# Reconfigurable Diplexer-Based Filtenna for Tx/Rx Operation in Mobile Satellite Terminals
## 3 Antenna Design
The main objective of using these band-stop filters was to isolate the two bands, that is, preventing that the downlink signal (20 GHz), received by the antenna not to reach port 2, going directly to port 1. Similarly, in transmission mode (29 GHz), it was avoiding the signal from port 2 to reach port 1, going directly to the antenna. To build such a blocking structure, to 29 GHz, a stop-band filter was designed that ensures that when this signal reaches the intersection with the antenna's feed line, it will see an extremely high impedance in the port 1 direction. This way, most of the signal will follow the antenna. For a better understanding, the structure is shown in Figure 8. To design this stop-band filter, a radial stub that forces a short circuit (SC) to 29 GHz was used followed by a \(\lambda\)/4 length line at the same frequency, allowing an open circuit (OC) at the end of this line. So, the signal will observe high impedance (theoretically infinite), and if the diode is reversed-biased, the antenna will radiate it. To improve the antenna matching, an open stub between the 20 GHz radial stub and the 29 GHz hairpin filter was placed, reducing the reflection of the 29 GHz signal. Regarding the downlink signal, the same analysis can be done; when the signal reaches the intersection with the antenna's feed line, it will see an OC to 20 GHz toward port 2, being received through port 1. A radial stub at 20 GHz is placed at \(\lambda\)/4 distance of the line's intersection, creating high impedance. In addition, an open stub was inserted between the 29 GHz radial stub and the 20 GHz hairpin filter to improve the antenna matching. The stop-band filter structure was simulated and optimized, and its final dimensions are reported in Table 2.  
Using the technique described above, it was possible to develop a reconfigurable printed antenna that is capable of operating in two different frequency bands, at 20 GHz and 29 GHz, connected to two independent transmission channels, and therefore two ports, through the activation of a PIN diode. This configuration also allows guaranteeing the isolation between bands through the inclusion of two filters in the path of the antenna feeding, making it a filtenna. The designed filtenna is presented in Figure 2:0 Note that the structure that is going to be integrated into mobile devices is the one limited with the gray dashed line with the dimensions of \(21.9\times 9.8\) mm\({}^{2}\) \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(ls_{20}\) & \(ds_{20}\) & \(ls_{29}\) & \(ds_{29}\) & \(r_{20}\) & \(r_{29}\) \\ \hline
0.50 & 2.51 & 1.20 & 2.91 & 1.73 & 1.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main dimensions of the matching and stop-band filter structure (in mm). Figure 8: Microstrip feeding structure of the dual-frequency antenna using two stop-band filters at each frequency (open circuit, or OC: radial strub + \(\lambda\)/4 line). As it is possible to see in Figure 9, the filtenna is composed of a main patch surrounded by an outer metal structure, with the ability to operate in the satellite's downlink and uplink bands. Two \(\lambda\)/4 length transformers for both frequencies (with characteristic impedance of 67 \(\Omega\)) were used to convert the 90 \(\Omega\) to the standard 50 \(\Omega\) lines. The red numbers correspond to the numbers that identifying the two ports.