# 1-Bit Reconfigurable Transmitarray Antenna with Out-of-Band RCS Reduction +
## 3 Element Design
### 1-Bit RTA Element  
After the design and analysis of the AFST, the next step is to design the 1-bit RTA element. As mentioned in Section 2, the RTA structure must meet two key requirements. The first is to provide low-insertion-loss transmission and 1-bit phase regulation at \(f_{M}=7.5\) GHz to enable in-band high-gain and beam scanning performance. The second is to maintain perfect reflection characteristics near \(f_{L}=4\) GHz and \(f_{H}=10.5\) GHz to ensure the out-of-band high absorptivity performance of the AFST.  
The proposed 1-bit RTA element is shown in Figure 7, which employs a receiver-transmitter structure with an active receiving dipole and a passive asymmetric transmitting dipole. Both dipoles, separated by a common ground and connected by a metallized via-hole, are linearly polarized and aligned in the orthogonal direction. Two pin diodes are integrated on the active dipole and biased in opposite states to reverse the current on the active dipole, which essentially results in a 180\({}^{\circ}\) phase difference, thus enabling a wideband 1-bit phase. The ON and OFF states of the pin diode are modeled as \(RL\) and \(RLC\) series circuits, respectively [26]. Specifically, in the ON state, the resistance and inductance are \(R_{on}=1\)\(\Omega\) and \(L_{on}=0.45\) nH, respectively. In the OFF state, the resistance, inductance, and capacitance are \(R_{off}=10\)\(\Omega\), \(L_{off}=0.45\) nH, and \(C_{off}=0.11\) pF, respectively. The element has a period \(P\) of 20 mm, a thickness \(t_{2}\) of 4 mm, and a substrate with a relative dielectric constant of 3.  
Figure 6: Simulated S parameters and absorptivity of the proposed dual-polarized AFST.  
Figure 7: Element of the proposed 1-bit RTA: (**a**) perspective view, (**b**) top view, and (**c**) bottom view.  
Figure 8 illustrates the simulated S parameters of the designed RTA, including its amplitude and phase. The results indicate that the structure has achieved a good polarization converter transmission performance in 7.3\(\sim\)7.75 GHz, with two resonant frequencies in the band. These resonant frequencies are generated by the half-wavelength resonance of the top dipole, which are then differentiated into two frequencies through the coupling of the metallic via-hole. The insertion loss of the passband is approximately 1.2 dB, which is mainly attributed to the resistance loss of the pin diode. Furthermore, Figure 8 depicts the transmission phase under different states, demonstrating that a 180\({}^{\circ}\) phase difference can be constructed within the transmission band through the pin diode, thus achieving a 1-bit electronic RTA.  
However, it should be noted that the structure has an unexpected resonance point near 5.3 GHz, which may deteriorate the absorption performance of the AFST at this frequency. Therefore, it is necessary to eliminate this resonance frequency. Through observing the current distribution at the resonant frequency of 5.3 GHz, as shown in Figure 9, an unwanted resonant path is generated through the bottom dipole, via-hole and the top dipole, as indicated by the black dotted line. To break this resonance, a shorted metallic via-hole is added to the top dipole to forcibly change the resonant path, as shown in Figure 9. It can be seen that the resonant path is broken because it is connected to GND through the shorted via-hole, thus the polarization converter transmission performance will be eliminated. Figure 9 shows the simulated S parameter after adding the shorted via-hole. It can be observed that although the resonant frequency at 5.3 GHz moves to 6.25 GHz, the co-polarized reflection coefficient at this frequency is greater than \(-1\) dB, which has little influence on the out-of-band absorption performance. Notably, the shorted via-hole can also be reused as a ground through-hole for the biasing voltage, providing a 0 V voltage to the pin diode.  
Figure 8: Simulated S parameters of the 1-bit RTA element under different states: (a) amplitudes and (b) phase.  
Figure 9: \(Cont\).