A brief description of the theory of passive and active absorbers is presented followed by details of an experimental study into a new design of adaptive absorber. The absorber is a single-layer planar structure based upon the topology of a Salisbury screen, but in which the conventional resistive layer is replaced by an active frequency selective surface (FSS) controlled by pin diodes. The resulting structure has superior reflectivity-bandwidth characteristics compared to conventional passive absorbers of corresponding thickness. Experimental results are presented and compared to those obtained from a transmission line model, and show that the reflectivity response of the absorber can be dynamically controlled over the frequency band from 9-13GHz.
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