Broadband Frequency-Selective Rasorber With Varactor-Tunable Interabsorption Band Transmission Window

In this paper, we present a frequency-selective rasorber with tunable transmission window located within the absorption band. The rasorber maintains a low reflectivity, which is less than -10 dB in broadband during the tuning process. An equivalent circuit model is provided as a guideline for the tunable rasorber design. Based on the model, a varactor-tunable rasorber composed of three stacked metallic layers is constructed and investigated. These layers include a frequency selective surface (FSS) loaded with resistors and varactors, a wire grid and a varactor-loaded bandpass FSS. The wire grids in the middle and bottom layers in combination with metallic vias provide the bias and grounding for the varactors, respectively. This configuration avoids using two separated bias networks for varactors on different layers, thus, averting undesirable responses associated with having more bias grids. A tunable rasorber prototype was fabricated and measured, showing that the passband can be tuned from 2.2 to 3.3 GHz by changing bias voltage from 4 to 22 V, with an insertion loss between 6.6 and 3.3 dB. A wide low-reflectivity band under normal incidence for double polarizations from 1.9 to 5.4 GHz is realized. This tunable rasorber has promising applications in broadband stealth facilities with integrated hopping communication functionality.