# Reconfigurable Diplexer-Based Filtenna for Tx/Rx Operation in Mobile Satellite Terminals
## 3 Antenna Design
Regarding the bias circuit attached to the patch, it was confirmed by simulation that the best position was the center of the patch length, as it is the location where the patch has the lowest impedance (theoretically zero), and therefore, the appropriate location to place in parallel an extremely high impedance, minimizing any impact. As for the bias circuit added to the patch's feed line, several simulations were carried out varying its location, evaluating the place where this copper structure would have the least effect on the overall characteristics of the antenna. Once the diode is placed on the antenna, Figure 5 shows the simulated response of the projected antenna (presented in Figure 2) for the two states of the diode, ON and OFF, for which the antenna operates in the 20 and 29 GHz band, respectively. It is possible to verify that the antenna is appropriately matched for both states, with an S\({}_{11}\) lower than \(-24\) dB, and for a theoretical 90 \(\Omega\) impedance characteristic of the line, the simulated value for the input impedance is 84 \(\Omega\)@20 GHz and 103 \(\Omega\)@29 GHz, which is an excellent compromise for both frequencies. Figure 6 shows the simulated radiation pattern of the microstrip patch, in the two main planes, for the two states of the diode: (a) ON--20 GHz (b) OFF--29 GHz. It is possible to verify that the element maintains an almost semi-spherical radiation, which is typical of these antennas, with a maximum gain of 7.12 dBi at 20 GHz and 5.45 dBi at 29 GHz. Figure 5: Simulated S\({}_{11}\) of the individual patch antenna for both states of the PIN diode. Figure 6: Simulated radiation pattern of the individual patch antenna in the two main radiation planes, for both states of the PIN diode: (a) ON mode (20 GHz) and (b) OFF mode (29 GHz). The microstrip hairpin filter can be used at microwave and millimeter-wave frequencies. Its name comes from its \"U\" shape and it is composed by \(\uplambda\)2 folded parallel coupled lines. With this topology, it is possible to obtain greater bandwidths with a small structure, since compared with others filter topologies, the hairpin filter is more compact [14]. To design the filter, some issues must be considered, such as its order, since it is a compromise between the size and performance. In this filtenna, two third-order hairpin filters were designed, one for each frequency (20 GHz and 29 GHz) with \(Z_{0}=90\)\(\Omega\) transmission lines, as it is the characteristic impedance of the antenna's feed line. Each U section comprising the filter is a \(\uplambda\)2 microstrip line folded with a separation between ends of \(\text{a}_{\text{n}}\) and spaced from the next units of \(\text{b}_{\text{n}}\). As they are third-order filters, it is possible to observe three \"U' sections. The structures of the two designed hairpin filters are shown in Figure 7a,b. The two microstrip filters were designed and optimized in the electromagnetic simulator to find the ideal dimensions of the gaps (\(\text{a}_{\text{n}}\) and \(\text{b}_{\text{n}}\)) ensuring proper operation. The final dimensions of the filters designed using 90 \(\Omega\) characteristic impedance microstrip lines are \(\text{a}_{1}\), \(\text{b}_{1}\), \(\text{a}_{2}\), and \(\text{b}_{2}\) of 0.54 mm, 0.2 mm, 0.14 mm, and 0.2 mm, respectively. These values allowed creating filters with good performance, maintaining a good reflection coefficient of the overall structure in both terminals, and a good transmission in the desired band, which are properties that can be observed in Figure 7c,d. To associate two signals of different frequencies to the same antenna, a method to isolate them had to be found. Two structures working as a two stop-band filters were designed and integrated in the signal paths, as shown in Figure 8.  
Figure 7: Structure of hairpin filters and its S-Parameters simulation: (**a**) filter in 20 GHz band; (**b**) filter in 29 GHz band; (**c**) 20 GHz hairpin filter response; and (**d**) 29 GHz hairpin filter response.