## 2 Design and Optimization of Proposed Unit Cell
### Design of Proposed Unit Cell Structure  
A hexagonal meander-shaped broadband reconfigurable transmitarray unit cell structure design is proposed, and its basic unit structure is shown in Figure 1. The dimensions of the offered unit cell are 8 mm \(\times\) 8 mm \(\times\) 2.6 mm\({}^{3}\) (0.45\(\lambda_{0}\)\(\times\) 0.45\(\lambda_{0}\)\(\times\) 0.13\(\lambda_{0}\)) at 16.5 GHz, where \(\lambda_{0}\) is the free space wavelength. The architecture of the unit cell is composed of five metal patches etched on three similar substrates. The receiver and transmitter metal layers are named MTL and MRL1, as shown in Figure 1a,d. The hexagonal patch is used with O-slot to integrate the PIN diode to achieve phase resolution for the transmitter (Tx). Two hexagonal patches have been used for the receiver (Rx), one for receiving the electromagnetic waves and the other to enhance the receiver's impedance matching. The receiver (Rx) MRL1 layer and transmitter (Tx) MTL are joined with a 0.3 mm metallic interface integrated at the center of the unit cell and separated by a ground layer (MGL). There are two bonding films used to join the substrates. Transmitter layer MTL, receiver layers MRL1 and MRL2 are etched on Roger RO6002 (\(\varepsilon_{\mathrm{r}}\) = 2.94, tan\(\delta\) = 0.0012) with a thickness of 0.78 mm. A total of 2 bonding films FR 27 (\(\varepsilon_{\mathrm{r}}\) = 2.94, tan\(\delta\) = 0.001) join 3 Roger RO6002 substrate layers. However, the thickness of each bonding film is 0.1 mm. All the dimensions are tabulated in Table 1.  
The top view of the transmitting layer (T\({}_{\mathrm{x}}\)), as demonstrated in Figure 1a, is used to transmit radiating electromagnetic waves into the free space. The O-slot on the transmitter layer divides the patch into internal and external hexagonal patches, which integrate with the two anti-parallel diodes. Two PIN diodes are mounted in the same direction along the \(x\)-axis to transmit electromagnetic waves with horizontal polarization. A one-bit phase resolution was obtained using two PIN diodes added to an active unit cell. The bias lines used on the transmitter and the bonding film to control the positive polarity of the diode. At the same time, negative polarity is accomplished with the ground. The serial number of the PIN diodes used in the design is (MADP-000907-14020). The diode requires +1.3 V for forward bias and \(-\)1.3 V for reverse biasing. The value of series resistance R = 7.8 \(\Omega\) with inductor value L = 30 pH for on state, whereas the values of capacitance are C = 25 fF and L = 30 pH in series for off state. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Unit cell size (UA) & 8 mm \(\times\) 8 mm \\ Transmitter outer patch diameter (TOPD) & 4.8 mm \\ Transmitter inner patch diameter (TIPD) & 2.28 mm \\ Height of the substrate (Hs) & 0.7 mm \\ Height of the bonding film (Hf) & 0.0114 mm \\ Inner patch diameter (IPD) & 2.5 mm \\ Slot size (S) & 1.5 mm \\ Via hole (dv) & 0.3 mm \\ Ground opening (dg) & 0.6 mm \\ Impedance-matching receiver outer parasitic passive patch (OPP) & 1.75 mm \\ Impedance-matching receiver inner parasitic passive patch (IPP) & 1.5 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of DLDRS. The PIN diodes integrated into the active elements are always biased in the opposite direction. One diode operates in the ON state while the other works in the OFF state. This design ensures the 2 diodes remain on and off all the time, permitting a 180-degree phase difference by reversing the current on the structure. These switching states produce two states directed to the opposite surface current, generating \(0^{\circ}\) and \(180^{\circ}\) phase shifts. The bias lines are etched on bonding film as illustrated in Figure 1b and are used to operate the PIN diodes. The DC bias network consists of two capacitive plates to separate RF signals from biasing voltages of diodes.