Radar-absorbing materials are used in stealth technologies for concealment of an object from radar detection. Resistive and/or magnetic composite materials are used to reduce the backscattered microwave signals. However, nontunable characteristics or the required complex structure hampered the application of these materials. Here, multilayer graphene-based frequency selective surfaces (MLGFSS), which reach a size of 150 mm $x$ 150 mm, are designed and fabricated. By properly changing the growth temperature of MLG using the chemical vapor deposition approach and designing the pattern of graphene layer, the impedance matching condition can be satisfied at different frequencies. As a result, two kinds of absorbers with different working bandwidths are realized. The performances of the proposed absorbers are analyzed using full-wave simulation and are also tested with experimental results. Our method of fabricating large-area MLGFSS avoids the direct contact between the stencil mask and graphene, and guarantees the integrity and quality of patterned graphene structure. A good agreement between simulation and measurement results demonstrates that such ultrathin MLGFSS is very useful in the design of graphene-based functional devices at microwave frequencies.

In this paper, a dynamically tunable substrate-integrated-waveguide (SIW) attenuator using graphene is first presented. Two graphene sandwich structures are placed inside an SIW, which serve as conductivity-tunable E-plane septum. By applying biased voltage to the graphene, the attenuation can be dynamically changed; meanwhile, the reflection can still keep at a relatively low level. A closed form of the attenuation is also proposed, and the mechanism of the attenuator is theoretically analyzed in detail. Furthermore, a prototype operating from 7 to 14.5 GHz has been fabricated and measured. Both the theoretical calculation and the numerical simulation results have a good agreement with the experimental results, which present the attenuation ranges from 2 to 15 dB with a stable 7.5-GHz wideband while the bias voltage changing from 0 to 4 V.

In this paper, a dynamically tunable graphene-based absorber in Ku band is proposed for the first time, the center frequency of which can be continuously tuned while maintaining an excellent absorption performance. It consists of a periodically patterned graphene sandwich structure (GSS) and periodic patches printed on the substrate backed with ground. The periodically patterned GSS are directly deposited on the top surface of patches. By using the external bias voltage, the proposed absorber can provide a dynamically tunable operating frequency from 13.9GHz to 16.4GHz, in which the absorptions remain above 0.92. To theoretically investigate the performance of the absorber, an equivalent circuit based on transmission-line is derived. We fabricate and measure some samples to experimentally verify the dynamical frequency tunability of the graphene-based absorber, which has never been really achieved before in microwave frequency. A good agreement between simulation and measurement is acquired. The whole thickness of absorber is equal to 0.06λ, which provides a path forward for realization of dynamically tunable and ultra-thin absorber. The design strategy can generally be applied to other wavelength regions applications such as mechanical deformation and bolometric detector.

A novel flexible and transparent broadband linear polarization converter based on inter-embedded lock-shaped metasurface was proposed. Three electromagnetic resonances were produced in an inter-embedded lock-shaped structure, which resulted in a considerable bandwidth expansion of the polarization converter. The simulation results revealed that the proposed metasurface could convert the x/y-polarized wave to the y/x-polarized wave in the frequency range of 9.5–18.5 GHz with a polarization conversion ratio (PCR) higher than 90%. The proposed broadband polarization converter was experimentally demonstrated by fabricating the prototype containing 20 × 20 units. The experimental results of the prototype were consistent with the simulated results. Furthermore, a conformal converter with a 100 mm curvature radius could experimentally obtain a PCR higher than 80% in the frequency range of 9.8–18.5 GHz.