## 4 Fabrication and Evaluation of Multi-Functional Transmitarray  
Based on the results of the simulations performed above, a transmitarray of a size of 120 mm \(\times\) 120 mm with an 8 \(\times\) 8 unit cell arrangement was fabricated, and an interface board for the bias control of each unit cell was configured to verify the performance of the proposed structure. Prior to experiment, an antenna horn with identical specifications from the above simulations was used, and the focal distance was set to 96 mm (F/D = 0.8) for maximum efficiency. The appearance of the fabricated PIN diode-based transmitarray is shown in Figure 6a,b, and the environment in which near-field measuring was performed for radiation pattern measurement to assess beam steering performance is illustrated in Figure 6c. Figure 5: Simulated results of proposed 8 \(\times\) 8 transmitarray (F/D = 0.64): (**a**) Simulation model (HFSS); (**b**) Broadside beam (V Pol. ); (**c**) 45\({}^{\circ}\) steered beam with PC OFF state (\(\varphi\) = 0\({}^{\circ}\)); (**d**) 45\({}^{\circ}\) steered beam (\(\varphi\) = 45\({}^{\circ}\)) under the ON and OFF state of PC. As demonstrated in Figure 6c, measurement was taken for altering the beam steering angle from 0 to 35 degrees to factor in the physical interference of the support frame that holds the antenna under test (AUT). The 1D beam steer result in comparison to the simulation result is shown in Figure 7a, and the result of the 2D beam steer according to the polarization control status (\(\varphi\) = 30\({}^{\circ}\), \(\theta\) = 30\({}^{\circ}\)) is shown in Figure 7b. Figure 6: Fabricated 8 \(\times\) 8 transmitarray: (**a**) top view (Tx patch); (**b**) bottom view (Rx patch); (**c**) far-field pattern measurement based on near-field antenna measurement system. Figure 7: Measured radiation pattern of multi-functional transmitarray. Results of: (**a**) scan angle in horizontal plane; (**b**) scan angle in diagonal planes with the state changes of polarization. The measured maximum gain was observed to be 14.6 dBi under the condition of the Bore-sight (\(\varphi=0^{\circ}\), \(\theta=0^{\circ}\)), and the beam width and side lobe level (SLL) are measured as 14.9\({}^{\circ}\) and 14.5 dB, respectively. The isolation level between the polarizations under bore-sight conditions is 14.7 dB. It was confirmed that both the polarization conversion and beam steering operate simultaneously without issues. According to the unit cell simulation and measurement results, the design margin of error is 160 MHz. This is considered because of the photoimageable solder resist (PSR) layer with a permittivity of 3.3 coated on both sides of the circuit board to prevent corrosion of the fabricated transmitarray bare board. Aperture efficiency calculated by the ratio of the measured benefit compared to the maximum directivity of \(\mathrm{D}_{\mathrm{max}}=4\pi\mathrm{A}/\lambda_{2}=22.4\) dBi that can be gained from the aperture area A is 16.4%. The gain-loss budget, which analyzes any factors contributing to the efficiency loss, is illustrated in Table 3 (where \(\lambda\) is the working frequency). Relatively significant loss due to fabrication and measurement was observed. This may be due to the resolution error over-etching of the copper line and fabrication error caused by the manual mounting of the PIN diode during the build-up process used to produce the transmitarray. These factors are believed to be improved if a high-precision fabrication process is used. Lastly, a performance comparison of the proposed design result to the previous studies of the transmitarray design based on a PIN diode is summarized as shown in Table 4.