Design of a Tunable Frequency-Selective Surface for Microwave Material Measurement Applications

In this work, a tunable frequency-selective surface is designed with a center frequency that can be varied with an applied DC voltage. An equivalent circuit representation of the FSS is derived from a finite-difference time-domain (FDTD) simulation of the passive FSS, allowing tuning circuits for the FSS to be designed in common circuit simulation tools such as SPICE. A comparison between the spectral response obtained from FDTD and equivalent circuit modeling (ECM) in SPICE shows that under certain conditions, ECM provides good agreement with full-wave analysis, is less computationally intensive, and provides physical insight. The ECM technique enables rapid design and analysis of various trade-offs, such as those between resonant frequency tunability and bandwidth. The ECM-designed circuit is then validated with full-wave analysis of the designed structure with active components using an FDTD-SPICE hybrid co-simulator. Finally, applicability of the chosen active FSS topology as a metasurface for free-space dielectric material characterization is discussed.