## 2 Pattern Reconfigurable Antenna Structure and Design
### RF MEMS Switch Structure and Design  
The RF MEMS switch plays a vital role in the pattern reconfigurable antenna. RF MEMS switches have very low loss, they do not consume any direct current (DC) power, and their linearity is excellent [13; 14]. The optimized geometry is shown in Figures 1b and 2a.The top view of the proposed RF MEMS switch is shown in Figure 1b, and the size is shown in Table 1. The beam thickness is \(1~{}\upmu\)m. The air gap between the RF MEMS switch beam and the signal lines is \(1.5~{}\upmu\)m. By varying this air gap, the switch can work in the up or the down state and determine whether the assistant patch is connected to the main patch (i.e., when the switch is in the down state, the assistant patch is detached from main patch, and vice versa). The RF MEMS switches were terminated by a \(\lambda_{g}/4\) sector open stub and a high resistivity bias line, shown in Figure 2a. The \(\lambda_{g}/4\) sector open stub is used to transform the open circuit to a short circuit for fear of causing a great impact on the resonant frequency and return loss of the antenna [15]. The advantages of a sector open stub over a microstrip line are miniaturization of structure and convenience of connection. The bias line is applied by direct current voltage to actuate the RF MEMS switch, as shown in Figure 2b. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode** & **State of RF MEMS Switches** & **Frequency** & **Main Lobe Direction (\(\mathbf{\phi}\) = 90\({}^{\circ}\))** \\ \hline
1 & Only left switch in the up state & 35 GHz & \(\theta=17.3^{\circ}\) \\
2 & Double switches in the up state & 35 GHz & \(\theta=-0.6^{\circ}\) \\
3 & Double switches in the down state & 35 GHz & \(\theta=-1.5^{\circ}\) \\
4 & Only right switch in the up state & 35 GHz & \(\theta=-17.2^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Four operating states and three reconfigurable radiating patterns. Figure 1: Proposed radiating pattern reconfigurable antenna using micro-electromechanical systems (MEMS) switches. (**a**) Structure of the reconfigurable antenna; (**b**) Structure of the switch used in the reconfigurable antenna; (**c**) Overall structure of the proposed antenna. The dielectric shown in Figure 2a is Si\({}_{3}\)N\({}_{4}\) with the thickness 0.16 \(\upmu\)m. The thickness of dielectric, the degree and radius of sector open stub are all related to the resonant frequency of the RF MEMS. In order to select the optimized parameters of the RF MEMS switch, some simulations of the switch were conducted under the conditions of various parameter combinations. The resonant frequency of the RF switch (in the down state) raises with the increase of the degree of sector stub, with the increase of the dielectric thickness and with the decrease of the radius of the sector stub, as shown in Figure 3. According to the requirement of optimal isolation and the antenna performance operating at 35 GHz, the combination of parameters is determined (i.e., the angle of the sector open stub is 25\({}^{\circ}\), the dielectric thickness is 0.16 \(\upmu\)m, and the radius of sector open stub is 580 \(\upmu\)m). The isolation of the RF MEMS switch in down state reaches 20 dB at an operating frequency of 35 GHz, and it can guarantee the performance of reconfigurable antenna. When the switch is in the up state, the resonant frequency of the switch is very high (beyond 40 GHz) and has slight changes with variations of the parameters, as shown in Figure 3b. Thus, only the relationship between structure configuration parameters of the up state switch and resonant frequency is analyzed without loss of generality. Figure 3: The relationship between resonant frequency (when the switch in the down state) and structure parameters. (**a**) The variation of the thickness of dielectric (isolation); (**b**) The variation of the thickness of dielectric (insertion loss); (**c**) The variation of the angle of the sector open stub; (**d**) The variation of the radius of the sector open stub. Figure 2: RF MEMS switch.