A dielectric spectrometer based on the quasi-static coaxial probe model and a miniature vector network analyser

Due to its fast and non-destructive characteristics, dielectric spectroscopy has been widely applied in the detection of agricultural and food products. However, traditional dielectric measurement systems, which rely on large and expensive vector network analysers (VNAs) and accompanying software, are expensive and restricted to the laboratory. To enable in-situ dielectric measurement, a quasi-static model for the relative complex permittivity was derived, and its performance was verified. Then, a dielectric spectrometer was developed, integrating a miniaturised VNA, a coaxial probe, and self-developed data acquisition software. Its stability and accuracy were evaluated using anhydrous ethanol and normal saline over the 200-6000 MHz frequency range. Finally, the potential of the developed dielectric spectrometer for component analysis and adulteration identification was evaluated. The results indicated that the model’s relative errors were within ±7% for both the dielectric constant ( ɛ ′ ) and dielectric loss factor ( ɛ ′ ′ ). The coefficients of variation for the ɛ ′ and ɛ ′ ′ measured by the developed spectrometer were less than 1% and 2%, respectively, across 200-6000 MHz. In milk component analysis, the developed spectrometer effectively identified changes in the dielectric spectra resulting from variations in water, fat, protein, and lactose contents. For adulteration identification, it clearly captured the spectral changes of honey after the addition of water or maltose syrup. This research demonstrates that fast, non-destructive, and on-site quality detection of agricultural and food products using dielectric spectroscopy is achievable. • The relative errors of the derived quasi-static model for ɛ ′ and ɛ ′ ′ were within ±7%. • A low-cost, portable dielectric spectrometer was developed with CVs less than 2%. • The spectral variations resulting from the main components of milk were identified. • The dielectric spectral changes of adulterated honey were clearly captured.