*3.2. Coding Metasurface Construction*

With the proposed unit cell, a 1-bit 16 × 16 coding metasurface is constructed and fabricated using printed circuit board technology, as shown in Figure 6. The metasurface has a total size of 176 mm × 176 mm and is precisely attached to the output pins of the control board as a sandwich structure. To enable the beam focusing ability, the control board plays a crucial role in coding metasurface, which provides an exact voltage to turn the PIN diode in each unit cell ON and OFF. The block diagram and prototype of the control board are presented in Figure 7.

**Figure 6.** The 1-bit 16 × 16 coding metasurface model and prototype: (**a**) metasurface model; (**b**) experimental set-up.

**Figure 7.** The control board of the coding metasurface: (**a**) block diagram; (**b**) the prototype.

As the coding metasurface has 256 unit cells, the control board has to control 256 output pins independently. In order to reduce the complexity of wiring the control board, we used two 8-bit shift registers (SR1 and SR2) to independently set the data for each row. Then, the data will be stored in the D flip flip (DFF), which is enabled by two 3 to 8 line decoders (DEC1 and DEC2). Therefore, by selecting the appropriate row, the data can be independently loaded to the intended row. Consequently, we can control the coding metasurface with any ON/OFF state of the unit cell. The LED is parallel connected to the PIN diode to indicate the state of the unit cell, which gives an observable view of the active ON/OFF pattern of the coding metasurface. All the input data are provided by the data acquisition (DAQ) that is controlled by a LabVIEW program.

#### **4. Results**

#### *4.1. Simulation Results*

Before fabricating the prototype, a 16 × 16 coding metasurface is modeled and simulated using CST Studio software to verify the theory. In this simulation, the metasurface is in the XOY plane, and a horn serves as an EM source, which is located at (−5.7 cm, 0 cm, 10 cm) or at (−30◦, 0◦) with respect to the metasurface. Then, an ON/OFF pattern matrix for steering to (40◦, 0◦) is loaded to the PIN diode of each unit cell. Finally, the simulation results are exported and shown in Figures 8 and 9.

**Figure 8.** The simulation results: (**a**) The 3D gain total with the horn and metasurface; (**b**) the 2D radiation pattern.

It is clearly observed that the coding metasurface is capable of steering the beam to the desired direction with 1◦ error in the test case. Furthermore, the surface current distribution in Figure 9 is almost coincident with the input ON/OFF coding pattern. Specifically, we can notice that the current distribution of the unit cell in OFF state is considerable compared to the one with the ON state.

As the proposed system transfers the power via EM waves, human exposure and the specific absorption rate (SAR) level should be considered. Figures 10 and 11 present the simulation SAR level (averaged over 1 gram of human tissue) and beam shape when a human head is exposed closely to the metasurface. In this simulation, the metasurface is encoded to focus the beam to (30◦, 0◦) at the human head, which is placed 50 cm away from the metasurface. The transmitted power is 27 dBm. Figure 10 demonstrates that SAR level at the operating frequency is within the specified limit of 1.6 W/kg regarding to FCC limit. In addition, in the case of human exposure, the beam shape is a bit wider compared to the one without a human head. This results in degrading the gain from 17.3 to around 15 dBi, as shown in Figure 11.

**Figure 9.** The ON/OFF coding pattern of the coding metasurface: (**a**) calculated phase distribution matrix; (**b**) simulated current distribution.

**Figure 10.** Simulation SAR results with a human head (50 cm away from the metasurface).

**Figure 11.** Effect of human exposure in beam shaping: (**a**) without a human head; (**b**) with a human head.
