# An Ultra-Wideband Frequency Selective Rasorber with Low Infrared Emissivity
## 1 Introduction
One approach involves the utilization of foam structures to achieve dual absorption of microwave and infrared electromagnetic waves, such as Ni\(-\)MXene/MF [32], Fe/3D carbon [33], and Fe/C/carbon foam [34]. The other entails coating the microwave absorber with low infrared emissivity materials like metal and nano-materials [35; 36]. However, these methods fail to attain broadband absorption in the microwave band. In recent years, significant advancements in metamaterials have provided enhanced design flexibility for multi-spectrum composite stealth applications. Several multi-spectrum composite metamaterials with good absorption characteristics have been reported [37; 38; 39; 40; 41; 42]. For example, [PERSON] et al. developed a metamaterial utilizing quartz and ITO, which exhibits a 90% absorption rate within the frequency range of 5.1-19.2 GHz and an IR emissivity of 0.15 [41]. [PERSON] et al. proposed a transparent and flexible structure using ITO and polyvinyl chloride, achieving 90% absorption at 7.7-18 GHz with maintained stability at oblique incidence of 40 \({}^{\circ}\)C, while exhibiting an IR emissivity of only 0.23 [42]. Nevertheless, these aforementioned structures lack wave transmission functionality and cannot be applied to antenna system stealth designs. In the past two years, there has been some research on designs featuring an infrared dual stealth communication radar [43; 44; 45; 46]. For example, [PERSON] et al. introduced a metamaterial wave absorber based on graphene, which exhibits adjustable absorption performance and radar infrared composite stealth characteristics, showing promising application prospects [45]. However, this design lacks wave transmission functionality. Addressing this limitation, [PERSON] et al.presented an actively tunable rasorber with broadband RCS reduction and low infrared emissivity. The transmission frequency can be continuously tuned from 1.8 to 4.5 GHz by applying a bias voltage of 0-15 V [46]. However, due to the absence of parallel resonance characteristics in the design structure, it cannot achieve a transmittance frequency higher than the absorption frequency. Additionally, while offering tunability through varac diodes, the structure also incurs significant wave penetration loss and narrow wave transmission bandwidth--rendering it unsuitable for high-efficiency communication scenarios in military applications. To conclude, despite the distinctive and intriguing features demonstrated in the aforementioned studies, there remain certain limitations:(1) In the current research of FSR, achieving wideband absorption and wideband transmission simultaneously is often challenging due to their inherent incompatibility. (2) There is limited research on the integrated design of communication-radar infrared bistealth. From an engineering perspective, the existing design [46] has certain limitations: the center frequency of transmission does not align with the operating frequency of the micro detector, and the transmission bandwidth fails to meet the operating bandwidth requirements of the antenna. In consideration of the constraints of the aforementioned study and the requirements for the practical implementation of micro detectors, the paper proposes an ultra-wideband FSR with low infrared emissivity. Firstly, by considering the absorption and transmission conditions based on the EC, we analyzed that impedance mismatch is essentially responsible for limiting the transmission bandwidth, and then we introduce a novel design approach utilizing non-frequency characteristic structures to optimize the imaginary part derivative of impedance and implement the design accordingly. By employing sinusoidal microstrip lines as parallel resonance structures, we achieve an ultra-wide transmission band along with an ultra-wide absorption band.