# An Ultra-Wideband Frequency Selective Rasorber with Low Infrared Emissivity
## 1 Introduction  
Frequency selective rasorber (FSR) has garnered significant attention due to its potential in addressing the challenges associated with microwave transmission and stealth compatibility. The concept originated from a patent in 1995 [1], which asserts that the absorption and transmission of electromagnetic waves within a specific frequency band can be achieved through the placement of lossy material on a frequency selective surface (FSS) structure. The specific implementation concept was proposed by [PERSON] in 2009 [2]. However, the actual structure was not provided for some reason. In 2012, [PERSON] quantitatively derived the absorption and transmission conditions based on the equivalent circuit (EC) [3], which established a theoretical foundation for the structural design of FSR. Since then, numerous interesting findings on FSR have been published over the course of more than a decade of diligent research and development [4, 5, 6, 7, 8, 9, 10, 11]. According to the relative position of the absorption band and the transmission band on the spectrum, three categories can be distinguished for FSR: low-transmission and high-absorption FSR (TA-FSR) [12, 13, 14, 15, 16], high-transmission and low-absorption FSR (AT-FSR) [17, 18, 19, 20, 21, 22, 23, 24, 25], and a transmission band between high and low-absorption bands (ATA-FSR) [26, 27, 28, 29]. According to the unit structure dimension, FSR can be also classified into 3D-FSR [30, 31] and 2D-FSR. While 3D-FSR offers greater design flexibility and structural strength, it poses challenges in processing, particularly in reducing the height of the longitudinal section, so there is a lack of mature application conditions for its implementation. Consequently, the current research primarily focuses on 2D-FSR. Currently, the primary operating frequency range for a ground-based detection radar is 2-12 GHz, with a trend toward lower frequencies. Most micro-detectors on flight platforms such as satellites operate in the frequency band above 12 GHz, and there is a trend toward higher frequencies. Given this situation, AT-FSR, the metamaterial that incorporates high-frequency transmission and low-frequency absorption, has become an important focus for future stealth design. According to [PERSON]'s equivalent circuit analysis theory [3], the AT-FSR requires a parallel resonant structure in order to achieve the high-frequency transmission window. However, expanding the bandwidth of the transmission window to meet practical application needs has become another challenge. Some designs promote using spiral microstrip lines instead of lumped inductors and capacitors, which can effectively reduce the insertion loss in the transmission band [18; 19; 20]. However, these results still do not meet the bandwidth requirements of micro-detectors. To address this issue, a design scheme based on densely arranged metal microstrip lines was proposed [29]. By perforating via holes in the dielectric plate of the RS and simultaneously loading metal microstrip lines densely on both sides of the dielectric plate connections, a significant broadening of the transmission bandwidth of AT-FSR is achieved. However, loading metal microstrip lines densely requires advanced processing technology and introduces significant processing errors; moreover, loading via holes can unpredictably affect the electromagnetic characteristics of the structure. Additionally, this design does not achieve broadband absorption while ensuring wideband transmission--further limiting its practical application value. Therefore, it is imperative to investigate the compatibility of AT-FSR in wideband transmission and absorption, in order to enhance its applicability in the stealth design of satellites and other aerospace platforms. Furthermore, to achieve bistealth in microwave and infrared bands, two conventional approaches are employed.