. The experimentally extracted dielectric constants were in an excellent agreement with the reference data (for pure cyclohexane and chloroform) or those obtained by mixture formulas.
Highlights
Dielectric constant or relative permittivity is one of the most important characteristics of materials whose accurate determination is crucial in various areas such as the food industry, agriculture, medicine, health-care, and military and defense[1,2]
Earlier designs of electrically-small planar resonator (ESPR) were based on the split-ring resonator (SRR) and the complementary split-ring resonator (CSRR), because these designs were the classic building blocks used for metamaterials[10,11,12,13,14,15,16,17,18,19,20,21,22]
Since the fields are highly localized in the proximity of the CSRR, the field interaction with the material under test (MUT) saturates beyond a certain thickness; the sensor response does not change considerably
Summary
Dielectric constant or relative permittivity is one of the most important characteristics of materials whose accurate determination is crucial in various areas such as the food industry, agriculture, medicine, health-care, and military and defense[1,2]. The advent of metamaterials, which can summarily be described as an ensemble of electrically-small resonators (ESR), inspired a silent revolution in the design of sensors, material characterization techniques, and even imaging These new class of resonators are essentially electrically-small planar resonator (ESPR) circuits with high field localization[8,9]. In another work[33], using a 3-D parallel plate capacitor, the over capacitance of the sensor was increased which resulted in an enhancement in the stored electric energy in the sensing volume, improving the sensitivity This technique, can increase the complexity of fabrication and the overall physical profile of the sensor. The design of fractals does not follow a systematic and streamlined procedure
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