# A Frequency-Selective Reconfigurable Antenna for Wireless Applications in the S and C Bands
## 3 Simulation Results  
The antenna simulation was conducted using CST Studio Suite 2019 with open-space boundary conditions. To provide excitation, a waveguide port was positioned at the edge of the microstrip feed line, with a port extension coefficient of 7.2. For the substrate material, lossy FR-4 was used, complemented by parts of annealed copper. The model of the PIN diode used is Skyworks SMP1302 in an SC-79 package. According to its datasheet, the diode can be used in applications from 10 MHz to beyond 10 GHz. In the simulation, the diode had to be represented by its equivalent circuit, because the simulation tool does not support nonlinear elements. The diode can be represented by an RL circuit when it is forward-biased (ON state) or by an RLC circuit when reverse-biased (OFF State), as shown in Figure 2. In the case of forward biasing, it acts as a current-controlled resistor, while in the case of reverse biasing, it acts as a capacitor in parallel with a high-value resistor. Inductance depends on the package used. In the simulation, the resistance (R) and the capacitance (C) values were selected from the diode's datasheet graphs, considering the voltage on the diode to be between 0 and 900 mV. L is always selected as 0.7 nH, as defined for the SC-79 diode package. Simulations encompassed the entire 0 to 900 mV range, corresponding to values of R ranging from 1 to 1000 \(\Omega\) under forward-bias conditions. Notably, three distinct cases stood out, distinguished by the conspicuously low S11 parameter values observed at their primary resonant frequencies. These cases were denoted as modes 1, 2, and 3. Mode 1 corresponds to 0 voltage on the diode, with values C = 0.3 pF and R = 5 k\(\Omega\). Modes 2 and 3 correspond to a forward biasing of the diode with no C and R = 160 \(\Omega\) or R = 4 \(\Omega\), respectively. The simulated S11 parameter and VSWR results for these three modes are illustrated in Figure 3. \begin{table}
\begin{tabular}{c c c c} \hline
**Design Parameter** & **Dimensions (mm)** & **Design Parameter** & **Dimensions (mm)** \\ \hline Ws & 70 & Lsl1 & 26 \\ Ls & 60 & Wsl2 & 24 \\ Wp1 & 25 & Lsl2 & 2 \\ Lp1 & 21 & Lg1 & 29.35 \\ Wp2 & 15 & Lg2 & 13.35 \\ Lp2 & 15 & a & 1.675 \\ Wf & 3 & b & 1.3 \\ Lf & 19.5 & c & 5.68 \\ Wsl1 & 15 & d & 5.5 \\ \hline \end{tabular}
\end{table} 
Table 1: The antenna’s design parameters. Figure 2: The equivalent circuit of PIN diode when (**a**) reverse-biased, and (**b**) forward-biased. As indicated by Figure 3, the primary resonant frequency for mode 1 is 2 GHz, with a return loss (S11) of \(-\)26.4 dB, which is about 9.5 dB lower than the next (secondary) resonance seen at 6.4 GHz. VSWR of the 2 GHz resonance for this mode was equal to 1.11. Subsequently, mode 2 shows a better main resonance at 2.04 GHz frequency than mode 1 in the 2 GHz band. It is characterized by a good return loss of \(-\)39.5 dB and a VSWR equal to 1.02. Amongst the three modes, this one has the best resonance in the 4 GHz band, at 4.44 GHz specifically. The return loss for this resonance is \(-\)22.3 dB, and the VSWR is 1.17. Finally, mode 3's primary resonance is found at 6.42 GHz, with an excellent return loss of \(-\)60.6 dB, in addition to a very low VSWR found at 1.01 for this resonant frequency. The following, secondary, resonance is seen at 0.91 GHz, with a return loss of about 33.5 dB higher than that of the main resonance. Overall, according to the simulation results, the antenna can operate in the 2, 4, and 6 GHz bands. Mode 2 is the most convenient for operation in the 2 and 4 GHz bands at frequencies of 2.04 and 4.44 GHz, while mode 3 is the best for operation in the 6 GHz band at 6.42 GHz. The \(-\)10 dB defined bandwidth for the frequencies 2.04, 4.44, and 6.42 GHz is 211, 941, and 452 MHz, respectively. The radiation patterns for these three cases are depicted in Figure 4, where the positive \(x\)-axis extends towards the front direction of the antenna. The azimuth angle will be referred to as \(\varphi\), while the elevation angle as \(\theta\).