Design of CSRR-Based Electronically Tunable Compact RF Sensor for Material Testing

A novel concept of designing electronically tunable compact planar RF sensor operating in the S-hand (2.7-3.1 GHz) for material testing is presented. The proposed design makes use of the complementary split ring resonator (CSRR) loaded microstrip line coupled to the varactor diode, where the electronic tuning is facilitated by reverse biased varactor diode. Using the improved design topology, proposed structure is able to provide material testing over a wide tunable range using only the single unit of CSRR resonator. The proposed tunable sensor is designed using the CST-MWS, and accordingly various parameters are optimized to obtain the desired performance. Equivalent circuit of the resonant structure is obtained using the advanced design system. The proposed structure is fabricated on a FR-4 substrate, and accordingly the S-parameters are measured under unloaded condition with a biasing voltage range of 0-28 V, using the network analyzer. The testing of the fabricated structure shows a close match between the measured and the simulated scattering parameters. Finally the dielectric properties of various standard test samples are measured over a number of frequency points in S-band using the proposed sensor, where the newly developed empirical formulas are employed to relate the dielectric properties with the corresponding measured resonance frequencies.