# Reconfigurable Diplexer-Based Filtenna for Tx/Rx Operation in Mobile Satellite Terminals
## 3 Antenna Design  
The design of the filtenna started with the construction of the radiating element, the reconfigurable antenna. A microstrip main patch was designed to the higher frequency, followed by the design of a secondary metal structure that allows the antenna to operate at the satellite receiver (lower) frequency band, when connected to the main patch. A rectangular microstrip patch structure was used as it is simple and the feeding techniques are widely known and easy to implement. To adapt the same antenna to two different frequencies, the inset feed technique was selected. This method consists of creating a reentrance in the main patch until the input impedance equals the characteristic impedance of the feed line [11]. The main rectangular patch that allows the antenna to operate at 29 GHz was designed first with the dimensions of \(2.41\times 3.25\) mm\({}^{2}\). The matching was made to 90 \(\Omega\), since this impedance allowed having a feed line with an adequate (thin) width, weighing its dimensions, the feasibility, and the losses of the structure. A second rectangular metal structure was designed around the main patch and, when both are connected via a PIN diode, the antenna resonates at 20 GHz. The structure of the antenna can be observed in the Figure 2, and its main dimensions are presented in Table 1. \begin{table}
\begin{tabular}{c c c c c c} \hline \(l_{20}\) & \(w_{20}\) & \(l_{29}\) & \(w_{29}\) & \(y\) & \(s\) \\ \hline
3.32 & 4.33 & 2.41 & 3.25 & 0.79 & 0.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Main dimensions of the designed microstrip antenna structure (in mm). Figure 2: Dual frequency microstrip antenna structure (main patch and outer structure) using a PIN diode and two bias circuits. The dielectric substrate used was the RO4350B, with a dielectric constant \(\varepsilon_{\rm r}\) of 3.48, thickness of 0.762 mm, and loss tangent of 0.0037 (@10 GHz). The frequency reconfigurability is achieved by connecting or disconnecting the main patch (Patch 1) from the outer structure (Patch 2) through a switch, which can be in an ON or OFF state. So, if the diode is reversed-biased (OFF), the patch is disconnected, and the antenna operates at 29 GHz. Otherwise, if the diode is forward-biased (ON), the patches are connected, working as a single patch, and the structure is resonating at 20 GHz. The switching element was the MA4 PBL027 [15], which is a MACOM silicon beam PIN diode. This element has low parasitic elements: series resistance, capacitance, and inductance. The switching speed is extremely fast and has low insertion loss and high isolation, making it perfect to be used in microwave and millimeter wave switch designs. Its equivalent model is shown in Figure 3, and it was used in the electromagnetic simulator. The intrinsic parasite values of this model correspond to a maximum series resistance of 4.0 \(\Omega\), a series inductance of 0.15 nH, a junction capacitance of 0.04 pF, and a parasitic capacitance of 8.0 fF. To turn the diode forward-biased or reverse-biased, it is necessary to apply a DC voltage to its terminals. A bias circuit was designed to supply this voltage to the switch diode, which was composed of radial stubs and \(\lambda\)/4 transformers. It is crucial that this circuit does not affect the performance of the antenna, and so, the RF signal should not cross it. To accomplish that, the input impedance of the bias circuit should be extremely high, that is, theoretically infinite, corresponding to an open circuit (OC) for both frequencies (20 GHz and 29 GHz). Therefore, the circuit acts as an RF blocker, preventing the RF signal from reaching the DC source. Figure 4 shows the structure of the designed dual-frequency bias circuit. To place the bias circuit in the structure of the antenna, a detailed parametric study was made in order to identify the most appropriate location so that it does not affect the performance of the  
Figure 4: Designed structure of the dual frequency bias circuit to the PIN diodes. Figure 3: PIN diode equivalent model. antenna.