# An Ultra-Wideband Frequency Selective Rasorber with Low Infrared Emissivity
## 2 Analysis, Design and Experimental Verification
### Design of the FSR
#### 2.2.3 Ultra-Wide Transmission/Absorption Band FSR  
The FSR structure is cascaded by RS and FSS. Under normal circumstances, a Floquet port corresponds to an RS unit of the same size as an FSS unit in order to facilitate simulation, leading to a slight difference between \(f_{t_{2}}\) and \(f_{t}\). This is not conducive to ensuring the wave transmission bandwidth and makes it difficult to reduce \(d_{f}\). To address this issue, this paper proposes designing one RS unit for four FSS units. This approach can ensure sufficient simulation efficiency while effectively increasing the resonant frequency \(f_{t_{2}}\) of FSS to reduce \(d_{f}\). Based on the aforementioned analysis, the RS and FSS designed above are cascaded and the DS between them is air, with a thickness of \(d_{a}\) = 11 mm. Figure (a)a depicts the structural schematic diagram of the FSR, while Figure (b)b illustrates its unit cell. The simulation results are depicted in Figure 11. According to relevant standards [1], \(S_{21}\geq-1\) dB is considered to indicate high transmission performance, while \(S_{11}\leq-10\) dB is considered to indicate high wave absorption performance. The FSR demonstrates a high absorption band of 107.4% relative bandwidth in the 2.65-8.80 GHz, and a high transmission band of 64.1% relative bandwidth in the 9.15-17.71 GHz. The FSR also shows good angular stability when the incident angle varies with \(\pm 30^{\circ}\). Figure 10: The structure and working principle of the whole FSR (**a**) and its unit cell (**b**). Figure 9: S-Parameters of the FSS vary with \(\pm 60^{\circ}\) TM (**a**) and TE (**b**) wave irradiation. Furthermore, due to the well-balanced symmetry, although the S parameters of the TE polarization are a little more stable than those of TM polarization, as is shown in Figure 11, the bandwidths are almost the same, so it can be considered that the FSR is polarization-insensitive. The influencing factors of the transmission effect have been discussed in Section 2.2.1, and now we will delve into the analysis of influencing factors affecting the absorption effect. According to the EC analysis method mentioned earlier, key determinants impacting FSR's absorption include: DS thickness (\(d_{a}\)) and resistance value (\(R\)). Their respective effects on the absorption effect are shown in Figure 12.  
With an increase in \(R\), there is a noticeable increase in peak absorption rate within the absorption band; however, this also leads to a decrease in absorption bandwidth. The optimal compromise is achieved at \(R=160\)\(\Omega\). Within a certain range, an increase in \(d_{a}\) results in a decrease in minimum frequency within the absorption band; however, it also leads to a decrease in maximum frequency. Therefore, to ensure sufficient absorption bandwidth, \(d_{a}=11\) mm is selected. Additionally, it is noteworthy that based on the simulation results in Sections 2.2.1 and 2.2.2, both RS and FSS exhibit miniaturization characteristics, with an angular stability of up to 60\({}^{\circ}\). This is attributed to the effective miniaturization function of sinusoidal microstrip results, which effectively suppresses grating lobes. Furthermore, the 4:1 design of FSS and RS significantly reduces the unit size of FSS, thereby enhancing its angular stability. Figure 11: S-parameters of FSR vary with \(\pm\)30\({}^{\circ}\) in TE polarization **(a)** and TM polarization **(b)**. Figure 12: S-parameters of FSR vary with different \(R\) (**a**) and \(d_{a}\) (**b**). However, as depicted in Figure 12, it can be observed that the angular stability of the FSR formed after cascading RS and FSS tends to deteriorate at 30\({}^{\circ}\).