Abstract
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.
Highlights
Frequency-selective rasorber (FSR) has attracted growing attention due to its unique transmission and absorption characteristics [1]
EXPERIMENTAL RESULTS AND DISCUSSION A photograph of our fabricated tunable FSR prototype consisting of eight cells in one direction with the size of 24 × 196 mm2 is shown in Fig. 4, with several supporting plastic nylon screws and spacers, which maintains a uniform gap of 14 mm
A tunable FSR based on square-loop array has been successfully designed and verified at the microwave band
Summary
Frequency-selective rasorber (FSR) has attracted growing attention due to its unique transmission and absorption characteristics [1]. A conventional method of designing FSR structures is based on multi-layered frequency selective surfaces (FSSs) combined with lossy components or materials [4]–[12]. In order to overcome the above issues, we present a doublelayer varactor-based tunable FSR with two absorption bands at both sides of the transmission window in this paper. A typical bandpass FSS layer is utilized to realize the transmission window within the absorption band Both layers are constructed with varactors to achieve the coordinated tunable characteristic. In each unit cell of the tunable rasorber proposed in this paper, the lossy layer consists of a square-loop structure loaded with lumped resistors into four metallic strips. The ABCD matrix of the cascaded two-port network for the air spacer and bandpass FSS can be written as: AB cos θ2 jZ2 sin θ2
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