Complex Permittivity Retrieval of Dispersive Liquids Based on Varactor-Loaded Split-Ring Resonator

A novel method has been evaluated to retrieve the dispersive dielectric constant versus frequency using a tunable metamaterial biosensor, which is designed and fabricated in this study. Varactor-loaded metamaterial resonators coupled micro-strip line form the main part of the structure. The presented structure consists of a split-ring resonator (SRR) and diode varactors loaded on SRRs. Diode varactor bias network is attached to the top layer, and the liquid under the test is contained inside the microfluidic channel. To retrieve the dispersive permittivity of the liquid, we considered a tunable sensor. The resonant frequency (
${f}_{r}$
) is affected by two parameters: sensor equivalent capacitance and inductance. We keep the second parameter constant and tune the first parameter by changing the varactor value from 0.44 pF up to 4.71 pF. The equivalent capacitance is also affected by the liquid permittivity value, which lets us measure the permittivity dispersion versus frequency. Our presented sensor can measure relative permittivity up to 100. As an example, four resonant frequencies, including 1.35, 1.5, 1.7, and 2.2 GHz, were selected to validate the sensor operation. In the experiment, varactor values are changed until resonant frequencies are shifted to the intended
${f}_{rs}$
, then because of the known value of varicap the permittivities are estimated precisely according to the previously simulated results. This idea has been verified by water, oil, and ethanol to validate the sensor.