A differential microwave sensor based on measurement of complex permittivity is presented to meet the needs of detecting concentration variations in mixed solutions.In order to suppress environmental interference

the sensor employs radio frequency switching technology which performs differential processing between the reference by-pass and the sensing by-pass.The improved microstrip complementary curved ring resonator enhances the electric field by increasing capacitive coupling which can improve the sensor’s sensitivity greatly.When detecting binary liquid mixtures(water-ethanol)of different volume fractions

the fitted empirical formulas regardin

g complex permittivity are derived from the measurements of the sensor’s resonant frequency and quality factor

Q

.The formulas are then validated by injecting water-methanol mixtures

and the calculated results show great agreement with the Debye function.Experimental results confirm that the sensor possesses excellent sensitivity and precision for the concentration detection of mixed solutions.