A Tunable Frequency Selective Absorber with Reflection Function

A broadband continuously tunable absorber capable of switching to total reflection state for X-band applications is presented in this paper. The tunable absorber structure is composed of a metallic FSS layer, a ground plane, and an air spacer layer between them for matching the input impedance of the structure to free space. In order to achieve a good tunable effect and maintain broadband absorption during the tuning process, we use a variable RC-parallel circuit to achieve the above functions. PIN diodes are used to practically implement tunable function, which can be equivalent to a mutative RC-parallel circuit when the forward bias voltage of the PIN diode is changing. The simulation results show that continuous absorption over 97% can be achieved within more than 1GHz. A reflection coefficient greater than 97.7% in the entire X-band is also attained.