Tunable Frequency-Selective Rasorber Based on Varactor-Embedded Square-Loop Array

This paper presents a varactor-based tunable frequency-selective rasorber (FSR) with embedded bias grid. The proposed structure consists of a lossy layer based on square-loop arrays and a lossless layer. Each layer has an embedded wire grid that provides the bias voltage for the varactors through metallic vias. This arrangement can avoid unnecessary effects associated with the additional biasing network. An equivalent circuit model is developed to explain how the structure can provide a tunable transmission window within the absorption band. Simple design guidelines for the tunable FSR are then provided. The performance of the proposed structure is measured in a parallel-plate waveguide setup. Both simulated and measured results show that by changing the bias voltage from 16 to 4 V, the center transmission frequency of the FSR can be tuned from 5.2 to 3.8 GHz continuously. The insertion loss of the FSR within the transmission window is only 0.59 dB at 5.2 GHz and a fractional bandwidth of 93.1% (2.4-6.58 GHz) for -10 dB reflectivity is achieved under the normal incidence.