  In this paper, a novel water-based reconfigurable frequency selective
rasorber (FSR) at microwave band is proposed, which has a thermally tunable
absorption band above the transmission band. The water-based FSR consists of a
bandpass type frequency selective surface (FSS) and a 3D printing container.
The water substrate is filled into the sealed space constructed by the above
two structures. The numerical simulation results show that the FSR can achieve
absorption with high absorptivity from 8.3 to 15.2 GHz, and obtain a
transmission band of 5.2 to 7.0 GHz. The minimum insertion loss of the
transmission band reaches 0.72 dB at 6.14 GHz. In addition, the FSR has the
reconfigurable characteristics of absorbing or reflecting electromagnetic waves
by filling with water or not. The proposed water-based FSR shows its good
transmission/absorption performance under different polarizations and oblique
incident angles. Due to the Debye model of water, the absorption band can be
adjusted by water temperature, while the passband remains stable. At last,
prototype of the FSR based on water has been fabricated, and the experimental
results are presented to demonstrate the validity of the proposed structure.
