Highly Sensitive Lossy Tunable Permittivity Sensor

This letter presents a novel lossy tunable permittivity sensor that enhances sensitivity through a controllable conductivity method using a junction field-effect transistor (JFET) in the ohmic region. The sensor, a one-port reflective-mode device, includes a coplanar waveguide (CPW) ending in a step impedance resonator (SIR) as the sensing element, with the JFET bridging the SIR patch and ground. The high sensitivity is achieved by virtue of the strong shift in the phase of the reflection coefficient at resonance experienced by varying the dielectric constant of the material under test (MUT). This device not only serves as a permittivity sensor but also for tracking variables linked to permittivity changes. Our experimental results uniquely demonstrate that sensitivity to minute perturbations can be significantly enhanced by tuning losses. The prototype, designed and constructed for detecting submillimeter proximity changes with a dielectric slab, achieves a peak sensitivity of 2291°/mm within a compact $8.5\times12$ -mm footprint.