Design of a Tunable Polarization-Insensitive Absorber for L and S Bands Using Active Frequency-Selective Surface

A tunable polarization-insensitive absorber based on an active frequency-selective surface is proposed for electromagnetic (EM) wave absorption in the L and S bands. The frequency-selective surface consists of a diamond-shaped loop that is interconnected by varactor diodes. The proposed design is significant because of its inclusion of wideband tuning and its ability to easily change from single-band tunable absorption to dual-band absorption. The simulation results demonstrate that the absorption response (> 80%) can vary from 1.62 GHz to 3.66 GHz under the reverse bias voltage, thus creating a tunable range of 125.9%. The absorption region (> 90%) spans from 1.86 GHz to 3.31 GHz and generates a 78% tunable range. The single-band tunable absorber is characterised by an equivalent circuit model to obtain the circuit parameters and predict the absorption frequency. Furthermore, a dual-band absorbing structure is obtained by adding a metasurface directly to the single-layer structure, which can easily be changed from single-band absorption to dual-band absorption by separately controlling the bias voltages of the upper and lower unit cells. The two fabricated prototypes were measured using free-space technology, and the experimental results closely mirror the simulation results. Results show that the proposed structure achieves the dynamic control of EM wave absorption.