# 1-Bit Reconfigurable Transmitarray Antenna with Out-of-Band RCS Reduction +
## 3 Element Design
### AFST Element  
In the first step of the low-RCS 1-bit RTA design process, the AFST element is analyzed and designed. The AFST needs to have the abilities of out-of-band absorption and in-band transmission while maintaining a low profile and simple structure. To achieve these goals, multi-resonant modes on the metallic strip are used, with different functions assigned to different modes using a loading resistor. The element of the AFST is shown in Figure 2, where a bent metallic strip is printed on a dielectric substrate with a relative dielectric constant of 3, and a lumped resistor is loaded in the center of the strip. The element has a period \(P\) of 20 mm, with approximately a half wavelength corresponding to the frequency \(f_{M}\), and the total length \(L_{s}=2\times(L_{s1}+L_{s2})\) of the metallic strip is 40 mm, which is roughly equal to one wavelength corresponding to the frequency \(f_{M}\). The other physical parameters are listed as follows: \(L_{s1}=12\) mm, \(L_{s2}=7.5\) mm, \(d_{s}=6\) mm, \(t_{1}=0.5\) mm, \(W_{s}=0.6\) mm, and \(R=250\)\(\Omega\). Figure 1: Schematic of the proposed low-RCS 1-bit RTA with various features at different frequencies: (a) \(f_{L}\), (b) \(f_{M}\), and (c) \(f_{H}\). Figure 2: Element of the AFST in (a) perspective view and (b) top view. The designed AFST structure was simulated to verify its abilities of out-of-band absorption and in-band transmission. In order to obtain the simulated S parameters of AFST, the element structure shown in Figure 2 was modeled in the HFSS 2021.R1 commercial software, and then the periodic boundary condition was set to fit the infinite array situation, and Floquet port excitation was used to fit the plane wave incidence [24]. Moreover, the incident wave was \(y\)-polarization. Figure 3 shows the simulated reflection and transmission parameters. It was found that the incident wave around \(f_{L}=4\) GHz and \(f_{H}=10.5\) GHz was absorbed, although the absorptivity was not very high, which can be significantly improved by adding a ground under the structure. In contrast, at \(f_{M}=7.5\) GHz, the incident wave passes through with low insertion loss. Therefore, the designed AFST structure can be applied to the low-RCS 1-bit RTA to achieve out-of-band RCS reduction. In addition, it is worth mentioning that although the three resonant frequencies are multiples, \(f_{L}\) and \(f_{H}\) can be separately regulated by adding stubs at appropriate locations [25]. The working principle of the AFST's absorption-transmission-absorption can be analyzed by observing its current distribution at the central frequency of \(f_{L}\), \(f_{M}\), and \(f_{H}\), as shown in Figure 4. Through observing the current intensity at the position of the resistor, it is found that with the change in the resonant frequency, the current intensity presents variations in the state of antinode-node-antinode. Specifically, when the resistor is in the current wave antinode state, the incoming wave will be absorbed, and when the resistor is in the current node state, the resistor is ineffective. On the other hand, the working principle of AFST can be further analyzed with an equivalent circuit model. The half- and one-and-half-wavelength resonances of the metallic strip can be equivalent to an \(RLC\) series circuit, with the current reaching its maximum value at resonance. In contrast, for one-wavelength resonance, it is equivalent to an \(LC\) parallel circuit in series with \(R\), resulting in the current tending to zero at resonance. Next, in order to achieve dual-polarized out-of-band RCS reduction, four single-polarized metallic strips are rebent and interwoven to form a central rotationally symmetric structure, as shown in Figure 5. Similarly, the resistor is still located in the center of the metallic strip, and its value is 200 \(\Omega\). The segmented lengths of metallic strip are \(L_{1}=9.5\) mm, \(L_{2}=8.5\) mm, and \(L_{3}=9\) mm, and the interval \(d\) is 1 mm. In addition, the dielectric constant of the substrate is 3, and its thickness \(t_{1}\) is 0.5 mm.