Dual frequency tunable frequency selective absorber based on pin diode and water-based coupling

This paper proposes a dual-band tunable frequency selective absorber based on the synergistic coupling of modified PIN diodes and a water dielectric layer. The five-layer metamaterial structure enables independent control over its two absorption bands by adjusting the resistance (R) of PIN diodes within a parallel capacitance and the thickness (h_water) of the water layer. Continuous tuning of the first resonant band from 5.7 GHz to 9.6 GHz and the second resonant band from 14.83 GHz to 15.83 GHz is achieved. Independent tuning is demonstrated, with the low-frequency center shifting by 2.01 GHz and the high-frequency peak shifting by 1.08 GHz. Mechanism analysis reveals that the ITO layer acts as the primary absorber, the PIN diodes compensate for high-resistance losses and dominate low-frequency tuning, while the water layer facilitates high-frequency tuning and absorption enhancement.