# 1-Bit Reconfigurable Transmitarray Antenna with Out-of-Band RCS Reduction +
## 4 Experimental Verification  
### Antenna Prototype and Measurement Setup  
The operating frequency and the size of the array can be expanded according to the actual application scenario. Certainly, larger arrays and a higher operating frequency are bound to bring about rapid increases in manufacturing difficulty and cost, and also the number of control hardware and logical complexity. Therefore, the array size and operating frequency should be assessed and balanced according to the actual situation. As an example, a 16 \(\times\) 16 low-RCS 1-bit RTA prototype was designed, fabricated, and measured based on the element structure, as illustrated in Figure 12. The prototype has an effective aperture of 320 mm \(\times\) 320 mm and integrates 512 pin diodes (SMP1340-040 LF, Skyworks, Shanghai, China). Additionally, there are 4 \(\times\) 64 pin connectors located on the sides of the array, which are connected to the FPGA control board to enable the electronic control of each element's state. The AFST layer is placed above the RTA layer using 16 plastic struts. It should be pointed out that compared with the traditional transmitarray antenna, the designed low-RCS 1-bit RTA significantly reduces the out-of-band RCS and has the in-band beam scanning capability, which greatly broadens its application range, although an additional AFST layer and PIN diode are added, resulting in complex manufacturing and increased cost. Moreover, the AFST layer uses a multimode resonant structure, which simplifies the structure complexity and reduces the antenna profile. Moreover, a linearly polarized horn antenna is used as the feed to excite the RTA, with a focal-diameter ratio \(F/D\) of 0.8. In the case of beam scanning, the actual compensation phase \(\Delta\phi_{mn}\) for each element \((m,n)\) in a different beam scanning angle \((\theta_{t},\varphi_{t})\) can be calculated using the following formulas [27]:  
\[\Delta\phi_{mn}=\Delta\varphi_{f}+\Delta\varphi_{t}+\Delta\varphi \tag{1}\]  
\[\Delta\varphi_{f}=\frac{2\pi}{\lambda}\bigg{(}\sqrt{F^{2}+(mP)^{2}+(nP)^{2}}- F\bigg{)} \tag{2}\]  
\[\Delta\varphi_{t}=-\frac{2\pi}{\lambda}(mP\sin\theta_{t}\cos\varphi_{t}+nP \sin\theta_{t}\sin\varphi_{t}) \tag{3}\]  
where \(\Delta\varphi_{f}\) and \(\Delta\varphi_{t}\) are the phase differences of the element position and beam scanning, \(\Delta\varphi\) is the reference phase, and \(\lambda\) is the wavelength corresponding to 7.5 GHz. Then, the bit state of each element can be obtained using the following formula [28]:  
\[bit_{mn}= \begin{cases}0,&\Delta\varphi_{mn}\in[0,\pi]\pm 2n\pi\\ 1,&\Delta\varphi_{mn}\in[\pi,2\pi]\pm 2n\pi\end{cases}  \tag{4}\]  
Figure 11: Simulated S parameters of the low-RCS 1-bit RTA element under different bit states: (a) amplitudes, (b) phase. The measurement process consists of two parts: RCS reduction evaluation and radiation performance measurement. Therefore, two different measurement systems are set up to complete the measurement process, as illustrated in Figure 12. The setup shown in Figure 12a, has the prototype in a near-field anechoic chamber and acts as a transmitting antenna to measure its beam scanning performance. On the other hand, the fabricated prototype was placed in an anechoic chamber that was irradiated with a pair of linearly polarized horn antennas. Then, the reflection coefficient differences caused by adding an AFST layer to the RTA layer was calculated, which can be used to characterize the RCS reduction, as shown in Figure 12b. During the measurements, the environment noise of the anechoic chamber is about \(-80\) dBm, and the signal-to-noise ratio can be guaranteed to be about 30 dB. Furthermore, the following two aspects were applied to ensure the measurement accuracy. First, the feed antenna and the receiving waveguide should be aligned with the center of the array. Therefore, a bracket was machined using 3D printed technology to ensure the accuracy of antenna's erection, as shown in Figure 12a.