Non-contact liquid sensing using high resolution microwave microstrip resonator

Here, non-contact liquid sensing using high-resolution microwave sensor is reported. The proposed sensor is based on a passive ring resonator with an active feedback loop to generate negative resistance and compensate for the resonator's loss, which in return significantly increases the quality factor of the system. High quality factor offers high-resolution readout with large field penetration depth and thus enables distant, non-contact sensing of liquid materials in a career tube. The active loop technique increases the quality factor of the resonator from 240 to 200000 in air. It also provides flexibility of adjusting the quality factor by varying DC voltage. To our knowledge, this is the first time that a non-contact liquid measurement technique using such high quality factor is reported.